Table of contents

We observed the Sh 2-233IR (S233IR) region with better sensitivity in the near-infrared than in previous studies of this region. By applying statistical subtraction of the background stars, we identified member sources and derived the age and mass of three distinguishable sub-groups in this region: Sh 2-233IR NE, Sh 2-233IR SW, and the "distributed stars" over the whole cloud. Star formation may occur sequentially with a relatively small age difference (∼0.2–0.3 Myr) between subclusters. We found that the slopes for the initial mass function (Γ ∼ −0.5) of two subclusters are flatter than those of Salpeter, which suggests that more massive stars were preferentially formed in those clusters compared to other Galactic star-forming regions. These subclusters may not result from the overall collapse of the whole cloud, but have formed by triggering before the previous star formation activities disturbed the natal molecular cloud. Additionally, high star formation efficiency (≳40%) of the subclusters may also suggest that stars form very efficiently in the center of the northeast.

In this work, we revisit the all-sky Galactic diffuse γ-ray emission taking into account new measurements of the cosmic ray (CR) electron/positron spectrum by PAMELA, ATIC, and Fermi, which show excesses of cosmic electrons/positrons beyond the expected fluxes in the conventional model. Since the origins of the extra electrons/positrons are not clear, we consider three different scenarios to account for the excesses: astrophysical sources such as Galactic pulsars, dark matter (DM) decay, and DM annihilation. Further, new results from Fermi-LAT on the (extra-)Galactic diffuse γ-rays are adopted. The background CRs without the new sources give lower diffuse γ-rays compared to the Fermi-LAT observation, which is consistent with previous analysis. The scenario with astrophysical sources predicts diffuse γ-rays with little difference with the background. The DM annihilation models with τ^± final state are disfavored by the Fermi diffuse γ-ray data, while there are only a few constraints on the decaying DM scenario. Furthermore, there is always a bump at higher energies (∼TeV) of the diffuse γ-ray spectra for the DM scenarios due to final state radiation. Finally, we find that the Fermi-LAT diffuse γ-ray data can be explained by simply enlarging the normalization of the electron spectrum without introducing any new sources, which may indicate that the current constraints on the DM models can be much stronger given a precise background estimate.

Combining archival data taken at radio and infrared wavelengths with state-of-the-art measurements at X-ray and gamma-ray energies, we assembled a broadband spectral energy distribution (SED) of Cas A, a young supernova remnant. Except for strong thermal emission at infrared and X-ray wavelengths, the SED is dominated by non-thermal radiation. We attempted to model the non-thermal SED with a two-zone leptonic model which assumes that the radio emission is produced by electrons that are uniformly distributed throughout the remnant while the non-thermal X-ray emission is produced by electrons that are localized in regions near the forward shock. Synchrotron emission from the electrons can account for data from radio to X-ray wavelengths. Much of the GeV–TeV emission can also be explained by a combination of bremsstrahlung emission and inverse Compton scattering of infrared photons. However, the model cannot fit a distinct feature at GeV energies. This feature can be well accounted for by adding a π⁰ emission component to the model, providing evidence for cosmic ray production in Cas A. We discuss the implications of these results.

The Fermi Large Area Telescope (LAT) data have confirmed the pulsed emission from all six high-confidence gamma-ray pulsars previously known from the EGRET observations. We report results obtained from the analysis of 13 months of LAT data for three of these pulsars (PSR J1057−5226, PSR J1709−4429, and PSR J1952+3252) each of which had some unique feature among the EGRET pulsars. The excellent sensitivity of LAT allows more detailed analysis of the evolution of the pulse profile with energy and also of the variation of the spectral shape with phase. We measure the cutoff energy of the pulsed emission from these pulsars for the first time and provide a more complete picture of the emission mechanism. The results confirm some, but not all, of the features seen in the EGRET data.

We perform multi-frequency polarimetry toward 0836+710 using data from the Very Long Baseline Array. These observations allow us to measure both the distributions of the polarization position angle and the Faraday rotation measure (RM). We find a systematic gradient in the RM distribution as has been reported in several observations of relativistic jets emanating from active galactic nuclei. The RM corresponds to the line-of-sight component of the magnetic field. Thus, a systematic gradient of the RM along the transverse direction of the jet implies the existence of helical magnetic components associated with the jet itself. We derive the pitch angle of the helical magnetic field independently from the distribution of the projected magnetic field and from the RM data. Their discrepancies can be understood in a spine/sheath structure of the jet.

IC 2118, also known as the Witch Head Nebula, is a wispy, roughly cometary, ∼5 degree long reflection nebula, and is thought to be a site of triggered star formation. In order to search for new young stellar objects (YSOs), we have observed this region in seven mid- and far-infrared bands using the Spitzer Space Telescope and in four bands in the optical using the U. S. Naval Observatory 40 inch telescope. We find infrared excesses in four of the six previously known T Tauri stars in our combined infrared maps, and we find six entirely new candidate YSOs, one of which may be an edge-on disk. Most of the YSOs seen in the infrared are Class II objects, and they are all in the "head" of the nebula, within the most massive molecular cloud of the region.

The outflow driven by the Class 0 protostar, IRAS 16253−2429, is associated with bipolar cavities visible in scattered mid-infrared light, which we refer to as the Wasp-Waist Nebula. InfraRed Spectometer (IRS) scan mapping with the Spitzer Space Telescope of a ∼1' × 2' area centered on the protostar was carried out. The outflow is imaged in six pure rotational (0-0 S(2) through 0-0 S(7)) H₂ lines, revealing a distinct, S-shaped morphology in all maps. A source map in the 11.3 μm polycyclic aromatic hydrocarbon (PAH) feature is presented in which the protostellar envelope appears in absorption. This is the first detection of absorption in the 11.3 μm PAH feature. Spatially resolved excitation analysis of positions in the blue- and redshifted outflow lobes, with extinction-corrections determined from archival Spitzer 8 μm imaging, shows remarkably constant temperatures of ∼1000 K in the shocked gas. The radiated luminosity in the observed H₂ transitions is found to be 1.94 ± 0.05 × 10⁻⁵L_☉ in the redshifted lobe and 1.86 ± 0.04 × 10⁻⁵L_☉ in the blueshifted lobe. These values are comparable to the mechanical luminosity of the flow. By contrast, the mass of hot (T ∼ 1000 K) H₂ gas is 7.95 ± 0.19 × 10⁻⁷M_☉ in the redshifted lobe and 5.78 ± 0.17 × 10⁻⁷M_☉ in the blueshifted lobe. This is just a tiny fraction, of order 10⁻³, of the gas in the cold (30 K), swept-up gas mass derived from millimeter CO observations. The H₂ortho/para ratio of 3:1 found at all mapped points in this flow suggests previous passages of shocks through the gas. Comparison of the H₂ data with detailed shock models of Wilgenbus et al. shows the emitting gas is passing through Jump (J-type) shocks. Pre-shock densities of 10⁴ cm⁻³⩽ n_H⩽ 10⁵ cm⁻³ are inferred for the redshifted lobe and n_H⩽ 10³ cm⁻³ for the blueshifted lobe. Shock velocities are 5 km s⁻¹ ⩽  v_s⩽ 10 km s⁻¹ for the redshifted gas and v_s= 10 km s⁻¹ for the blueshifted gas. Initial transverse (to the shock) magnetic field strengths for the redshifted lobe are in the range 10–32 μG, and just 3 μG for the blueshifted lobe. A cookbook for using the CUBISM contributed software for IRS spectral mapping data is presented in the Appendix.

We present Spitzer MIPS 24 μm observations of sixteen 0.4 < z < 0.8 galaxy clusters drawn from the ESO Distant Cluster Survey. This is the first large 24 μm survey of clusters at intermediate redshift. The depth of our imaging corresponds to a total IR luminosity of 8 × 10¹⁰ L_☉, just below the luminosity of luminous infrared galaxies (LIRGs), and 6⁺¹₋₁% of M_V < −19 cluster members show 24 μm emission at or above this level. We compare with a large sample of coeval field galaxies and find that while the fraction of cluster LIRGs lies significantly below that of the field, the IR luminosities of the field and cluster galaxies are consistent. However, the stellar masses of the EDisCS LIRGs are systematically higher than those of the field LIRGs. A comparison with optical data reveals that ∼80% of cluster LIRGs are blue and the remaining 20% lie on the red sequence. Of LIRGs with optical spectra, 88⁺⁴₋₅% show [O ii] emission with EW([O ii]) > 5 Å, and ∼75% exhibit optical signatures of dusty starbursts. On average, the fraction of cluster LIRGs increases with projected clustercentric radius but remains systematically lower than the field fraction over the area probed (<1.5× R₂₀₀). The amount of obscured star formation declines significantly over the 2.4 Gyr interval spanned by the EDisCS sample, and the rate of decline is the same for the cluster and field populations. Our results are consistent with an exponentially declining LIRG fraction, with the decline in the field delayed by ∼1 Gyr relative to the clusters.

We explore the effects of the deflagration to detonation transition (DDT) density on the production of ⁵⁶Ni in thermonuclear supernova (SN) explosions (Type Ia supernovae). Within the DDT paradigm, the transition density sets the amount of expansion during the deflagration phase of the explosion and therefore the amount of nuclear statistical equilibrium (NSE) material produced. We employ a theoretical framework for a well-controlled statistical study of two-dimensional simulations of thermonuclear SNe with randomized initial conditions that can, with a particular choice of transition density, produce a similar average and range of ⁵⁶Ni masses to those inferred from observations. Within this framework, we utilize a more realistic "simmered" white dwarf progenitor model with a flame model and energetics scheme to calculate the amount of ⁵⁶Ni and NSE material synthesized for a suite of simulated explosions in which the transition density is varied in the range (1–3) ×10⁷ g cm⁻³. We find a quadratic dependence of the NSE yield on the log of the transition density, which is determined by the competition between plume rise and stellar expansion. By considering the effect of metallicity on the transition density, we find the NSE yield decreases by 0.055 ± 0.004 M_☉ for a 1 Z_☉ increase in metallicity evaluated about solar metallicity. For the same change in metallicity, this result translates to a 0.067 ± 0.004 M_☉ decrease in the ⁵⁶Ni yield, slightly stronger than that due to the variation in electron fraction from the initial composition. Observations testing the dependence of the yield on metallicity remain somewhat ambiguous, but the dependence we find is comparable to that inferred from some studies.

The SpitzerSpace Telescope routinely detects asteroids in astrophysical observations near the ecliptic plane. For the galactic or extragalactic astronomer, these solar system bodies can introduce appreciable uncertainty into the source identification process. We discuss an infrared color discrimination tool that may be used to distinguish between solar system objects and extrasolar sources. We employ four Spitzer Legacy data sets, the First Look Survey-Ecliptic Plane Component (FLS-EPC), SCOSMOS, SWIRE, and GOODS. We use the Standard Thermal Model to derive FLS-EPC main belt asteroid (MBA) diameters of 1–4 km for the numbered asteroids in our sample and note that several of our solar system sources may have fainter absolute magnitude values than previously thought. A number of the MBAs are detected at flux densities as low as a few tens of μJy at 3.6 μm. As the FLS-EPC provides the only 3.6–24.0 μm observations of individual asteroids to date, we are able to use this data set to carry out a detailed study of asteroid color in comparison to astrophysical sources observed by SCOSMOS, SWIRE, and GOODS. Both SCOSMOS and SWIRE have identified a significant number of asteroids in their data, and we investigate the effectiveness of using relative color to distinguish between asteroids and background objects. We find a notable difference in color in the IRAC 3.6–8.0 mm and MIPS 24 μm bands between the majority of MBAs, stars, galaxies, and active galactic nuclei, though this variation is less significant when comparing fluxes in individual bands. We find median colors for the FLS-EPC asteroids to be [F(5.8/3.6), F(8.0/4.5), F(24/8)] = (4.9 ± 1.8, 8.9 ± 7.4, 6.4 ± 2.3). Finally, we consider the utility of this technique for other mid-infrared observations that are sensitive to near-Earth objects, MBAs, and trans-Neptunian objects. We consider the potential of using color to differentiate between solar system and background sources for several space-based observatories, including Warm Spitzer, Herschel, and WISE.

We report on the study of a fast coronal mass ejection (CME)-driven shock associated with the solar eruption of 2002 March 22. This event was observed in the intermediate corona both in white light and the extreme ultraviolet (EUV) by the LASCO and UVCS instruments on board the Solar and Heliospheric Observatory, as well as in metric and decametric wavelengths through space- and ground-based radio observatories. Clear signatures of shock transit are (1) strong type II emission lanes observed after the CME initiation, (2) strong O vi λλ1032, 1037 line profile broadenings (up to ∼2 × 10⁷ K) associated with the shock transit across the UVCS slit field of view, and (3) a density enhancement located in LASCO images above the CME front. Since the UVCS slit was centered at 4.1 R_☉, in correspondence with the flank of the expanding CME, this observation represents the highest UV detection of a shock obtained so far with the UVCS instrument. White-light and EUV data have been combined in order to estimate not only the shock compression ratio and the plasma temperature, but also the strength of the involved coronal magnetic fields, by applying the Rankine–Hugoniot equations for the general case of oblique shocks. Results show that, for a compression ratio X = 2.06 as derived from LASCO data, the coronal plasma is heated across the shock from an initial temperature of 2.3 ×  10⁵ K up to 1.9 ×  10⁶ K, while at the same time the magnetic field undergoes a compression from a pre-shock value of ∼0.02 G up to a post-shock field of ∼0.04 G. Magnetic and kinetic energy density increases at the shock are comparable (in agreement with the idea of equipartition of energy), and both are more than two times larger than the thermal energy density increase. This is the first time that a complete characterization of pre- and post-shock plasma physical parameters has been derived in the solar corona.

