Table of contents

We propose a new dynamical picture of galactic stellar and gas spirals, based on hydrodynamic simulations in a "live" stellar disk. We focus especially on spiral structures excited in an isolated galactic disk without a stellar bar. Using high-resolution, three-dimensional N-body/smoothed particle hydrodynamic simulations, we found that the spiral features of the gas in galactic disks are formed by essentially different mechanisms from the galactic shock in stellar density waves. The stellar spiral arms and the interstellar matter on average corotate in a galactic potential at any radii. Unlike the stream motions in the galactic shock, the interstellar matter flows into the local potential minima with irregular motions. The flows converge to form dense gas clouds/filaments near the bottom of the stellar spirals, whose global structures resemble dust lanes seen in late-type spiral galaxies. The stellar arms are non-steady; they are wound and stretched by the galactic shear, and thus local densities of the arm change on a timescale of ∼100 Myr, due to bifurcating or merging with other arms. This makes the gas spirals associated with the stellar arms non-steady. The association of dense gas clouds is eventually dissolved into inter-arm regions with non-circular motions. Star clusters are formed from the cold, dense gases, whose ages are less than ∼30 Myr, and they are roughly associated with the background stellar arms without a clear spatial offset between gas spiral arms and distribution of young stars.

The brightness of gamma-ray burst (GRB) afterglows and their occurrence in young, blue galaxies make them excellent probes to study star-forming regions in the distant universe. We here elucidate dust extinction properties in the early universe through the analysis of the afterglows of all known z > 6 GRBs: GRB 090423, 080913, and 050904, at z = 8.2, 6.69, and 6.295, respectively. We gather all available optical and near-infrared photometry, spectroscopy, and X-ray data to construct spectral energy distributions (SEDs) at multiple epochs. We then fit the SEDs at all epochs with a dust-attenuated power law or broken power law. We find no evidence for dust extinction in GRB 050904 and GRB 090423, with possible evidence for a low level of extinction in GRB 080913. We compare the high redshift GRBs to a sample of lower redshift GRB extinctions and find a lack of even moderately extinguished events (A_V ∼ 0.3) above z ≳ 4. In spite of the biased selection and small number statistics, this result hints at a decrease in dust content in star-forming environments at high redshifts.

We propose an off-axis relativistic jet model for the Type Ic supernova SN 2007gr. Most of the energy (∼2 × 10⁵¹ erg) in the explosion is contained in non-relativistic ejecta which produces the supernova (SN). The optical emission comes from the decay process of ⁵⁶Ni synthesized in the bulk SN ejecta. Only very little energy (∼10⁴⁸ erg) is contained in the relativistic jet with initial velocity about 0.94 times the speed of light. The radio and X-ray emission comes from this relativistic jet. With some typical parameters of a Wolf–Rayet star (progenitor of Type Ic SN), i.e., the mass-loss rate $\dot{M}=1.0\times 10^{-5} \;M_{\odot } \;\rm yr^{-1}$ and the wind velocity v_w = 1.5 × 10³ km s⁻¹ together with an observing angle of θ_obs = 633, we can obtain multiband light curves that fit the observations well. All the observed data are consistent with our model. Thus, we conclude that SN 2007gr contains a weak relativistic jet and we observe the jet from off-axis.

We formulate an analytical model of the coronal-phase acceleration observed in the beginning of major solar energetic particle (SEP) events, before the main-phase acceleration associated with coronal mass ejections (CMEs) in solar wind. The model is driven and constrained by the broadband observations of SEPs and CMEs, in particular SEP data from the particle telescope of the Energetic and Relativistic Nuclei and Electron (ERNE) experiment on the Solar and Heliospheric Observatory (SOHO) spacecraft, solar radio spectrograms, and low-corona observations of CMEs. The model is also verified against observations of solar high-energy neutrons and neutron-decay protons. The model suggests SEP acceleration above ∼ 50 MeV nucleon⁻¹ by coronal shock and the shock-amplified turbulence in closed magnetic structures, and particle release at magnetic reconnection between the closed structure of expanding CME and pre-existing open magnetic flux tubes. The analytical model connects parameters of coronal shocks and structures and the SEP parameters in space, which facilitates analysis of multiwavelength data and will assist in further development of coronal acceleration models.