On 2009 September 26, a dramatic and large filament (LF) eruption and a small filament (SF) eruption were observed in the He ii 304 Å line by the two EUVI telescopes aboard the STEREOA and B spacecraft. The LF heads out into space and becomes the bright core of a gradual coronal mass ejection (CME), while the eruption of the SF is characterized by motions of the filament materials. Using stereoscopic analysis of EUVI data, we reconstruct the three-dimensional shape and evolution of two eruptive filaments. For the first time, we investigate the true velocities and accelerations of 12 points along the axis of the LF, and find that the velocity and acceleration vary with the measured location. The highest points among the 12 points are the fastest in the first half hour, and then the points at the low-latitude leg of the LF become the fastest. For the SF, it is an asymmetric whip-like filament eruption, and the downward motions of the material lead to the disappearance of the former high-latitude endpoint and the formation of a new low-latitude endpoint. Based on the temporal evolution of the two filaments, we infer that the two filaments lie in the same filament channel. By combining the EUVI, COR1, and COR2 data of STEREO A together, we find that there is no impulsive or fast acceleration in this event. It displays a weak and persistent acceleration for more than 17 hr. The average velocity and acceleration of the LF are 101.8 km s⁻¹ and 2.9 m s⁻², respectively. The filament eruptions are associated with a slow CME with an average velocity of 177.4 km s⁻¹. The velocity of the CME is nearly 1.6 times as large as that of the filament material. This event is one example of a gradual filament eruption associated with a gradual CME. In addition, the moving direction of the LF changes from a non-radial to a nearly radial direction with a variation of inclination angle of nearly 382.

The standard planetary formation models assume that primitive materials, such as carbonaceous chondrites, are the precursor materials of evolved planetesimals. Past chronological studies have revealed that planetesimals of several hundred kilometers in size, such as the Howardite–Eucrite–Diogenite (HED) parent body (Vesta) and angrite parent body, began their differentiation as early as ∼3 million years of the solar system formation, and continued for at least several million years. However, the timescale of planetesimal formation in distinct regions of the inner solar system, as well as the isotopic characteristics of the reservoirs from which they evolved, remains unclear. Here we present the first report for the precise ⁵³Mn–⁵³Cr ages of monomict ureilites. Chemically separated phases from one monomict ureilite (NWA 766) yielded the Mn–Cr age of 4564.60 ± 0.67 Ma, identical within error to the oldest age preserved in other achondrites, such as angrites and eucrites. The ⁵⁴Cr isotopic data for this and seven additional bulk ureilites show homogeneous ε⁵⁴Cr of ∼−0.9, a value distinct from other achondrites and chondrites. Using the ε⁵⁴Cr signatures of Earth, Mars, and Vesta (HED), we noticed a linear decrease in the ε⁵⁴Cr value with the heliocentric distance in the inner region of the solar system. If this trend can be extrapolated into the outer asteroid belt, the ε⁵⁴Cr signatures of monomict ureilites will place the position of the ureilite parent body at ∼2.8 AU. These observations imply that the differentiation of achondrite parent bodies began nearly simultaneously at ∼4565 Ma in different regions of the inner solar system. The distinct ε⁵⁴Cr value between ureilite and carbonaceous chondrite also implies that a genetic link commonly proposed between the two is unlikely.

We present a detailed analysis of Spitzer-IRAC images obtained toward six Herbig–Haro objects (HH 54/211/212, L 1157/1448, and BHR 71). Our analysis includes (1) comparisons of morphology between the four IRAC bands (3.6, 4.5, 5.8, and 8.0 μm) and H₂ 1–0 S(1) at 2.12 μm for three out of six objects, (2) measurements of spectral energy distributions (SEDs) at selected positions, and (3) comparisons of these results with calculations of thermal H₂ emission at LTE (207 lines in four bands) and non-LTE (32–45 lines, depending on the particle for collisions). We show that the morphologies observed at 3.6 and 4.5 μm are similar to each other and to H₂ 1–0 S(1). This is well explained by thermal H₂ emission at non-LTE if the dissociation rate is significantly larger than 0.002–0.02, allowing thermal collisions to be dominated by atomic hydrogen. In contrast, the 5.8 and 8.0 μm emission shows different morphologies from the others in some regions. This emission appears to be more enhanced at the wakes in bow shocks, or less enhanced in patchy structures in the jet. These tendencies are explained by the fact that thermal H₂ emission in the 5.8 and 8.0 μm band is enhanced in regions at lower densities and temperatures. Throughout, the observed similarities and differences in morphology between four bands and 1–0 S(1) are well explained by thermal H₂ emission. The observed SEDs are categorized into type-A, those in which the flux monotonically increases with wavelength, and type-B, those with excess emission at 4.5 μm. The type-A SEDs are explained by thermal H₂ emission, in particular with simple shock models with a power-law cooling function (Λ ∝ T^s). Our calculations suggest that the type-B SEDs require extra contaminating emission in the 4.5 μm band. The CO vibrational emission is the most promising candidate, and the other contaminants discussed to date (H i, [Fe ii], fluorescent H₂, and polycyclic aromatic hydrocarbon) are not likely to explain the observed SEDs.

Rate constants were measured at 300 K for the reactions of the hydride anion, H⁻, with neutral molecules C₂H₂, H₂O, CH₃CN, CH₃OH, (CH₃)₂CO, CH₃CHO, N₂O, CO₂, O₂, CO, CH₃Cl, (CH₃)₃CCl, (CH₃CH₂)₂O, C₆H₆, and D₂ using a flowing-afterglow instrument. Experimental work was supplemented by ab initio calculations to provide insight into the viability of reaction pathways. Our reported rate constants should prove useful to models of astrophysical environments where conditions prevail for the existence of both H⁻ and neutral species. The variety of neutral reactants studied includes representative species from prototypical chemical groups, effectively mapping reactivity trends for the hydride anion.

We use a simple two-layer outer gap model, whose accelerator consists of a primary region and a screening region, to discuss the γ-ray spectrum of mature pulsars detected by Fermi. By solving the Poisson equation with an assumed simple step-function distribution for the charge density in these two regions, the distribution of the electric field and the curvature radiation process of the accelerated particles can be calculated. In our model, the properties of the phase-averaged spectrum can be completely specified by three gap parameters, i.e., the fractional gap size in the outer magnetosphere, the gap current in the primary region, and the gap size ratio between the primary region and the total gap size. We discuss how these parameters affect the spectral properties. We argue that although the radiation mechanism in the outer gap is a curvature radiation process, the observed gamma-ray spectrum can substantially deviate from the simple curvature spectrum because the overall spectrum consists of two components, i.e., a primary region and a screening region. In some pulsars, the radiation from the screening region is so strong that the photon index from 100 MeV to several GeV can be as flat as ∼2. We show that the fitting fractional gap thickness of the canonical pulsars increases with the spin-down age. We find that the total gap current is about 50% of the Goldreich–Julian value and the thickness of the screening region is a few percent of the total gap thickness. We also find that the predicted γ-ray luminosity is less dependent on the spin-down power (L_sd) for pulsars with L_sd ≳ 10³⁶ erg s⁻¹, while the γ-ray luminosity decreases with the spin-down power for pulsars with L_sd ≲ 10³⁶ erg s⁻¹. This relation may imply that the major gap closure mechanism is a photon–photon pair-creation process for pulsars with L_sd ≳ 10³⁶ erg s⁻¹, while it is a magnetic pair-creation process for pulsars with L_sd ≲ 10³⁶ erg s⁻¹.

The relationships between color, characterized with respect to the g − r red sequence; stellar structure, as determined using the i-band Sérsic index; and group membership are explored using the Sloan Digital Sky Survey (SDSS). The new results place novel constraints on theories of galaxy evolution, despite the strong correlation between color and stellar structure. Observed correlations are of three independent types—those based on stellar structure, on the color of disk-like galaxies, and on the color of elliptical galaxies. Of particular note, the fraction of galaxies residing on the red sequence measured among galaxies with disk-like light profiles is enhanced for satellite galaxies compared to central galaxies. This fraction increases with group mass. When these new results are considered, theoretical treatments of galaxy evolution that adopt a gas accretion model centered on the hot galactic halo cannot consistently account for all observations of disk galaxies. The hypothesis is advanced that inflow within the extended H i disk prolongs star formation in satellite galaxies. When combined with partial ram pressure stripping (RPS) of this disk, this new scenario is consistent with the observations. This is demonstrated by applying an analytical model of RPS of the extended H i disk to the SDSS groups. These results motivate incorporating more complex modes of gas accretion into models of galaxy evolution, including cold mode accretion, an improved treatment of gas dynamics within disks, and disk stripping.

A number of neutron star low-mass X-ray binaries (LMXBs) have recently been discovered to show broad, asymmetric Fe K emission lines in their X-ray spectra. These lines are generally thought to be the most prominent part of a reflection spectrum, originating in the inner part of the accretion disk where strong relativistic effects can broaden emission lines. We present a comprehensive, systematic analysis of Suzaku and XMM-Newton spectra of 10 neutron star LMXBs, all of which display broad Fe K emission lines. Of the 10 sources, 4 are Z sources, 4 are atolls, and 2 are accreting millisecond X-ray pulsars (also atolls). The Fe K lines are fit well by a relativistic line model for a Schwarzschild metric, and imply a narrow range of inner disk radii (6–15 GM/c²) in most cases. This implies that the accretion disk extends close to the neutron star surface over a range of luminosities. Continuum modeling shows that for the majority of observations, a blackbody component (plausibly associated with the boundary layer) dominates the X-ray emission from 8 to 20 keV. Thus it appears likely that this spectral component produces the majority of the ionizing flux that illuminates the accretion disk. Therefore, we also fit the spectra with a blurred reflection model, wherein a blackbody component illuminates the disk. This model fits well in most cases, supporting the idea that the boundary layer illuminates a geometrically thin disk.

The high-density star formation typical of the merger/starburst events that power the large IR luminosities of ultraluminous infrared galaxies (ULIRGs) (L_IR(8–1000 μm) ≳10¹² L_☉) throughout the universe results in extraordinarily high cosmic-ray (CR) energy densities of U_CR ∼ few ×(10³–10⁴) U_CR,Gal permeating their interstellar medium, a direct consequence of the large supernova remnant number densities in such systems. Unlike far-UV photons emanating from numerous star-forming (SF) sites, these large CR energy densities in ULIRGs will volumetrically heat and raise the ionization fraction of dense (n > 10⁴ cm⁻³) UV-shielded gas cores throughout their compact SF volumes. Such conditions can turn most of the large molecular gas masses found in such systems and their high redshift counterparts (∼10⁹–10¹⁰ M_☉) into giant CR-dominated regions (CRDRs) rather than ensembles of photon-dominated regions (PDRs) which dominate in less IR-luminous systems where star formation and molecular gas distributions are much more extended. The molecular gas in CRDRs will have a minimum temperature of T_kin ∼ (80–160) K, and very high ionization fractions of x(e) > 10⁻⁶ throughout its UV-shielded dense core, which in turn will fundamentally alter the initial conditions for star formation in such systems. Observational tests of CRDRs can be provided by high-J CO and ¹³CO lines or multi-J transitions of any heavy rotor molecules (e.g., HCN) and their isotopologs. Chemical signatures of very high ionization fractions in dense UV-shielded gas such as low [DCO⁺]/[HCO⁺] and high [HCO⁺]/[CO] abundance ratios would be good probes of CRDRs in extreme starbursts. These tests, along with direct measurements of the high CO line brightness temperatures expected over the areas of compact dense gas disks found in ULIRGs, will soon be feasible as sub-arcsecond interferometric imaging capabilities and sensitivity at millimeter/submillimeter wavelengths improve in the era of ALMA.

We present a radiative magnetohydrodynamics simulation of the formation of an active region (AR) on the solar surface. The simulation models the rise of a buoyant magnetic flux bundle from a depth of 7.5 Mm in the convection zone up into the solar photosphere. The rise of the magnetic plasma in the convection zone is accompanied by predominantly horizontal expansion. Such an expansion leads to a scaling relation between the plasma density and the magnetic field strength such that B ∝ ϱ^1/2. The emergence of magnetic flux into the photosphere appears as a complex magnetic pattern, which results from the interaction of the rising magnetic field with the turbulent convective flows. Small-scale magnetic elements at the surface first appear, followed by their gradual coalescence into larger magnetic concentrations, which eventually results in the formation of a pair of opposite polarity spots. Although the mean flow pattern in the vicinity of the developing spots is directed radially outward, correlations between the magnetic field and velocity field fluctuations allow the spots to accumulate flux. Such correlations result from the Lorentz-force-driven, counterstreaming motion of opposite polarity fragments. The formation of the simulated AR is accompanied by transient light bridges between umbrae and umbral dots. Together with recent sunspot modeling, this work highlights the common magnetoconvective origin of umbral dots, light bridges, and penumbral filaments.