We present the highest redshift detections of resolved Lyα emission, using Hubble Space Telescope (HST)/Advanced Camera for Surveys F658N narrowband-imaging data taken in parallel with the Wide Field Camera 3 Early Release Science program in the GOODS Chandra Deep Field-South. We detect Lyα emission from three spectroscopically confirmed z = 4.4 Lyα emitting galaxies (LAEs), more than doubling the sample of LAEs with resolved Lyα emission. Comparing the light distribution between the rest-frame ultraviolet continuum and narrowband images, we investigate the escape of Lyα photons at high redshift. While our data do not support a positional offset between the Lyα and rest-frame ultraviolet (UV) continuum emission, the half-light radius in one out of the three galaxies is significantly (>1σ) larger in Lyα than in the rest-frame UV continuum. Stacking the three LAEs in both the narrowband and UV continuum images, we find that the Lyα light appears larger than the rest-frame UV at 4.2σ significance. This Lyα flux detected with HST is a factor of 4–10 less than observed in similar filters from the ground. These results together imply that the Lyα emission is not strictly confined to its indigenous star-forming regions. Rather, for at least one object the Lyα emission is more extended, with the missing HST flux possibly existing in a diffuse outer halo. This suggests that the radiative transfer of Lyα photons in high-redshift LAEs is complicated, with the interstellar-medium geometry and/or outflows playing a significant role in galaxies at these redshifts.

A systematic investigation of dust emission associated with ionized gas has so far been performed only in our Galaxy and for wavelengths longer than 60 μm. Newly available Spitzer data now offer the opportunity to carry out a similar analysis in the Large Magellanic Cloud (LMC). By cross-correlating SpitzerSurveying the Agents of a Galaxy's Evolution (SAGE) data with the Australia Telescope Compact Array/Parkes H i 21 cm data, the NANTEN ¹²CO (J = 1–0) data, and both the Southern H-Alpha Sky Survey Atlas Hα and the Parkes 6 cm data, we investigate the physical properties of dust associated with the different phases of the gas (atomic, molecular, and ionized). In particular, we study the presence and nature of dust from 3.6 to 160 μm and for various regimes of ionized gas, spanning emission measures from ∼1 pc cm⁻⁶ (diffuse component) to ∼10³pc cm⁻⁶ (H ii regions). Using a dust emission model and testing our results with several radiation field spectra, we show that dust in ionized gas is warmer than dust associated with other phases (atomic and molecular). We also find a decrease of the polycyclic aromatic hydrocarbon (PAH) relative abundance with respect to big grains, as well as an increase of the near-infrared (NIR) continuum. These three results (i.e., warmer temperature, decrease of PAH abundance, and increase of the NIR continuum) are found consistently for all regimes of ionized gas. On the contrary, the molecular phase appears to provide favorable conditions for the survival of PAHs. Furthermore, the very small grain relative abundance tends to increase in the ionized phase, especially in bright H ii regions. Last but not least, our analysis shows that the emissivity of dust associated with ionized gas is lower in the LMC than in our Galaxy and that this difference is not accounted for by the lower metallicity of the LMC.

Images of the solar corona obtained by the Solar-Terrestrial Relations Observatory (STEREO) provide high-cadence, high-resolution observations of a compression wave forming ahead of a fast (940 km s⁻¹) coronal mass ejection (CME) that erupted at ∼9:00 UT on 2010 April 03. The passage of this wave at 1 AU is detected in situ by the Advanced Composition Explorer and Wind spacecraft at 08:00 UT on April 05 as a shock followed by a turbulent and heated sheath. These unprecedented and complementary observations of a shock–sheath region from the Sun to 1 AU are used to investigate the onset of a Solar Energetic Particle (SEP) event measured at the first Lagrange point (L1) and at STEREO-Behind (STB). The spatial extent, radial coordinates, and speed of the ejection are measured from STEREO observations and used as inputs to a numerical simulation of the CME propagation in the background solar wind. The simulated magnetic and plasma properties of the shock and sheath region at L1 agree very well with the in situ measurements. These simulation results reveal that L1 and STB are magnetically connected to the western and eastern edges of the driven shock, respectively. They also show that the 12 hr delay between the eruption time of the ejection and the SEP onset at L1 corresponds to the time required for the bow shock to reach the magnetic field lines connected with L1. The simulated shock compression ratio increases along these magnetic field lines until the maximum flux of high-energy particles is observed.

Whether protoplanetary disks accrete at observationally significant rates by the magnetorotational instability (MRI) depends on how well ionized they are. Disk surface layers ionized by stellar X-rays are susceptible to charge neutralization by small condensates, ranging from ∼0.01 μm sized grains to angstrom-sized polycyclic aromatic hydrocarbons (PAHs). Ion densities in X-ray-irradiated surfaces are so low that ambipolar diffusion weakens the MRI. Here we show that ionization by stellar far-ultraviolet (FUV) radiation enables full-blown MRI turbulence in disk surface layers. Far-UV ionization of atomic carbon and sulfur produces a plasma so dense that it is immune to ion recombination on grains and PAHs. The FUV-ionized layer, of thickness 0.01–0.1 g cm⁻², behaves in the ideal magnetohydrodynamic limit and can accrete at observationally significant rates at radii ≳ 1–10 AU. Surface layer accretion driven by FUV ionization can reproduce the trend of increasing accretion rate with increasing hole size seen in transitional disks. At radii ≲1–10 AU, FUV-ionized surface layers cannot sustain the accretion rates generated at larger distance, and unless turbulent mixing of plasma can thicken the MRI-active layer, an additional means of transport is needed. In the case of transitional disks, it could be provided by planets.

Electron–positron pairs may be produced near accreting black holes by a variety of physical processes, and the resulting pair plasma may be accelerated and collimated into a relativistic jet. Here, we use a self-consistent dynamical and radiative model to investigate pair production by γγ collisions in weakly radiative accretion flows around a black hole of mass M and accretion rate $\dot{M}$. Our flow model is drawn from general relativistic magnetohydrodynamic simulations, and our radiation field is computed by a Monte Carlo transport scheme assuming the electron distribution function is thermal. We argue that the pair production rate scales as $r^{-6} M^{-1} \dot{M}^{6}$. We confirm this numerically and calibrate the scaling relation. This relation is self-consistent in a wedge in $M, \dot{M}$ parameter space. If $\dot{M}$ is too low the implied pair density over the poles of the black hole is below the Goldreich–Julian density and γγ pair production is relatively unimportant; if $\dot{M}$ is too high the models are radiatively efficient. We also argue that for a power-law spectrum the pair production rate should scale with the observables L_X ≡ X-ray luminosity and M as L²_XM⁻⁴. We confirm this numerically and argue that this relation likely holds even for radiatively efficient flows. The pair production rates are sensitive to black hole spin and to the ion–electron temperature ratio which are fixed in this exploratory calculation. We finish with a brief discussion of the implications for Sgr A* and M87.

Most detected planet-bearing binaries are in wide orbits, for which a high inclination, i_B, between the binary orbital plane and the plane of the planetary disk around the primary is likely to be common. In this paper, we investigate the intermediate stages—from planetesimals to planetary embryos/cores—of planet formation in such highly inclined cases. Our focus is on the effects of gas drag on the planetesimals' orbital evolution, in particular on the evolution of the planetesimals' semimajor axis distribution and their mutual relative velocities. We first demonstrate that a non-evolving axisymmetric disk model is a good approximation for studying the effects of gas drag on a planetesimal in the highly inclined case (30° < i_B < 150°). We then find that gas drag plays a crucial role, and the results can be generally divided into two categories, i.e., the Kozai-on regime and the Kozai-off regime, depending on the specific value of i_B. For both regimes, a robust outcome over a wide range of parameters is that planetesimals migrate/jump inward and pile up, leading to a severely truncated and dense planetesimal disk around the primary. In this compact and dense disk, collision rates are high but relative velocities are low, providing conditions that are favorable for planetesimal growth and potentially allow for the subsequent formation of planets.