We report spectroscopic confirmation and high-resolution infrared imaging of a z = 2.79 triply imaged galaxy behind the Bullet Cluster. This source, a Spitzer-selected luminous infrared galaxy, is confirmed via polycyclic aromatic hydrocarbon (PAH) features using the Spitzer Infrared Spectrograph (IRS) and resolved with Hubble Space Telescope Wide Field Camera 3 imaging. In this galaxy, which with a stellar mass M_* ≈ 4 × 10⁹ M_☉ is one of the two least massive ones studied with IRS at z>2, we also detect H₂ S(4) and H₂ S(5) pure rotational lines (at 3.1σ and 2.1σ)—the first detection of these molecular hydrogen lines in a high-redshift galaxy. From the molecular hydrogen lines we infer an excitation temperature T = 377⁺⁶⁸₋₈₄ K. The detection of these lines indicates that the warm molecular gas mass is 6⁺³⁶₋₄% of the stellar mass and implies the likely existence of a substantial reservoir of cold molecular gas in the galaxy. Future spectral observations at longer wavelengths with facilities such as the HerschelSpace Observatory, the Large Millimeter Telescope, and the Atacama Pathfinder Experiment thus hold the promise of precisely determining the total molecular gas mass. Given the redshift, and using refined astrometric positions from the high-resolution imaging, we also update the magnification estimate and derived fundamental physical properties of this system. The previously published values for L_IR, star formation rate, and dust temperature are confirmed modulo the revised magnification; however, we find that PAH emission is roughly a factor of 5 stronger than would be predicted by the relations between L_IR and L_PAH reported for SMGs and starbursts in Pope et al.

We present 7.5–14.2 μm low-resolution spectroscopy, obtained with the Spitzer Infrared Spectrograph, of the T8.5 dwarf Wolf 940 B, which is a companion to an M4 dwarf with a projected separation of 400 AU. We combine these data with previously published near-infrared spectroscopy and mid-infrared photometry to produce the spectral energy distribution for the very low temperature T dwarf. We use atmospheric models to derive the bolometric correction and obtain a luminosity of log L/L_☉ = −6.01 ± 0.05 (the observed spectra make up 47% of the total flux). Evolutionary models are used with the luminosity to constrain the values of effective temperature (T_eff) and surface gravity and hence mass and age for the T dwarf. We ensure that the spectral models used to determine the bolometric correction have T_eff and gravity values consistent with the luminosity-implied values. We further restrict the allowed range of T_eff and gravity using age constraints implied by the M dwarf primary and refine the physical properties of the T dwarf by comparison of the observed and modeled spectroscopy and photometry. This comparison indicates that Wolf 940 B has a metallicity within ∼0.2 dex of solar, as more extreme values give poor fits to the data—lower metallicity produces a poor fit at λ>2 μm, while higher metallicity produces a poor fit at λ < 2 μm. This is consistent with the independently derived value of [m/H] =+0.24 ± 0.09 for the primary star, using the Johnson & Apps M_K: V − K relationship. We find that the T dwarf atmosphere is undergoing vigorous mixing, with an eddy diffusion coefficient K_zz of 10⁴ to 10⁶ cm² s⁻¹. We derive an effective temperature of 585 K to 625 K, and surface gravity log  g = 4.83 to 5.22 (cm s⁻²), for an age range of 3 Gyr to 10 Gyr, as implied by the kinematic and Hα properties of the M dwarf primary. Gravity and temperature are correlated such that the lower gravity corresponds to the lower temperature and younger age for the system and the higher values to the higher temperature and older age. The mass of the T dwarf is 24 M_Jupiter to 45 M_Jupiter for the younger to older age limit.

Data from the Five College Radio Astronomy Observatory CO Mapping Survey of the Taurus molecular cloud are combined with extinction data for a sample of 292 background field stars to investigate the uptake of CO from the gas to icy grain mantles on dust within the cloud. On the assumption that the reservoir of CO in the ices is represented well by the combined abundances of solid CO and solid CO₂ (which forms by oxidation of CO on the dust), we find that the total column density (gas + solid) correlates tightly with visual extinction (A_V) over the range 5 mag < A_V < 30 mag, i.e., up to the highest extinctions covered by our sample. The mean depletion of gas-phase CO, expressed as δ(CO) = N(CO)_ice/N(CO)_total, increases monotonically from negligible levels for A_V ≲ 5 to ∼ 0.3 at A_V = 10 and ∼ 0.6 at A_V = 30. As these results refer to line-of-sight averages, they must be considered lower limits to the actual depletion at loci deep within the cloud, which may approach unity. We show that it is plausible for such high levels of depletion to be reached in dense cores on timescales ∼0.6 Myr, comparable with their expected lifetimes. Dispersal of cores during star formation may be effective in maintaining observable levels of gaseous CO on the longer timescales estimated for the age of the cloud.

We present observations of HESS J1640−465 with the Fermi-Large Area Telescope. The source is detected with high confidence as an emitter of high-energy gamma-rays. The spectrum lacks any evidence for the characteristic cutoff associated with emission from pulsars, indicating that the emission arises primarily from the pulsar wind nebula (PWN). Broadband modeling implies an evolved nebula with a low magnetic field resulting in a high γ-ray to X-ray flux ratio. The Fermi emission exceeds predictions of the broadband model, and has a steeper spectrum, possibly resulting from a distinct excess of low energy electrons similar to what is inferred for both the Vela X and Crab PWNe.

We report on the Fermi-LAT observations of the Geminga pulsar, the second brightest non-variable GeV source in the γ-ray sky and the first example of a radio-quiet γ-ray pulsar. The observations cover one year, from the launch of the Fermi satellite through 2009 June 15. A data sample of over 60,000 photons enabled us to build a timing solution based solely on γ-rays. Timing analysis shows two prominent peaks, separated by Δϕ = 0.497 ± 0.004 in phase, which narrow with increasing energy. Pulsed γ-rays are observed beyond 18 GeV, precluding emission below 2.7 stellar radii because of magnetic absorption. The phase-averaged spectrum was fitted with a power law with exponential cutoff of spectral index Γ = (1.30 ± 0.01 ± 0.04), cutoff energy E₀ = (2.46 ± 0.04 ± 0.17) GeV, and an integral photon flux above 0.1 GeV of (4.14 ± 0.02 ± 0.32) × 10⁻⁶ cm⁻² s⁻¹. The first uncertainties are statistical and the second ones are systematic. The phase-resolved spectroscopy shows a clear evolution of the spectral parameters, with the spectral index reaching a minimum value just before the leading peak and the cutoff energy having maxima around the peaks. The phase-resolved spectroscopy reveals that pulsar emission is present at all rotational phases. The spectral shape, broad pulse profile, and maximum photon energy favor the outer magnetospheric emission scenarios.

We present composite 3.6 and 4.5 μm luminosity functions (LFs) for cluster galaxies measured from the Spitzer Deep, Wide-Field Survey for 0.3 < z < 2. We compare the evolution of m* for these LFs to models for passively evolving stellar populations to constrain the primary epoch of star formation in massive cluster galaxies. At low redshifts (z ≲ 1.3), our results agree well with models with no mass assembly and passively evolving stellar populations with a luminosity-weighted mean formation redshift z_f = 2.4 assuming a Kroupa initial mass function (IMF). We conduct a thorough investigation of systematic biases that might influence our results, and estimate systematic uncertainties of Δz_f = ^+0.16_−0.18 (model normalization), Δz_f = ^+0.40_−0.05 (α), and Δz_f = ^+0.30_−0.45 (choice of stellar population model). For a Salpeter-type IMF, the typical formation epoch is thus strongly constrained to be z ∼ 2–3. Higher formation redshifts can only be made consistent with the data if one permits an evolving IMF that is bottom-light at high redshift, as suggested by van Dokkum. At high redshifts (z ≳ 1.3), we also witness a statistically significant (>5σ) disagreement between the measured LF and the continuation of the passive evolution model from lower redshifts. After considering potential systematic biases that might influence our highest redshift data points, we interpret the observed deviation as potential evidence for ongoing mass assembly at this epoch.

We perform a cross-correlation analysis of microwave data from the Wilkinson Microwave Anisotropy Probe and photometric quasars from the Sloan Digital Sky Survey, testing for the Sunyaev–Zeldovich (SZ) effect from quasars. A statistically significant (2.5σ) temperature decrement exists in the 41 GHz microwave band. A two-component fit to the cross-correlation spectrum incorporating both dust emission and SZ yields a best-fit y parameter of (7.0 ± 3.4) × 10⁻⁷. A similar cross-correlation analysis with the luminous red galaxy sample from Sloan gives a best-fit y parameter of (5.3 ±  2.5) × 10⁻⁷. We discuss the possible physical origin of these signals, which is likely a combination of SZ effects from quasars and galaxy clusters. Both the Planck Surveyor satellite and the current ground-based arcminute-resolution microwave experiments will detect this signal with a higher statistical significance.

The dynamics and the energy balance of large-scale fluctuations in a coronal loop are studied. The loop is represented by a simplified structure where the curvature is neglected and the background magnetic field is uniform. In a previous paper, we studied a similar model where a uniform background density was assumed. The present paper represents a generalization of the previous one and it has the purpose of investigating possible modifications to the large-scale energy balance and dynamics due to a more realistic longitudinally nonuniform density. Large-scale fluctuations are dominated by coherent eigenmodes that nonlinearly couple to produce an energy cascade to smaller scales. Eigenmodes properties are calculated by a simplified linear dissipative model, deriving an expression for the input energy flux that is not substantially modified by the presence of the density inhomogeneity and is independent of dissipation. For typical values of the parameters, the derived input energy flux is comparable with that required to heat the active region corona. Nonlinear couplings are dominated by coherence effects due to the symmetry properties of eigenmodes; the consequences are that the system is in a weakly nonlinear regime that produces fluctuating energy storage in the loop, and that the kinetic and magnetic nonlinear energy fluxes are of the same order, despite the dominance of magnetic energy at large scales. From the energy balance, an expression for the velocity fluctuation is derived, which is valid in the more general case of a nonuniform background density; this estimate is in agreement both with measures of nonthermal velocities in the solar corona and with previous numerical results.

We report on a dusty Mg ii absorber associated with the quasar SDSS J003545.13+011441.2 (hereafter J0035+0114) at z = 1.5501, the strongest of the three Mg ii absorbers along the sight line of the quasar. The two low-redshift intervening absorbers are at z = 0.7436 and 0.5436. Based on the photometric and spectroscopic data of the Sloan Digital Sky Survey (SDSS), we infer that the rest-frame color excess E(B − V) due to the associated dust is more than 0.07 by assuming a Small Magellanic Cloud (SMC) type extinction curve. Our follow-up moderate resolution spectroscopic observation with the ESI spectrometer at the 10 m Keck telescope enables us to reliably identify most of the important metal elements, such as Zn, Fe, Mn, Mg, Al, Si, Cr, and Ni, in the associated system. We measure the column density of each species and detect significant dust depletion. In addition, we develop a simulation technique to gauge the significance of a 2175 Å dust absorption bump in the SDSS quasar spectra. By using this technique, we analyze the SDSS spectrum of J0035+0114 for the presence of an associated 2175 Å extinction feature and report a tentative detection at a ∼2σ significant level.

We report the discovery of a new transiting close-in giant planet, WASP-24 b, in a 2.341 day orbit, 0.037 AU from its F8-9 type host star. By matching the star's spectrum with theoretical models, we infer an effective temperature T_eff = 6075 ± 100 K and a surface gravity of log g = 4.15 ± 0.10. A comparison of these parameters with theoretical isochrones and evolutionary mass tracks places only weak constraints on the age of the host star, which we estimate to be 3.8^+1.3_−1.2 Gyr. The planetary nature of the companion was confirmed by radial velocity measurements and additional photometric observations. These data were fit simultaneously in order to determine the most probable parameter set for the system, from which we infer a planetary mass of 1.071^+0.036_−0.038 M_Jup and radius 1.3^+0.039_−0.037 R_Jup.

GJ436b is a unique member of the transiting extrasolar planet population being one of the smallest and least irradiated and possessing an eccentric orbit. Because of its size, mass, and density, GJ436b could plausibly have an atmospheric metallicity similar to Neptune (20–60 times solar abundances), which makes it an ideal target to study the effects of atmospheric metallicity on dynamics and radiative transfer in an extrasolar planetary atmosphere. We present three-dimensional atmospheric circulation models that include realistic non-gray radiative transfer for 1, 3, 10, 30, and 50 times solar atmospheric metallicity cases of GJ436b. Low metallicity models (1 and 3 times solar) show little day/night temperature variation and strong high-latitude jets. In contrast, higher metallicity models (30 and 50 times solar) exhibit day/night temperature variations and a strong equatorial jet. Spectra and light curves produced from these simulations show strong orbital phase dependencies in the 50 times solar case and negligible variations with orbital phase in the 1 times solar case. Comparisons between the predicted planet/star flux ratio from these models and current secondary eclipse measurements support a high metallicity atmosphere (30–50 times solar abundances) with disequilibrium carbon chemistry at play for GJ436b. Regardless of the actual atmospheric composition of GJ436b, our models serve to illuminate how metallicity influences the atmospheric circulation for a broad range of warm extrasolar planets.