We present scaling relations between jet power and radio power measured using the Giant Metrewave Radio Telescope (GMRT), Chandra, and XMM-Newton, for a sample of nine galaxy groups combined with the Bîrzan et al. sample of clusters. Cavity power is used as a proxy for mechanical jet power. Radio power is measured at 235 MHz and 1.4 GHz, and the integrated 10 MHz–10 GHz radio luminosity is estimated from the GMRT 610–235 MHz spectral index. The use of consistently analyzed, high-resolution low-frequency radio data from a single observatory makes the radio powers for the groups more reliable than those used by previous studies, and the combined sample covers 6–7 decades in radio power and 5 decades in cavity power. We find a relation of the form P_jet∝ L^∼0.7_radio for integrated radio luminosity, with a total scatter of σ_Lrad = 0.63 and an intrinsic scatter of σ_{i, Lrad} = 0.59. A similar relation is found for 235 MHz power, but a slightly flatter relation with greater scatter is found for 1.4 GHz power, suggesting that low-frequency or broadband radio measurements are superior jet power indicators. We find our low-frequency relations to be in good agreement with previous observational results. Comparison with jet models shows reasonable agreement, which may be improved if radio sources have a significant low-energy electron population. We consider possible factors that could bias our results or render them more uncertain, and find that correcting for such factors in those groups we are able to study in detail leads to a flattening of the P_jet:L_radio relation.

Observations of the globular clusters (GCs) NGC 6388 and M15 were carried out by the High Energy Stereoscopic System array of Cherenkov telescopes for a live time of 27.2 and 15.2 hr, respectively. No gamma-ray signal is found at the nominal target position of NGC 6388 and M15. In the primordial formation scenario, GCs are formed in a dark matter (DM) halo and DM could still be present in the baryon-dominated environment of GCs. This opens the possibility of observing a DM self-annihilation signal. The DM content of the GCs NGC 6388 and M15 is modeled taking into account the astrophysical processes that can be expected to influence the DM distribution during the evolution of the GC: the adiabatic contraction of DM by baryons, the adiabatic growth of a black hole in the DM halo, and the kinetic heating of DM by stars. Ninety-five percent confidence level exclusion limits on the DM particle velocity-weighted annihilation cross section are derived for these DM halos. In the TeV range, the limits on the velocity-weighted annihilation cross section are derived at the 10⁻²⁵ cm³ s⁻¹ level and a few 10⁻²⁴ cm³ s⁻¹ for NGC 6388 and M15, respectively.

The Wolf-Rayet star WR 46 is known to exhibit a very complex variability pattern on relatively short timescales of a few hours. Periodic but intermittent radial velocity shifts of optical lines as well as multiple photometric periods have been found in the past. Non-radial pulsations, rapid rotational modulation, or the presence of a putative low-mass companion have been proposed to explain the short-term behavior. In an effort to unveil its true nature, we observed WR 46 with the Far Ultraviolet Spectroscopic Explorer (FUSE) over several short-term variability cycles. We found significant variations on a timescale of ∼8 hr in the far-ultraviolet (FUV) continuum, in the blue edge of the absorption trough of the O vi λλ1032, 1038 doublet P Cygni profile and in the S vi λλ933, 944 P Cygni absorption profile. We complemented these observations with X-ray and UV light curves and an X-ray spectrum from archival X-ray Multi-Mirror Mission-Newton Space Telescope (XMM-Newton) data. The X-ray and UV light curves show variations on a timescale similar to the variability found in the FUV. We discuss our results in the context of the different scenarios suggested to explain the short-term variability of this object and reiterate that non-radial pulsations is the scenario most likely to occur.