The holy grail of exoplanet searches is an exo-Earth, an Earth mass planet in the habitable zone (HZ) around a nearby star. Mass is one of the most important characteristics of a planet and can only be measured by observing the motion of the star around the planet–star center of gravity. The planet's orbit can be measured either by imaging the planet at multiple epochs or by measuring the position of the star at multiple epochs by space-based astrometry. The measurement of an exoplanet's orbit by direct imaging is complicated by a number of factors. One is the inner working angle (IWA). A space coronagraph or interferometer imaging an exo-Earth can separate the light from the planet from the light from the star only when the star–planet separation is larger than the IWA. Second, the apparent brightness of a planet depends on the orbital phase. A single image of a planet cannot tell us whether the planet is in the HZ or distinguish whether it is an exo-Earth or a Neptune-mass planet. Third is the confusion that may arise from the presence of multiple planets. With two images of a multiple planet system, it is not possible to assign a dot to a planet based only on the photometry and color of the planet. Finally, the planet–star contrast must exceed a certain minimum value in order for the planet to be detected. The planet may be unobservable even when it is outside the IWA, such as when the bright side of the planet is facing away from us in a "crescent" phase. In this paper we address the question: "Can a prior astrometric mission that can identify which stars have Earth-like planets significantly improve the science yield of a mission to image exo-Earths?" In the case of the Occulting Ozone Observatory, a small external occulter mission that cannot measure spectra, we find that the occulter mission could confirm the orbits of ∼4 to ∼5 times as many exo-Earths if an astrometric mission preceded it to identify which stars had such planets. In the case of an internal coronagraph we find that a survey of the nearest ∼60 stars could be done with a telescope half the size if an astrometric mission had first identified the presence of Earth-like planets in the HZ and measured their orbital parameters.

We extend color–magnitude relations for moderate-luminosity X-ray active galactic nucleus (AGN) hosts and non-AGN galaxies through the galaxy formation epoch (z ≈ 1–4) in the Chandra Deep Field-North and Chandra Deep Field-South (CDF-N and CDF-S, respectively; jointly CDFs) surveys. This study was enabled by the deepest available X-ray data from the 2 Ms CDF surveys as well as complementary ultradeep multiwavelength data in these regions. We utilized analyses of color–magnitude diagrams (CMDs) to assess the role of moderate-luminosity AGNs in galaxy evolution. First, we confirm some previous results and extend them to higher redshifts, finding, for example, that (1) there is no apparent color bimodality (i.e., the lack of an obvious red sequence and blue cloud) for AGN hosts from z ≈ 0to2, but non-AGN galaxy color bimodality exists up to z ≈ 3 and the relative fraction of red-sequence galaxies generally increases as the redshift decreases (consistent with a blue-to-red migration of galaxies), (2) most AGNs reside in massive hosts and the AGN fraction rises strongly toward higher stellar mass, up to z ≈ 2–3, and (3) the colors of both AGN hosts and non-AGN galaxies become redder as the stellar mass increases, up to z ≈ 2–3. Second, we point out that, in order to obtain a complete and reliable picture, it is critical to use mass-matched samples to examine color–magnitude relations of AGN hosts and non-AGN galaxies. We show that for mass-matched samples up to z ≈ 2–3, AGN hosts lie in the same region of the CMD as non-AGN galaxies; i.e., there is no specific clustering of AGN hosts in the CMD around the red sequence, the top of the blue cloud, or the green valley in between. The AGN fraction (≈ 10%) is mostly independent of host-galaxy color, providing an indication of the duty cycle of supermassive black hole growth in typical massive galaxies. These results are in contrast to those obtained with non-mass-matched samples where there is apparent AGN clustering in the CMD and the AGN fraction generally increases as the color becomes redder. We also find, for mass-matched samples, that the star formation rates of AGN hosts are typically a factor of ≈ 2–3 larger than those of non-AGN galaxies at z ≈ 0–1, whereas this difference diminishes at z ≈ 1–3. With mass-selection effects taken into account, we find that almost all of the results obtained in this work can reasonably be explained by two main ingredients, color–mass correlation (i.e., X-ray AGNs preferentially reside in massive galaxies that generally tend to be redder than less-massive galaxies) and passive or secular evolution of galaxies. Our results show that the presence of moderate-luminosity AGN activity does not have a significant effect on the colors of galaxies and thus tightly constrain any effects from moderate-luminosity AGN feedback upon color–magnitude properties over the ≈ 80% of cosmic time during which most of galaxy formation occurred.

To investigate whether distinctions exist between low- and high-luminosity Class ii 6.7 GHz methanol masers, we have undertaken multi-line mapping observations of various molecular lines, including the NH₃ (1,1), (2,2), (3,3), (4,4), and ¹²CO (1–0) transitions, toward a sample of nine low-luminosity 6.7 GHz masers and ¹²CO (1–0) observations toward a sample of eight high-luminosity 6.7 GHz masers, for which we already had NH₃ spectral line data. Emission in the NH₃ (1,1), (2,2), and (3,3) transitions was detected in eight out of nine low-luminosity maser sources, in which 14 cores were identified. We derive densities, column densities, temperatures, core sizes, and masses of both low- and high-luminosity maser regions. A comparative analysis of the physical quantities reveals marked distinctions between the low-luminosity and high-luminosity groups: in general, cores associated with high-luminosity 6.7 GHz masers are larger and more massive than those traced by low-luminosity 6.7 GHz masers; regions traced by the high-luminosity masers have larger column densities but lower densities than those of the low-luminosity maser regions. Further, strong correlations between 6.7 GHz maser luminosity and NH₃ (1,1) and (2,2) line widths are found, indicating that internal motions in high-luminosity maser regions are more energetic than those in low-luminosity maser regions. A ¹²CO (1–0) outflow analysis also shows distinctions in that outflows associated with high-luminosity masers have wider line wings and larger sizes than those associated with low-luminosity masers.

Characterization of microlensing planets requires modeling of observed light curves including many parameters. Studying the dependency of the pattern of light curves on the lensing parameters and the correlations between the parameters is important to understand how the uncertainties of the planetary parameters are propagated from other parameters. In this paper, we show that despite the apparent complexity of the pattern of light curves of planetary lensing events, the correlations between the lensing parameters can be understood by studying how the parameters affect the characteristics of lensing light curves such as the height and width, the caustic-crossing time scale, and the location and duration of planetary perturbations. Based on analytic arguments about the dependency of light curve features on the parameters, we obtain the correlations for the two representative cases of planetary events. We also demonstrate the applicability of the correlations to general planetary events by actually obtaining the correlations from modelings of light curves produced by simulations.

We present results from multi-epoch neutral hydrogen (H i) absorption observations of six bright pulsars with the Arecibo telescope. Moving through the interstellar medium (ISM) with transverse velocities of 10–150 AU yr⁻¹, these pulsars have swept across 1–200 AU over the course of our experiment, allowing us to probe the existence and properties of the tiny-scale atomic structure (TSAS) in the cold neutral medium (CNM). While most of the observed pulsars show no significant change in their H i absorption spectra, we have identified at least two clear TSAS-induced opacity variations in the direction of B1929+10. These observations require strong spatial inhomogeneities in either the TSAS clouds' physical properties themselves or else in the clouds' galactic distribution. While TSAS is occasionally detected on spatial scales down to 10 AU, it is too rare to be characterized by a spectrum of turbulent CNM fluctuations on scales of 10¹–10³ AU, as previously suggested by some work. In the direction of B1929+10, an apparent correlation between TSAS and interstellar clouds inside the warm Local Bubble (LB) indicates that TSAS may be tracing the fragmentation of the LB wall via hydrodynamic instabilities. While similar fragmentation events occur frequently throughout the ISM, the warm medium surrounding these cold cloudlets induces a natural selection effect wherein small TSAS clouds evaporate quickly and are rare, while large clouds survive longer and become a general property of the ISM.

This is the first of a series of papers aimed at characterizing the populations detected in the high-latitude sky of the Fermi-LAT survey. In this work, we focus on the intrinsic spectral and flux properties of the source sample. We show that when selection effects are properly taken into account, Fermi sources are on average steeper than previously found (e.g., in the bright source list) with an average photon index of 2.40 ± 0.02 over the entire 0.1–100 GeV energy band. We confirm that flat spectrum radio quasars have steeper spectra than BL Lacertae objects with an average index of 2.48 ± 0.02 versus 2.18 ± 0.02. Using several methods, we build the deepest source count distribution at GeV energies, deriving that the intrinsic source (i.e., blazar) surface density at F₁₀₀ ⩾ 10⁻⁹ ph cm⁻² s⁻¹ is 0.12^+0.03_−0.02 deg⁻². The integration of the source count distribution yields that point sources contribute 16(±1.8)% (±7% systematic uncertainty) of the GeV isotropic diffuse background. At the fluxes currently reached by LAT, we can rule out the hypothesis that pointlike sources (i.e., blazars) produce a larger fraction of the diffuse emission.

The interaction between interplanetary small-scale magnetic flux ropes and the magnetic field in the ambient solar wind is an important topic in the understanding of the evolution of magnetic structures in the heliosphere. Through a survey of 125 previously reported small flux ropes from 1995 to 2005, we find that 44 of them reveal clear signatures of Alfvénic fluctuations and thus classify them as Alfvén wave trains rather than flux ropes. Signatures of magnetic reconnection, generally including a plasma jet of ∼30 km s⁻¹ within a magnetic field rotational region, are clearly present at boundaries of about 42% of the flux ropes and 14% of the wave trains. The reconnection exhausts are often observed to show a local increase in the proton temperature, density, and plasma beta. About 66% of the reconnection events at flux rope boundaries are associated with a magnetic field shear angle larger than 90° and 73% of them reveal a decrease of 20% or more in the magnetic field magnitude, suggesting a dominance of anti-parallel reconnection at flux rope boundaries. The occurrence rate of magnetic reconnection at flux rope boundaries through the years 1995–2005 is also investigated and we find that it is relatively low around the solar maximum and much higher when approaching solar minima. The average magnetic field depression and shear angle for reconnection events at flux rope boundaries also reveal a similar trend from 1995 to 2005. Our results demonstrate for the first time that boundaries of a substantial fraction of small-scale flux ropes have properties similar to those of magnetic clouds, in the sense that both of them exhibit signatures of magnetic reconnection. The observed reconnection signatures could be related either to the formation of small flux ropes or to the interaction between flux ropes and the interplanetary magnetic fields.

We present the results of rest-frame, UV slitless spectroscopic observations of a sample of 32 z ∼ 0.7 Lyman break galaxy (LBG) analogs in the COSMOS field. The spectroscopic search was performed with the Solar Blind Channel on the Hubble Space Telescope. We report the detection of leaking Lyman continuum (LyC) radiation from an active galactic nucleus–starburst composite. While we find no direct detections of LyC emission in the remainder of our sample, we achieve individual lower limits (3σ) of the observed non-ionizing UV-to-LyC flux density ratios, f_ν (1500 Å)/f_ν(830 Å) of 20 to 204 (median of 73.5) and 378.7 for the stack. Assuming an intrinsic Lyman break of 3.4 and an intergalactic medium transmission of LyC photons along the line of sight to the galaxy of 85%, we report an upper limit for the relative escape fraction in individual galaxies of 0.02–0.19 and a stacked 3σ upper limit of 0.01. We find no indication of a relative escape fraction near unity as seen in some LBGs at z ∼ 3. Our UV spectra achieve the deepest limits to date at any redshift for the escape fraction in individual sources. The contrast between these z ∼ 0.7 low escape fraction LBG analogs with z ∼ 3 LBGs suggests that either the processes conducive to high f_esc are not being selected for in the z ≲ 1 samples or the average escape fraction is decreasing from z ∼ 3 to z ∼ 1. We discuss possible mechanisms that could affect the escape of LyC photons.

Chemistry plays an important role in the structure and evolution of protoplanetary disks, with implications for the composition of comets and planets. This is the first of a series of papers based on data from DISCS, a Submillimeter Array survey of the chemical composition of protoplanetary disks. The six Taurus sources in the program (DM Tau, AA Tau, LkCa 15, GM Aur, CQ Tau, and MWC 480) range in stellar spectral type from M1 to A4 and offer an opportunity to test the effects of stellar luminosity on the disk chemistry. The disks were observed in 10 different lines at ∼3'' resolution and an rms of ∼100 mJy beam⁻¹ at ∼0.5 km s⁻¹. The four brightest lines are CO 2–1, HCO⁺ 3–2, CN 2_3 3/4/2 − 1_2 2/3/1, and HCN 3–2, and these are detected toward all sources (except for HCN toward CQ Tau). The weaker lines of CN 2_2 2–1_1 1, DCO⁺ 3–2, N₂H⁺ 3–2, H₂CO 3_0 3–2_0 2, and 4_1 4–3_1 3 are detected toward two to three disks each, and DCN 3–2 only toward LkCa 15. CH₃OH 4_2 1–3_1 2 and c-C₃H₂ are not detected. There is no obvious difference between the T Tauri and Herbig Ae sources with regard to CN and HCN intensities. In contrast, DCO⁺, DCN, N₂H⁺, and H₂CO are detected only toward the T Tauri stars, suggesting that the disks around Herbig Ae stars lack cold regions for long enough timescales to allow for efficient deuterium chemistry, CO freeze-out, and grain chemistry.