We report here our search for molecular outflows from young very low mass stars and brown dwarfs in Taurus and ρ Ophiuchi. Using the Submillimeter Array and the Combined Array for Research in Millimeter-wave Astronomy, we have observed four targets at 1.3 mm wavelength (230 GHz) to search for CO J = 2 → 1 outflows. A young very low mass star MHO 5 (in Taurus) with an estimated mass of 90 M_J, which is just above the hydrogen-burning limit, shows two gas lobes that are likely outflows. While the CO map of MHO 5 does not show a clear structure of outflow, possibly due to environment gas, its position–velocity diagram indicates two distinct blue- and redshifted components. We therefore conclude that they are components of a bipolar molecular outflow from MHO 5. We estimate an outflow mass of 7.0 × 10⁻⁵ M_☉ and a mass-loss rate of 9.0 × 10⁻¹⁰ M_☉. These values are over two orders of magnitude smaller than the typical ones for T Tauri stars and somewhat weaker than those we have observed in the young brown dwarf ISO-Oph 102 of 60 M_J in ρ Ophiuchi. This makes MHO 5 the first young very low mass star showing a bipolar molecular outflow in Taurus. The detection boosts the scenario that very low mass objects form like low-mass stars but in a version scaled down by a factor of over 100.

We investigate the formation and evolution of interstellar dust-grain ices under dark-cloud conditions, with a particular emphasis on CO₂. We use a three-phase model (gas/surface/mantle) to simulate the coupled gas–grain chemistry, allowing the distinction of the chemically active surface from the ice layers preserved in the mantle beneath. The model includes a treatment of the competition between barrier-mediated surface reactions and thermal-hopping processes. The results show excellent agreement with the observed behavior of CO₂, CO, and water ice in the interstellar medium. The reaction of the OH radical with CO is found to be efficient enough to account for CO₂ ice production in dark clouds. At low visual extinctions, with dust temperatures ≳12 K, CO₂ is formed by direct diffusion and reaction of CO with OH; we associate the resultant CO₂-rich ice with the observational polar CO₂ signature. CH₄ ice is well correlated with this component. At higher extinctions, with lower dust temperatures, CO is relatively immobile and thus abundant; however, the reaction of H and O atop a CO molecule allows OH and CO to meet rapidly enough to produce a CO:CO₂ ratio in the range ∼2–4, which we associate with apolar signatures. We suggest that the observational apolar CO₂/CO ice signatures in dark clouds result from a strongly segregated CO:H₂O ice, in which CO₂ resides almost exclusively within the CO component. Observed visual-extinction thresholds for CO₂, CO, and H₂O are well reproduced by depth-dependent models. Methanol formation is found to be strongly sensitive to dynamical timescales and dust temperatures.

Estimating the black hole mass at the center of galaxies is a fundamental step not only for understanding the physics of accretion, but also for the cosmological evolution of galaxies. Recently, a new method, based solely on X-ray data, was successfully applied to determine the black hole mass in Galactic systems. Since X-rays are thought to be produced via Comptonization process both in stellar and supermassive black holes, in principle, the same method may be applied to estimate the mass in supermassive black holes. In this work we test this hypothesis by performing a systematic analysis of a sample of active galactic nuclei, whose black hole mass has been already determined via reverberation mapping and which possess high-quality XMM-Newton archival data. The good agreement obtained between the black hole masses derived with this novel scaling technique and the reverberation mapping values suggests that this method is robust and works equally well on stellar and supermassive black holes, making it a truly scale-independent technique for black hole determination.