The minimum of solar cycle 24 is significantly different from most other minima in terms of its duration as well as its abnormally low levels of activity. Using available helioseismic data that cover epochs from the minimum of cycle 23 to now, we study the differences in the nature of the solar rotation between the minima of cycles 23 and 24. We find that there are significant differences between the rotation rates during the two minima. There are differences in the zonal-flow pattern too. We find that the band of fast rotating region close to the equator bifurcated around 2005 and recombined by 2008. This behavior is different from that during the cycle 23 minimum. By autocorrelating the zonal-flow pattern with a time shift, we find that in terms of solar dynamics, solar cycle 23 lasted for a period of 11.7 years, consistent with the result of Howe et al. (2009). The autocorrelation coefficient also confirms that the zonal-flow pattern penetrates through the convection zone.

We consider ion heating by turbulent Alfvén waves (AWs) and kinetic Alfvén waves (KAWs) with wavelengths (measured perpendicular to the magnetic field) that are comparable to the ion gyroradius and frequencies ω smaller than the ion cyclotron frequency Ω. We focus on plasmas in which β ≲ 1, where β is the ratio of plasma pressure to magnetic pressure. As in previous studies, we find that when the turbulence amplitude exceeds a certain threshold, an ion's orbit becomes chaotic. The ion then interacts stochastically with the time-varying electrostatic potential, and the ion's energy undergoes a random walk. Using phenomenological arguments, we derive an analytic expression for the rates at which different ion species are heated, which we test by simulating test particles interacting with a spectrum of randomly phased AWs and KAWs. We find that the stochastic heating rate depends sensitively on the quantity ε = δv_ρ/v_⊥, where v_⊥ (v_∥) is the component of the ion velocity perpendicular (parallel) to the background magnetic field B₀, and δv_ρ (δB_ρ) is the rms amplitude of the velocity (magnetic-field) fluctuations at the gyroradius scale. In the case of thermal protons, when ε ≪ ε_crit, where ε_crit is a constant, a proton's magnetic moment is nearly conserved and stochastic heating is extremely weak. However, when ε>ε_crit, the proton heating rate exceeds half the cascade power that would be present in strong balanced KAW turbulence with the same value of δv_ρ, and magnetic-moment conservation is violated even when ω ≪ Ω. For the random-phase waves in our test-particle simulations, ε_crit = 0.19. For protons in low-β plasmas, ε ≃ β^−1/2δB_ρ/B₀, and ε can exceed ε_crit even when δB_ρ/B₀ ≪ ε_crit. The heating is anisotropic, increasing v²_⊥ much more than v²_∥ when β ≪ 1. (In contrast, at β ≳ 1 Landau damping and transit-time damping of KAWs lead to strong parallel heating of protons.) At comparable temperatures, alpha particles and minor ions have larger values of ε than protons and are heated more efficiently as a result. We discuss the implications of our results for ion heating in coronal holes and the solar wind.

Elliptical, lenticular, and early-type spiral galaxies show a remarkably tight power-law correlation between the mass M_• of their central supermassive black hole (SMBH) and the number N_GC of globular clusters (GCs): M_• = m_•/⋆ × N^1.08±0.04_GC with m_•/⋆ = 1.7 × 10⁵ M_☉. Thus, to a good approximation the SMBH mass is the same as the total mass of the GCs. Based on a limited sample of 13 galaxies, this relation appears to be a better predictor of SMBH mass (rms scatter 0.2 dex) than the M_•–σ relation between SMBH mass and velocity dispersion σ. The small scatter reflects the fact that galaxies with high GC specific frequency S_N tend to harbor SMBHs that are more massive than expected from the M_•–σ relation.

We study the dependence of satellite galaxy properties on the distance to the host galaxy and the orbital motion (prograde and retrograde orbits) using the Sloan Digital Sky Survey (SDSS) data. From SDSS Data Release 7, we find 3515 isolated satellite systems of galaxies at z < 0.03 that contain 8904 satellite galaxies. Using this sample, we construct a catalog of 635 satellites associated with 215 host galaxies whose spin directions are determined by our inspection of the SDSS color images and/or by spectroscopic observations in the literature. We divide satellite galaxies into prograde and retrograde orbit subsamples depending on their orbital motion with respect to the spin direction of the host. We find that the number of galaxies in prograde orbit is nearly equal to that of retrograde orbit galaxies: the fraction of satellites in prograde orbit is 50% ± 2%. The velocity distribution of satellites with respect to their hosts is found to be almost symmetric: the median bulk rotation of satellites is −1 ± 8 km s⁻¹. It is found that the radial distribution of early-type satellites in prograde orbit is strongly concentrated toward the host while that of retrograde ones shows much less concentration. We also find the orbital speed of late-type satellites in prograde orbit increases as the projected distance to the host (R) decreases while the speed decreases for those in retrograde orbit. At R less than 0.1 times the host virial radius (R < 0.1r_vir,host), the orbital speed decreases in both prograde and retrograde orbit cases. Prograde satellites are on average fainter than retrograde satellites for both early and late morphological types. The u − r color becomes redder as R decreases for both prograde and retrograde orbit late-type satellites. The differences between prograde and retrograde orbit satellite galaxies may be attributed to their different origin or the different strength of physical processes that they have experienced through hydrodynamic interactions with their host galaxies.

The suprathermal tails on the distribution functions of ions in the solar wind are observed to have a common spectral shape in many different circumstances: a power law in particle speed with spectral index of −5. Three possible approaches for explaining these observations are considered: (1) the acceleration mechanism of Fisk & Gloeckler in which energy is redistributed from a core particle population into the suprathermal tail; (2) traditional stochastic acceleration in which particles are accelerated by damping turbulence; and (3) the statistical approach introduced by Schwadron et al. in which the −5 spectrum is formed by averaging over individual spectra. The acceleration mechanism of Fisk & Gloeckler has advantages: (1) it appears to occur in conditions that are readily satisfied: compressive turbulence that is thermally isolated (no large-scale spatial gradients), with a core distribution of particles with a sharp initial cutoff in particle speed, above which particles can spatially diffuse; and (2) it yields spectra that are consistent with observations. Traditional stochastic acceleration has the disadvantage that it is unlikely to yield spectral shapes consistent with observations, and the spectra are dependent upon the plasma conditions and thus unlikely to be the same in different circumstances. The statistical approach of Schwadron et al. can yield the −5 spectrum and is consistent with the results of Fisk & Gloeckler when the assumed distribution functions for individual events and the averaging technique are taken to be compatible with the assumptions and averaging in Fisk & Gloeckler.

We study the distribution of stars, H ii regions, molecular gas, and individual giant molecular clouds in M33 over a wide range of spatial scales. The clustering strength of these components is systematically estimated through the fractal dimension. We find scale-free behavior at small spatial scales and a transition to a larger correlation dimension (consistent with a nearly uniform distribution) at larger scales. The transition region lies in the range ∼500–1000 pc. This transition defines a characteristic size that separates the regime of small-scale turbulent motion from that of large-scale galactic dynamics. At small spatial scales, bright young stars and molecular gas are distributed with nearly the same three-dimensional fractal dimension (D_f,3D ≲ 1.9), whereas fainter stars and H ii regions exhibit higher values, D_f,3D ≃ 2.2–2.5. Our results indicate that the interstellar medium in M33 is on average more fragmented and irregular than in the Milky Way.

Low-frequency Alfvén-wave turbulence causes ion trajectories to become chaotic, or "stochastic," when the turbulence amplitude is sufficiently large. Stochastic orbits enable ions to absorb energy from the turbulence, increasing the perpendicular ion temperature T_⊥i even when the fluctuation frequencies are too small for a cyclotron resonance to occur. In this paper, an analytic expression for the stochastic heating rate is used in conjunction with an observationally constrained turbulence model to obtain an analytic formula for T_⊥i as a function of heliocentric distance r, ion mass, and ion charge in coronal holes at 2 R_☉ ≲ r ≲ 15 R_☉. The resulting temperature profiles provide a good fit to observations of protons and O⁺⁵ ions at 2 R_☉ ≲ r ≲ 3 R_☉ from the Ultraviolet Coronagraph Spectrometer (UVCS). Stochastic heating also offers a natural explanation for several detailed features of the UVCS observations, including the preferential and anisotropic heating of minor ions, the rapid radial increase in the O⁺⁵ temperature between 1.6 R_☉ and 1.9 R_☉, and the abrupt flattening of the O⁺⁵ temperature profile as r increases above 1.9 R_☉.

We present results from 4.8 GHz Very Large Array (VLA) and global very long baseline interferometry (VLBI) observations of the northern half of the moderate FIR luminosity (median L_IR = 10^11.01 L_☉) COLA sample of star-forming galaxies. VLBI sources are detected in a high fraction (20/90) of the galaxies observed. The radio luminosities of these cores (∼10²¹ W Hz⁻¹) are too large to be explained by radio supernovae or supernova remnants and we argue that they are instead powered by active galactic nuclei (AGNs). These sub-parsec scale radio cores are preferentially detected toward galaxies whose VLA maps show bright 100–500 parsec scale nuclear radio components. Since these latter structures tightly follow the FIR to radio-continuum correlation for star formation, we conclude that the AGN-powered VLBI sources are associated with compact nuclear starburst environments. The implications for possible starburst–AGN connections are discussed. The detected VLBI sources have a relatively narrow range of radio luminosity consistent with models in which intense compact Eddington-limited starbursts regulate the gas supply onto a central supermassive black hole. The high incidence of AGN radio cores in compact starbursts suggests little or no delay between the starburst phase and the onset of AGN activity.

We continue our deep optical imaging survey of the Virgo cluster using the CWRU Burrell Schmidt telescope by presenting B-band surface photometry of the core of the Virgo cluster in order to study the cluster's intracluster light (ICL). We find ICL features down to μ_B ≈29 mag arcsec⁻², confirming the results of Mihos et al., who saw a vast web of low surface brightness streams, arcs, plumes, and diffuse light in the Virgo cluster core using V-band imaging. By combining these two data sets, we are able to measure the optical colors of many of the cluster's low surface brightness features. While much of our imaging area is contaminated by galactic cirrus, the cluster core near the cD galaxy, M87, is unobscured. We trace the color profile of M87 out to over 2000'', and find a blueing trend with radius, continuing out to the largest radii. Moreover, we have measured the colors of several ICL features which extend beyond M87's outermost reaches and find that they have similar colors to the M87's halo itself, B − V ≈0.8. The common colors of these features suggest that the extended outer envelopes of cD galaxies, such as M87, may be formed from similar streams, created by tidal interactions within the cluster, that have since dissolved into a smooth background in the cluster potential.

We present an analysis of optical spectra for 29 DAO white dwarfs. First, we present our new up-to-date model atmosphere grids computed without the assumption of local thermodynamic equilibrium in which we have included carbon, nitrogen, and oxygen at solar abundances. We demonstrate that the addition of these metals in the model atmospheres is essential in overcoming the Balmer-line problem, which manifests itself as an inability to fit all the Balmer lines simultaneously with consistent atmospheric parameters. We then present the spectroscopic analysis of our sample of DAO white dwarfs for which we determine the effective temperature, surface gravity, and helium abundance. We also present 18 hot DA white dwarfs that also suffer from the Balmer-line problem. We analyze these stars with models analogous to those for the DAO white dwarfs save for the presence of helium. Systematic differences between our newly determined atmospheric parameters with respect to previous determinations are explored. Far-ultraviolet spectra from the FUSE archive are then examined to demonstrate that there exists a correlation between higher metallic abundances and instances of the Balmer-line problem. The implications of these findings for all hot, hydrogen-rich white dwarfs are discussed. Specifically, the possible evolutionary scenario for DAO white dwarfs is revised and post-extreme horizontal branch evolution is no longer needed to explain the evolution for the majority of the DAO stars. Finally, we discuss how the presence of metals might drive a weak stellar wind which in turn could explain the presence of helium in DAO white dwarfs.

We study the correlation of the velocity dispersion of the coexisting molecules H¹³CN and H¹³CO⁺ and the turbulent energy dissipation scale in the DR21(OH) star-forming region. The downshift of the H¹³CO⁺ spectrum relative to H¹³CN is consistent with the presence of ambipolar diffusion at dissipation length scales that helps the process of turbulent energy dissipation, but at a different cutoff for ions compared with the neutrals. We use our observational data to calculate a turbulent ambipolar diffusion length scale L' ≃ 17 mpc and a strength of B_pos ≃ 1.7 mG for the plane of the sky component of the magnetic field in DR21(OH).

We investigate the contamination of the Sunyaev–Zel'dovich (SZ) effect for six galaxy clusters, A1689, A1995, A2142, A2163, A2261, and A2390, observed by the Y. T. Lee Array for Microwave Background Anisotropy in 2007. With the range of baselines used, we find that the largest effect (of order 13%–50% of the central SZ flux density) comes from primary anisotropies in the cosmic microwave background and exceeds the thermal noise in all six cases. Contamination from discrete radio sources is estimated to be at a level of 3%–60% of the central SZ flux density. We use the statistics of these contaminating sources to estimate and correct the errors in the measured SZ effects of these clusters.

Bearing in mind the application to the collapsar models of gamma-ray bursts (GRBs), we develop a numerical scheme and code for estimating the deposition of energy and momentum due to the neutrino pair annihilation ($\nu + {\bar{\nu }} \rightarrow e^{-} + e^{+}$) in the vicinity of an accretion tori around a Kerr black hole. Our code is designed to solve the general relativistic (GR) neutrino transfer by a ray-tracing method. To solve the collisional Boltzmann equation in curved spacetime, we numerically integrate the so-called rendering equation along the null geodesics. We employ the Fehlberg (4,5) adaptive integrator in the Runge–Kutta method to perform the numerical integration accurately. For the neutrino opacity, the charged-current β-processes, which are dominant in the vicinity of the accretion tori, are taken into account. The numerical accuracy of the developed code is certified by several tests in which we show comparisons with the corresponding analytical solutions. In order to solve the energy-dependent ray-tracing transport, we propose that an adaptive-mesh-refinement approach, which we take for the two radiation angles (θ, ϕ) and the neutrino energy, is useful in reducing the computational cost significantly. Based on the hydrodynamical data in our collapsar simulation, we estimate the annihilation rates in a post-processing manner. Increasing the Kerr parameter from 0 to 1, it is found that the GR effect can increase the local energy deposition rate by about one order of magnitude, and the net energy deposition rate by several tens of percent. After the accretion disk settles into a stationary state (typically later than ∼9 s from the onset of gravitational collapse), we point out that the neutrino-heating timescale in the vicinity of the polar funnel region can be shorter than the dynamical timescale. Our results suggest that the neutrino pair annihilation is potentially as important as the conventional magnetohydrodynamic mechanism for igniting the GRB fireballs.

In this paper, we derive the Spitzer IRAC band period–luminosity (P–L) relations for the Small Magellanic Cloud (SMC) Cepheids, by matching the Spitzer archival SAGE-SMC data with the OGLE-III SMC Cepheids. We find that the 3.6 μm and 4.5 μm band P–L relations can be better described using two P–L relations with a break period at log(P) = 0.4: this is consistent with similar results at optical wavelengths for SMC P–L relations. The 5.8 μm and 8.0 μm band P–L relations do not extend to sufficiently short periods to enable a similar detection of a slope change at log(P) = 0.4. The slopes of the SMC P–L relations, for log(P) > 0.4, are consistent with their Large Magellanic Cloud counterparts that were derived from a similar data set. They are also in agreement with those obtained from a small sample of Galactic Cepheids with parallax measurements.

The hemispheric sign rule of current helicity is examined for large-scale magnetic fields, extending to 60° high in solar latitudes, at three different phases of solar cycle 23 and using three different instruments. The current helicity is calculated from the vector magnetic fields derived respectively from the longitudinal full-disk magnetograms obtained by the Michelson Doppler Imager on board the Solar and Heliospheric Observatory as well as the Kitt Peak Vacuum Telescope and the Synoptic Long-term Investigations of the Sun at the National Solar Observatory at Kitt Peak. We have used the same technique introduced by Pevtsov & Latushko to retrieve the vector magnetic field but have applied a different method to calculate the current helicity. Our analysis strengthens evidence that the large-scale magnetic fields show a clear and consistent current helicity pattern that follows the established hemispheric rule, that is, a positive helicity sign in the southern hemisphere and a negative helicity sign in the northern hemisphere. This hemispheric sign pattern is present everywhere in the global magnetic field, extending to 60° high in solar latitudes, independent of the instruments and parameters used, and is evident at both solar maximum and minimum phases.

Bias due to imperfect shear calibration is the biggest obstacle when constraints on cosmological parameters are to be extracted from large area weak lensing surveys such as Pan-STARRS-3π, DES, or future satellite missions like EUCLID. We demonstrate that bias present in existing shear measurement pipelines (e.g., KSB) can be almost entirely removed by means of neural networks. In this way, bias correction can depend on the properties of the individual galaxy instead of being a single global value. We present a procedure to train neural networks for shear estimation and apply this to subsets of simulated GREAT08 RealNoise data. We also show that circularization of the point-spread function (PSF) before measuring the shear reduces the scatter related to the PSF anisotropy correction and thus leads to improved measurements, particularly on low and medium signal-to-noise data. Our results are competitive with the best performers in the GREAT08 competition, especially for the medium and higher signal-to-noise sets. Expressed in terms of the quality parameter defined by GREAT08, we achieve a Q≈ 40, 140, and 1300 without and 50, 200, and 1300 with circularization for low, medium, and high signal-to-noise data sets, respectively.

Observations of the cores of nearby galaxy clusters show Hα and molecular emission-line filaments. We argue that these are the result of local thermal instability in a globally stable galaxy cluster core. We present local, high-resolution, two-dimensional magnetohydrodynamic simulations of thermal instability for conditions appropriate to the intracluster medium (ICM); the simulations include anisotropic thermal conduction along magnetic field lines and adiabatic cosmic rays. Thermal conduction suppresses thermal instability along magnetic field lines on scales smaller than the Field length (≳10 kpc for the hot, diffuse ICM). We show that the Field length in the cold medium must be resolved both along and perpendicular to the magnetic field in order to obtain numerically converged results. Because of negligible conduction perpendicular to the magnetic field, thermal instability leads to fine scale structure in the perpendicular direction. Filaments of cold gas along magnetic field lines are thus a natural consequence of thermal instability with anisotropic thermal conduction. This is true even in the fully nonlinear regime and even for dynamically weak magnetic fields. The filamentary structure in the cold gas is also imprinted on the diffuse X-ray-emitting plasma in the neighboring hot ICM. Nonlinearly, filaments of cold (∼10⁴ K) gas should have lengths (along the magnetic field) comparable to the Field length in the cold medium ∼10⁻⁴ pc! Observations show, however, that the atomic filaments in clusters are far more extended, ∼10 kpc. Cosmic-ray pressure support (or a small-scale turbulent magnetic pressure) may resolve this discrepancy: even a small cosmic-ray pressure in the diffuse ICM, ∼10⁻⁴ of the thermal pressure, can be adiabatically compressed to provide significant pressure support in cold filaments. This is qualitatively consistent with the large population of cosmic rays invoked to explain the atomic and molecular line ratios observed in filaments.

We report on atomic gas (H i) and molecular gas (as traced by CO(2–1)) redshifted absorption features toward the nuclear regions of the closest powerful radio galaxy, Centaurus A (NGC 5128). Our H i observations using the Very Long Baseline Array allow us to discern with unprecedented sub-parsec resolution H i absorption profiles toward different positions along the 21 cm continuum jet in the inner 03 (or 5.4 pc). In addition, our CO(2–1) data obtained with the Submillimeter Array probe the bulk of the absorbing molecular gas with little contamination by emission, which was not possible with previous CO single-dish observations. We shed light on the physical properties of the gas in the line of sight with these data, emphasizing the still open debate about the nature of the gas that produces the broad absorption line (∼55 km s⁻¹). First, the broad H i line is more prominent toward the central and brightest 21 cm continuum component than toward a region along the jet at a distance ∼20 mas (or 0.4 pc) further from the nucleus. This indicates that the broad absorption line arises from gas located close to the nucleus, rather than from diffuse and more distant gas. Second, the different velocity components detected in the CO(2–1) absorption spectrum match well with other molecular lines, such as those of HCO⁺(1–0), except the broad absorption line that is detected in HCO⁺(1–0) (and most likely related to that of the H i). Dissociation of molecular hydrogen due to the active galactic nucleus seems to be efficient at distances r ≲ 10 pc, which might contribute to the depth of the broad H i and molecular lines.

We present new images and photometry of the massive star-forming complex Cygnus X obtained with the Infrared Array Camera (IRAC) and the Multiband Imaging Photometer for Spitzer (MIPS) on board the Spitzer Space Telescope. A combination of IRAC, MIPS, UKIRT Deep Infrared Sky Survey, and Two Micron All Sky Survey data are used to identify and classify young stellar objects (YSOs). Of the 8231 sources detected exhibiting infrared excess in Cygnus X North, 670 are classified as class I and 7249 are classified as class II. Using spectra from the FAST Spectrograph at the Fred L. Whipple Observatory and Hectospec on the MMT, we spectrally typed 536 sources in the Cygnus X complex to identify the massive stars. We find that YSOs tend to be grouped in the neighborhoods of massive B stars (spectral types B0 to B9). We present a minimal spanning tree analysis of clusters in two regions in Cygnus X North. The fraction of infrared excess sources that belong to clusters with ⩾10 members is found to be 50%–70%. Most class II objects lie in dense clusters within blown out H ii regions, while class I sources tend to reside in more filamentary structures along the bright-rimmed clouds, indicating possible triggered star formation.

We explore the dependence on spatial dimension of the viability of the neutrino heating mechanism of core-collapse supernova explosions. We find that the tendency to explode is a monotonically increasing function of dimension, with three dimensions (3D) requiring ∼40%–50% lower driving neutrino luminosity than one dimension and ∼15%–25% lower driving neutrino luminosity than two dimensions (2D). Moreover, we find that the delay to explosion for a given neutrino luminosity is always shorter in 3D than 2D, sometimes by many hundreds of milliseconds. The magnitude of this dimensional effect is much larger than the purported magnitude of a variety of other effects, such as nuclear burning, inelastic scattering, or general relativity, which are sometimes invoked to bridge the gap between the current ambiguous and uncertain theoretical situation and the fact of robust supernova explosions. Since real supernovae occur in three dimensions, our finding may be an important step toward unraveling one of the most problematic puzzles in stellar astrophysics. In addition, even though in 3D, we do see pre-explosion instabilities and blast asymmetries, unlike the situation in 2D, we do not see an obvious axially symmetric dipolar shock oscillation. Rather, the free energy available to power instabilities seems to be shared by more and more degrees of freedom as the dimension increases. Hence, the strong dipolar axisymmetry seen in 2D and previously identified as a fundamental characteristic of the shock hydrodynamics may not survive in 3D as a prominent feature.

We present multi-band photometry and multi-epoch spectroscopy of the peculiar Type Ia supernova (SN Ia) 2007qd, discovered by the SDSS-II Supernova Survey. It possesses physical properties intermediate to those of the peculiar SN 2002cx and the extremely low-luminosity SN 2008ha. Optical photometry indicates that it had an extraordinarily fast rise time of ≲10 days and a peak absolute B magnitude of −15.4 ± 0.2 at most, making it one of the most subluminous SN Ia ever observed. Follow-up spectroscopy of SN 2007qd near maximum brightness unambiguously shows the presence of intermediate-mass elements which are likely caused by carbon/oxygen nuclear burning. Near maximum brightness, SN 2007qd had a photospheric velocity of only 2800 km s⁻¹, similar to that of SN 2008ha but about 4000 and 7000 km s⁻¹ less than that of SN 2002cx and normal SN Ia, respectively. We show that the peak luminosities of SN 2002cx like objects are highly correlated with both their light-curve stretch and photospheric velocities. Its strong apparent connection to other SN 2002cx like events suggests that SN 2007qd is also a pure deflagration of a white dwarf, although other mechanisms cannot be ruled out. It may be a critical link between SN 2008ha and the other members of the SN 2002cx like class of objects.

Line-of-sight magnetograms for 217 active regions (ARs) with different flare rates observed at the solar disk center from 1997 January until 2006 December are utilized to study the turbulence regime and its relationship to flare productivity. Data from the SOHO/MDI instrument recorded in the high-resolution mode and data from the BBSO magnetograph were used. The turbulence regime was probed via magnetic energy spectra and magnetic dissipation spectra. We found steeper energy spectra for ARs with higher flare productivity. We also report that both the power index, α, of the energy spectrum, E(k) ∼ k^−α, and the total spectral energy, W = ∫E(k)dk, are comparably correlated with the flare index, A, of an AR. The correlations are found to be stronger than those found between the flare index and the total unsigned flux. The flare index for an AR can be estimated based on measurements of α and W as A = 10^b(αW)^c, with b = −7.92 ± 0.58 and c = 1.85 ± 0.13. We found that the regime of the fully developed turbulence occurs in decaying ARs and in emerging ARs (at the very early stage of emergence). Well-developed ARs display underdeveloped turbulence with strong magnetic dissipation at all scales.

We set out to test the claim that the recently identified population of compact, massive, and quiescent galaxies at z ∼ 2.3 must undergo significant size evolution to match the properties of galaxies found in the local universe. Using data from the Sloan Digital Sky Survey (SDSS; Data Release 7), we have conducted a search for local red sequence galaxies with sizes and masses comparable to those found at z ∼ 2.3. The SDSS spectroscopic target selection algorithm excludes high surface brightness objects; we show that this makes incompleteness a concern for such massive, compact galaxies, particularly for low redshifts (z ≲ 0.05). We have identified 63 M_*>10^10.7 M_☉ (≈5 × 10¹⁰ M_☉) red sequence galaxies at 0.066 < z_spec < 0.12 which are smaller than the median size–mass relation by a factor of 2 or more. Consistent with expectations from the virial theorem, the median offset from the mass–velocity dispersion relation for these galaxies is 0.12 dex. We do not, however, find any galaxies with sizes and masses comparable to those observed at z ∼ 2.3, implying a decrease in the comoving number density of these galaxies, at fixed size and mass, by a factor of ≳5000. This result cannot be explained by incompleteness: in the 0.066 < z < 0.12 interval, we estimate that the SDSS spectroscopic sample should typically be ≳75% complete for galaxies with the sizes and masses seen at high redshift, although for the very smallest galaxies it may be as low as ∼20%. In order to confirm that the absence of such compact massive galaxies in SDSS is not produced by spectroscopic selection effects, we have also looked for such galaxies in the basic SDSS photometric catalog, using photometric redshifts. While we do find signs of a slight bias against massive, compact galaxies, this analysis suggests that the SDSS spectroscopic sample is missing at most a few objects in the regime we consider. Accepting the high-redshift results, it is clear that massive galaxies must undergo significant structural evolution over z ≲ 2 in order to match the population seen in the local universe. Our results suggest that a highly stochastic mechanism (e.g., major mergers) cannot be the primary driver of this strong size evolution.

We study compressible magnetohydrodynamic turbulence, which holds the key to many astrophysical processes, including star formation and cosmic-ray propagation. To account for the variations of the magnetic field in the strongly turbulent fluid, we use wavelet decomposition of the turbulent velocity field into Alfvén, slow, and fast modes, which presents an extension of the Cho & Lazarian decomposition approach based on Fourier transforms. The wavelets allow us to follow the variations of the local direction of the magnetic field and therefore improve the quality of the decomposition compared to the Fourier transforms, which are done in the mean field reference frame. For each resulting component, we calculate the spectra and two-point statistics such as longitudinal and transverse structure functions as well as higher order intermittency statistics. In addition, we perform a Helmholtz– Hodge decomposition of the velocity field into incompressible and compressible parts and analyze these components. We find that the turbulence intermittency is different for different components, and we show that the intermittency statistics depend on whether the phenomenon was studied in the global reference frame related to the mean magnetic field or in the frame defined by the local magnetic field. The dependencies of the measures we obtained are different for different components of the velocity; for instance, we show that while the Alfvén mode intermittency changes marginally with the Mach number, the intermittency of the fast mode is substantially affected by the change.

By examining many X-ray jets in Hinode/X-Ray Telescope coronal X-ray movies of the polar coronal holes, we found that there is a dichotomy of polar X-ray jets. About two thirds fit the standard reconnection picture for coronal jets, and about one third are another type. We present observations indicating that the non-standard jets are counterparts of erupting-loop Hα macrospicules, jets in which the jet-base magnetic arch undergoes a miniature version of the blowout eruptions that produce major coronal mass ejections. From the coronal X-ray movies we present in detail two typical standard X-ray jets and two typical blowout X-ray jets that were also caught in He ii 304 Å snapshots from STEREO/EUVI. The distinguishing features of blowout X-ray jets are (1) X-ray brightening inside the base arch in addition to the outside bright point that standard jets have, (2) blowout eruption of the base arch's core field, often carrying a filament of cool (T ∼ 10⁴ − 10⁵ K) plasma, and (3) an extra jet-spire strand rooted close to the bright point. We present cartoons showing how reconnection during blowout eruption of the base arch could produce the observed features of blowout X-ray jets. We infer that (1) the standard-jet/blowout-jet dichotomy of coronal jets results from the dichotomy of base arches that do not have and base arches that do have enough shear and twist to erupt open, and (2) there is a large class of spicules that are standard jets and a comparably large class of spicules that are blowout jets.

Anomalous resistivity is a critical parameter for triggering the fast magnetic reconnection and interpreting the eruption of solar flares in the nearly collisionless coronal plasma. However, the mechanism for the production of anomalous resistivity and its evolution are weakly understood. In this paper, the one-dimensional Vlasov equation was numerically solved with the typical solar coronal parameters and realistic mass ratio in the presence of strong inductive electric field, and the relationship between the anomalous resistivity and the reconnecting electric field was inferred for the area near the center of reconnecting current sheets. Our principal findings are summarized as follows. (1) The relationship between the anomalous resistivity and the reconnecting electric field E₀ may be represented by η_eff = [10.82–10.99 exp (−0.36 E₀)]Ω m. (2) If E₀ is small enough, it may be described by η_eff = [4.02 E₀ − 0.18]Ω m, which is basically consistent with the early experimental results on the plasma response to the applied electric field. (3) In comparison with theoretical formulas for the current-driven ion-acoustic and Buneman anomalous resistivities, if E₀ is small, the anomalous resistivity may be due to the ion-acoustic instability; if E₀ is large, the anomalous resistivity may be due to the Buneman instability. These results are also basically consistent with early experiments.

We examine spectra of the Ca ii H line, obtained under good seeing conditions with the VTT Echelle Spectrograph in 2007 June, and higher resolution data of the Ca ii λ8542 line from Fabry–Pérot instruments. The VTT targets were areas near disk center which included quiet Sun and some dispersed plage. The infrared data included quiet Sun and plage associated with small pores. Bright chromospheric network emission patches expand little with wavelength from line wing to line center, i.e., with increasing line opacity and height. We argue that this simple observation has implications for the force and energy balance of the chromosphere, since bright chromospheric network emission is traditionally associated with enhanced local mechanical heating which increases temperatures and pressures. Simple physical considerations then suggest that the network chromosphere may not be able to reach horizontal force balance with its surroundings, yet the network is a long-lived structure. We speculate on possible reasons for the observed behavior. By drawing attention to a potential conundrum, we hope to contribute to a better understanding of a long-standing unsolved problem: the heating of the chromospheric network.

Active galactic nuclei (AGNs) consist of an accretion disk around a supermassive black hole which in turn is surrounded by an obscuring torus of dust and gas. As the resulting geometry of this system affects the observable properties, quantifying isotropic indicators of intrinsic AGN luminosity is important in selecting unbiased samples of AGNs. In this paper, we consider five such proxies: the luminosities of the [O iii]λ5007 line, the [O iv]25.89 μm line, the mid-infrared (MIR) continuum emission by the torus, and the radio and hard X-ray (E > 10 keV) continuum emission. We compare these different proxies using two complete samples of low-redshift Type 2 AGNs selected in a homogeneous way based on different indicators: an optically selected [O iii] sample and an MIR-selected 12 μm sample. To assess the relative merits of these proxies, we have undertaken two analyses. First, we examine the correlations between all five different proxies, and find better agreement for the [O iv], MIR, and [O iii] luminosities than for the hard X-ray and radio luminosities. Next, we compare the ratios of the fluxes of the different proxies to their values in unobscured Type 1 AGNs. The agreement is best for the ratio of the [O iv] and MIR fluxes, while the ratios of the hard X-ray to [O iii], [O iv], and MIR fluxes are systematically low by about an order of magnitude in the Type 2 AGNs, indicating that hard X-ray-selected samples do not represent the full Type 2 AGN population. In a similar spirit, we compare different optical and MIR diagnostics of the relative energetic contributions of AGN and star formation processes in our samples of Type 2 AGNs. We find good agreement between the various diagnostic parameters, such as the equivalent width of the MIR polycyclic aromatic hydrocarbon features, the ratio of the MIR [O iv]/[Ne ii] emission lines, the spectral index of the MIR continuum, and the commonly used optical emission-line ratios. Finally, we test whether the presence of cold gas associated with star formation leads to an enhanced conversion efficiency of AGN ionizing radiation into [O iii] or [O iv] emission. We find that no compelling evidence exists for this scenario for the luminosities represented in this sample (L_bol ≈10⁹ − 8 × 10¹¹ L_☉).

The science goals for future ground-based all-sky surveys, such as the Dark Energy Survey, PanSTARRS, and the Large Synoptic Survey Telescope, require calibration of broadband photometry that is stable in time and uniform over the sky to precisions of 1% or better, and absolute calibration of color measurements that are similarly accurate. This performance will need to be achieved with measurements made from multiple images taken over the course of many years, and these surveys will observe in less than ideal conditions. This paper describes a technique to implement a new strategy to directly measure variations of atmospheric transmittance at optical wavelengths and application of these measurements to calibration of ground-based observations. This strategy makes use of measurements of the spectra of a small catalog of bright "probe" stars as they progress across the sky and back-light the atmosphere. The signatures of optical absorption by different atmospheric constituents are recognized in these spectra by their characteristic dependences on wavelength and airmass. State-of-the-art models of atmospheric radiation transport and modern codes are used to accurately compute atmospheric extinction over a wide range of observing conditions. We present results of an observing campaign that demonstrate that correction for extinction due to molecular constituents and aerosols can be done with precisions of a few millimagnitudes with this technique.

The physical processes that heat the solar corona and accelerate the solar wind remain unknown after many years of study. Some have suggested that the wind is driven by waves and turbulence in open magnetic flux tubes, and others have suggested that plasma is injected into the open tubes by magnetic reconnection with closed loops. In order to test the latter idea, we developed Monte Carlo simulations of the photospheric "magnetic carpet" and extrapolated the time-varying coronal field. These models were constructed for a range of different magnetic flux imbalance ratios. Completely balanced models represent quiet regions on the Sun and source regions of slow solar wind streams. Highly imbalanced models represent coronal holes and source regions of fast wind streams. The models agree with observed emergence rates, surface flux densities, and number distributions of magnetic elements. Despite having no imposed supergranular motions in the models, a realistic network of magnetic "funnels" appeared spontaneously. We computed the rate at which closed field lines open up (i.e., recycling times for open flux), and we estimated the energy flux released in reconnection events involving the opening up of closed flux tubes. For quiet regions and mixed-polarity coronal holes, these energy fluxes were found to be much lower than that which is required to accelerate the solar wind. For the most imbalanced coronal holes, the energy fluxes may be large enough to power the solar wind, but the recycling times are far longer than the time it takes the solar wind to accelerate into the low corona. Thus, it is unlikely that either the slow or fast solar wind is driven by reconnection and loop-opening processes in the magnetic carpet.

Using the Chandra X-ray Observatory, we have pinpointed the location of a faint X-ray point source (CXOU J182913.1–125113) and an associated diffuse nebula in the composite supernova remnant (SNR) G18.95–1.1. These objects appear to be the long-sought pulsar and its wind nebula. The X-ray spectrum of the point source is best described by an absorbed power-law model with Γ = 1.6 and an N_H of ∼1 × 10²² cm⁻². This model predicts a relatively low unabsorbed X-ray luminosity of about L_X(0.5–8.0 keV) ≃ 4.1 × 10³¹ D²₂ erg s⁻¹, where D₂ is the distance in units of 2 kpc. The best-fit model of the diffuse nebula is a combination of thermal (kT = 0.48 keV) and non-thermal (1.4 ⩽ Γ ⩽ 1.9) emission. The unabsorbed X-ray luminosity of L_X ≃ 5.4 × 10³³ D²₂ erg s⁻¹ in the 0.5–8 keV energy band seems to be largely dominated by the thermal component from the SNR, providing 87% of L_X in this band. No radio or X-ray pulsations have been reported for CXOU J182913.1–125113. If we assume an age of ∼5300 yr for G18.95–1.1 and use the X-ray luminosity for the pulsar and the wind nebula together with the relationship between spin-down luminosity (via magnetic dipole radiation) and period, we estimate the pulsar's period to be P ≃ 0.4 s. Compared to other rotation-powered pulsars, a magnetic field of 2.2 × 10¹³ G is implied by its location in the P–$\dot{P}$ diagram, a value which is close to that of the quantum critical field.

We discuss a mechanism for cosmic-ray penetration into an interplanetary magnetic flux rope, particularly the effect of the finite Larmor radius and magnetic field irregularities. First, we derive analytical solutions for cosmic-ray behavior inside a magnetic flux rope, on the basis of the Newton–Lorentz equation of a particle, to investigate how cosmic rays penetrate magnetic flux ropes under an assumption of there being no scattering by small-scale magnetic field irregularities. The results show that the behavior of a particle is determined by only one parameter f₀, that is, the ratio of the Larmor radius at the flux rope axis to the flux rope radius. The analytical solutions show that cosmic rays cannot penetrate into the inner region of a flux rope by only gyration and gradient-curvature drift in the case of small f₀. Next, we perform a numerical simulation of a cosmic-ray penetration into an interplanetary magnetic flux rope by adding small-scale magnetic field irregularities. The results show that cosmic rays can penetrate into a magnetic flux rope even in the case of small f₀ because of the effect of small-scale magnetic field irregularities. This simulation also shows that a cosmic-ray density distribution is greatly different from that deduced from a guiding center approximation because of the effect of the finite Larmor radius and magnetic field irregularities for the case of a moderate to large Larmor radius compared to the flux rope radius.

We present VLT/FORS2 spectroscopy and Gamma-ray Optical/Near-Infrared Detector optical/near-IR photometry of the afterglow of the bright Fermi/LAT GRB 090926A. The spectrum shows prominent Lyman-α absorption with $N_{\rm H\,\mathsc{i}} = 10^{21.73\pm 0.07}$ cm⁻² and a multitude of metal lines at a common redshift of z = 2.1062 ± 0.0004, which we associate with the redshift of the gamma-ray burst (GRB). The metallicity derived from S ii is log(Z/Z_☉) ≈ −1.9, one of the lowest values ever found in a GRB Damped Lyman-α (DLA) system. This confirms that the spread of metallicity in GRB-DLAs at z ≈ 2 is at least two orders of magnitude. We argue that this spread in metallicity does not require a similar range in abundances of the GRB progenitors, since the neutral interstellar medium probed by the DLA is expected to be at a significant distance from the explosion site. The hydrogen column density derived from the Swift/XRT afterglow spectrum (assuming log(Z/Z_☉) ≈ −1.9) is approx. ≈100 times higher than the $N_{\rm H\,\mathsc{i}}$ obtained from the Lyman-α absorptions. This suggests either a large column density of ionized gas or a higher metallicity of the circum-burst medium compared to the gas traced by the DLA. We also discuss the afterglow light curve evolution and energetics. The absence of a clear jet-break like steeping until at least 21 days post-burst suggests a beaming-corrected energy release of E_γ>3.5 × 10⁵² erg, indicating that GRB 090926A may have been one of the most energetic bursts ever detected.

The knowledge of accurate stellar parameters is paramount in several fields of stellar astrophysics, particularly in the study of extrasolar planets, where often the star is the only visible component and therefore used to infer the planet's fundamental parameters. Another important aspect of the analysis of planetary systems is the stellar activity and the possible star–planet interaction. Here, we present a self-consistent abundance analysis of the planet-hosting star WASP-12 and a high-precision search for a structured stellar magnetic field on the basis of spectropolarimetric observations obtained with the ESPaDOnS spectropolarimeter. Our results show that the star does not have a structured magnetic field, and that the obtained fundamental parameters are in good agreement with what was previously published. In addition, we derive improved constraints on the stellar age (1.0–2.65 Gyr), mass (1.23–1.49 M/M_☉), and distance (295–465 pc). WASP-12 is an ideal object in which to look for pollution signatures in the stellar atmosphere. We analyze the WASP-12 abundances as a function of the condensation temperature and compare them with those published by several other authors on planet-hosting and non-planet-hosting stars. We find hints of atmospheric pollution in WASP-12's photosphere but are unable to reach firm conclusions with our present data. We conclude that a differential analysis based on WASP-12 twins will probably clarify whether an atmospheric pollution is present as well as the nature of this pollution and its implications in planet formation and evolution. We also attempt the direct detection of the circumstellar disk through infrared excess, but without success.

We present a Spitzer InfraRed Spectrometer search for 10–36 μm molecular emission from a large sample of protoplanetary disks, including lines from H₂O, OH, C₂H₂, HCN, and CO₂. This paper describes the sample and data processing and derives the detection rate of mid-infrared molecular emission as a function of stellar mass. The sample covers a range of spectral type from early M to A, and is supplemented by archival spectra of disks around A and B stars. It is drawn from a variety of nearby star-forming regions, including Ophiuchus, Lupus, and Chamaeleon. Spectra showing strong emission lines are used to identify which lines are the best tracers of various physical and chemical conditions within the disks. In total, we identify 22 T Tauri stars with strong mid-infrared H₂O emission. Integrated water line luminosities, where water vapor is detected, range from 5 × 10⁻⁴ to 9 × 10⁻³ L_☉, likely making water the dominant line coolant of inner disk surfaces in classical T Tauri stars. None of the five transitional disks in the sample show detectable gaseous molecular emission with Spitzer upper limits at the 1% level in terms of line-to-continuum ratios (apart from H₂), but the sample is too small to conclude whether this is a general property of transitional disks. We find a strong dependence on detection rate with spectral type; no disks around our sample of 25 A and B stars were found to exhibit water emission, down to 1%–2% line-to-continuum ratios, in the mid-infrared, while more than half of disks around late-type stars (M-G) show sufficiently intense water emission to be detected by Spitzer, with a detection rate approaching 2/3 for disks around K stars. Some Herbig Ae/Be stars show tentative H₂O/OH emission features beyond 20 μm at the 1%–2% level, however, and one of them shows CO₂ in emission. We argue that the observed differences between T Tauri disks and Herbig Ae/Be disks are due to a difference in excitation and/or chemistry depending on spectral type and suggest that photochemistry may be playing an important role in the observable characteristics of mid-infrared molecular line emission from protoplanetary disks.

Light refracted by the planet's atmosphere is usually ignored in analysis of planetary transits. Here, we show that refraction can add shoulders to the transit light curve, i.e., an increase in the observed flux, mostly just before and after the transit. During transit, light may be refracted away from the observer. Therefore, even completely transparent planets will display a very similar signal to that of a standard transit, i.e., of an opaque planet. We provide analytical expression for the amount of additional light deflected toward the observer before the transit, and show that the effect may be as large as 10⁻⁴ of the stellar light and therefore measurable by current instruments. By observing this effect, we can directly measure the scale height of the planet's atmosphere. We also consider the attenuation of starlight in the planetary atmosphere due to Rayleigh scattering and discuss the conditions under which the atmospheric lensing effect is most prominent. We show that, for planets on orbital periods larger than about 70 days, the size of the transit is determined by refraction effects, and not by absorption within the planet.

Analysis is presented for 15 months of data taken with the Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope for 11 non-blazar active galactic nuclei (AGNs), including seven FRI radio galaxies and four FRII radio sources consisting of two FRII radio galaxies and two steep spectrum radio quasars. The broad line FRI radio galaxy 3C 120 is reported here as a γ-ray source for the first time. The analysis is based on directional associations of LAT sources with radio sources in the 3CR, 3CRR, and MS4 (collectively referred to as 3C-MS) catalogs. Seven of the eleven LAT sources associated with 3C-MS radio sources have spectral indices larger than 2.3 and, except for the FRI radio galaxy NGC 1275 that shows possible spectral curvature, are well described by a power law. No evidence for time variability is found for any sources other than NGC 1275. The γ-ray luminosities of FRI radio galaxies are significantly smaller than those of the BL Lac objects detected by the LAT, whereas the γ-ray luminosities of the FRII sources are quite similar to those of FSRQs, which could reflect different beaming factors for the γ-ray emission. A core dominance (CD) study of the 3CRR sample indicates that sources closer to the jet axis are preferentially detected with the Fermi LAT, insofar as the γ-ray-detected misaligned AGNs have larger CD at a given average radio flux. The results are discussed in view of the AGN unification scenario.

Gas has been detected in a number of debris disk systems. This gas may have arisen from grain sublimation or grain photodesorption. It interacts with the surrounding dust grains through a number of charge and heat exchanges. Studying the chemical composition and physical state of this gas can therefore reveal much about the dust component in these debris disks. We have produced a new code, ontario, to address gas emission from dusty gas-poor disks around A–F stars. This code computes the gas ionization and thermal balance self-consistently, with particular care taken of heating/cooling mechanisms. Line emission spectra are then produced for each species (up to zinc) by statistical equilibrium calculations of the atomic/ionic energy levels. For parameters that resemble the observed β Pictoris gas disk, we find that the gas is primarily heated by photoelectric emission from dust grains, and primarily cooled through the C ii 157.7 μm line emission. The gas can be heated to a temperature that is warmer than that of the dust and may in some cases reach temperature for thermal escape. The dominant cooling line, C ii 157.7 μm, should be detectable by Herschel in these disks, while the O i 63.2 μm line will be too faint. We also study the dependence of the cooling line fluxes on a variety of disk parameters, in light of the much improved sensitivity to thermal line emission in the mid/far-infrared and at submillimeter wavelengths provided by, in particular, Herschel, SOFIA, and ALMA. These new instruments will yield much new information about dusty debris disks.

We present abundances of C, O, Ti, and Fe for 11 M-giant stars in the trailing tidal arm of the Sagittarius dwarf (Sgr). The abundances were derived by comparing synthetic spectra with high-resolution infrared spectra obtained with the Phoenix spectrograph on the Gemini South telescope. The targeted stars are drawn from two regions of the Sgr trailing arm separated by 66° (five stars) and 132° (six stars) from the main body of Sgr. The trailing arm provides a more direct diagnostic of the chemical evolution of Sgr compared to the extensively phase-mixed leading arm. Within our restricted sample of ∼2–3 Gyr old stars, we find that the stream material exhibits a significant metallicity gradient of −(2.4 ± 0.3) × 10⁻³ dex/degree (−(9.4 ± 1.1) × 10⁻⁴ dex/kpc) away from the main body of Sgr. The tidal disruption of Sgr is a relatively recent event. We therefore interpret the presence of a metallicity gradient in the debris as indicative of a similar gradient in the progenitor. The fact that such a metallicity gradient survived for almost a Hubble time indicates that the efficiency of radial mixing was very low in the Sgr progenitor. No significant gradient is seen to exist in the [α/Fe] abundance ratio along the trailing arm. Our results may be accounted for by a radial decrease in star formation efficiency and/or radial increase in the efficiency of galactic wind-driven metal loss in the chemical evolution of the Sgr progenitor. The [Ti/Fe] and [O/Fe] abundance ratios observed within the stream are distinct from those of the Galactic halo. We conclude that the fraction of the intermediate to metal-rich halo population contributed by the recent dissolution (<3 Gyr) of Sgr-like dwarf galaxies cannot be substantial.

Formation of solar filament channels by photospheric magnetic reconnection is considered. A magnetohydrodynamic model for magnetic merging, driven by converging convective motions in the photosphere, is presented. Evolution of the axial magnetic field in a channel is analyzed. An exact time-dependent analytical solution for the field profile in a steady stagnation-point flow is derived. The maximum magnetic field in the channel is determined, and its dependence on the reconnection inflow speed is discussed. The quantitative results show that the maximum axial magnetic field in a forming channel is an indicator of the photospheric reconnection rate, in agreement with recent solar observations and laboratory experiments.

We present a detailed assessment of the various dynamical pathways leading to the coalescence of compact objects in globular clusters (GCs) and Short Gamma-ray Burst (SGRB) production. We consider primordial binaries, dynamically formed binaries (through tidal two-body and three-body exchange interactions), and direct impacts of compact objects (WD/NS/BH). Here, we show that if the primordial binary fraction is small, close encounters dominate the production rate of coalescing compact systems. We find that the two dominant channels are the interaction of field neutron stars (NSs) with dynamically formed binaries and two-body encounters. Under such conditions, we estimate the redshift distribution and host galaxy demographics of SGRB progenitors, and find that GCs can provide a significant contribution to the overall observed rate. Regarding the newly identified channel of close stellar encounters involving WD/NS/BH, we have carried out precise modeling of the hydrodynamical evolution, giving us a detailed description of the resulting merged system. Our calculations show that there is in principle no problem in accounting for the global energy budget of a typical SGRB. The particulars of each encounter, however, are variable in several aspects and can lead to interesting diversity. First and most importantly, the characteristics of the encounter are highly dependent on the impact parameter. This is in contrast to the merger scenario, where the masses of the compact objects dictate a typical length and luminosity scale for SGRB activity. Second, the nature of the compact star itself can produce very different outcomes. Finally, the presence of tidal tails in which material will fall back onto the central object at a later time is a robust feature of the present set of calculations. The mass involved in these structures is considerably larger than for binary mergers. It is thus possible to account generically in this scenario for a prompt episode of energy release, as well as for activity many dynamical time scales later.

We present new moderate-resolution, far-ultraviolet spectra from the Hubble Space Telescope/Cosmic Origins Spectrograph (HST/COS) of the BL Lac object 1ES 1553+113 covering the wavelength range 1135 Å < λ < 1795 Å. The data show a smooth continuum with a wealth of narrow (b < 100 km s⁻¹) absorption features arising in the interstellar medium and intergalactic medium. These features include 41 Lyα absorbers at 0 < z_abs < 0.43, 14 of which are detected in multiple Lyman lines and 6 of which show absorption in one or more metal lines. We analyze a metal-rich triplet (Δcz ∼ 1000 km s⁻¹) of Lyα absorbers at z_abs ≈ 0.188 in which O vi, N v, and C iii absorption is detected. Silicon ions (Si iii, Si iv) are not detected to fairly strong upper limits and we use the measured Si iii/C iii upper limit to derive an abundance limit (C/Si) ⩾ 4(C/Si)_☉ for the strongest component of the absorber complex. Galaxy redshift surveys show a number of massive galaxies at approximately the same redshift as this absorption complex, suggesting that it arises in a large-scale galaxy filament. As one of the brightest extragalactic X-ray and γ-ray sources, 1ES 1553+113 is of great interest to the high-energy astrophysics community. With no intrinsic emission or absorption features, 1ES 1553+113 has no direct redshift determination. We use intervening Lyα absorbers to place a direct limit on the redshift: z_em>0.395 based on a confirmed Lyα+O vi absorber and z_em>0.433 based on a single-line detection of Lyα. The current COS data are only sensitive to Lyα absorbers at z < 0.47, but we present statistical arguments that z_em ≲ 0.58 (at a 1σ confidence limit) based on the non-detection of any Lyβ absorbers at z>0.4.