We perform a time-dependent ionization analysis to constrain plasma heating requirements during a fast partial halo coronal mass ejection (CME) observed on 2000 June 28 by the Ultraviolet Coronagraph Spectrometer (UVCS) aboard the Solar and Heliospheric Observatory (SOHO). We use two methods to derive densities from the UVCS measurements, including a density sensitive O v line ratio at 1213.85 and 1218.35 Å, and radiative pumping of the O vi λλ1032, 1038 doublet by chromospheric emission lines. The most strongly constrained feature shows cumulative plasma heating comparable to or greater than the kinetic energy, while features observed earlier during the event show plasma heating of order or less than the kinetic energy. SOHO Michelson Doppler Imager observations are used to estimate the active region magnetic energy. We consider candidate plasma heating mechanisms and provide constraints when possible. Because this CME was associated with a relatively weak flare, the contribution from flare energy (e.g., through thermal conduction or energetic particles) is probably small; however, the flare may have been partially behind the limb. Wave heating by photospheric motions requires heating rates to be significantly larger than those previously inferred for coronal holes, but the eruption itself could drive waves that heat the plasma. Heating by small-scale reconnection in the flux rope or by the CME current sheet is not significantly constrained. UVCS line widths suggest that turbulence must be replenished continually and dissipated on timescales shorter than the propagation time in order to be an intermediate step in CME heating.

We report the detection of color gradients in six massive (stellar mass (M_star) > 10¹⁰ M_☉) and passively evolving (specific star formation rate <10⁻¹¹ yr⁻¹) galaxies at redshift 1.3 < z < 2.5 identified in the Hubble Ultra Deep Field using ultra-deep Hubble Space Telescope (HST) Advanced Camera for Surveys and WFC3/IR images. After carefully matching the different point-spread functions, we obtain color maps and multi-band optical/near-IR photometry (BVizYJH) in concentric annuli, from the smallest resolved radial distance (≈1.7 kpc) up to several times the H-band effective radius. We find that the inner regions of these galaxies have redder rest-frame UV–optical colors (U − V, U − B, and B − V) than the outer parts. The slopes of the color gradient have no obvious dependence on the redshift and on the stellar mass of the galaxies. They do mildly depend, however, on the overall dust obscuration (E(B − V)) and rest-frame (U − V) color, with more obscured or redder galaxies having steeper color gradients. The z ∼ 2 color gradients are also steeper than those of local early-type ones. The gradient of a single parameter (age, extinction, or metallicity) cannot fully explain the observed color gradients. Fitting the spatially resolved HST seven-band photometry to stellar population synthesis models, we find that, regardless of assumptions on the metallicity gradient, the redder inner regions of the galaxies have slightly higher dust obscuration than the bluer outer regions, implying that dust partly contributes to the observed color gradients, although the magnitude depends on the assumed extinction law. Due to the age–metallicity degeneracy, the derived age gradient depends on the assumptions for the metallicity gradient. We discuss the implications of a number of assumptions for metallicity gradients on the formation and evolution of these galaxies. We find that the evolution of the mass–size relationship from z ∼ 2 to the present cannot be driven by in situ extended star formation, which implies that accretion or merger is mostly responsible for the growth of their stellar mass and size. The lack of a correlation between the strength of the color gradient and the stellar mass argues against the metallicity gradient predicted by the monolithic-collapse scenario, which would require significant major mergers to evolve into the one observed at the present.

We report 350 and 230 GHz observations of molecular gas and dust in the starburst nucleus of NGC 253 at 20–40 pc (1''–2'') resolution. The data contain CO(3–2), HCN(4–3), CO(2–1), ¹³CO(2–1), C¹⁸O(2–1), and continuum at 0.87 mm and 1.3 mm toward the central kiloparsec. The CO(2–1) size of the galaxy's central molecular zone (CMZ) is measured to be about 300 pc×100 pc at the half-maximum of intensity. Five clumps of dense and warm gas stand out in the CMZ at arcsecond resolution, and they are associated with compact radio sources due to recent massive star formation. They contribute one-third of the CO emission in the central 300 pc and have ¹²CO peak brightness temperatures around 50 K, molecular gas column densities on the order of 10⁴ M_☉ pc⁻², gas masses on the order of 10⁷ M_☉ in the size scale of 20 pc, volume-averaged gas densities of $\mbox{$n_{\rm H_2}$}\sim 4000$ cm⁻³, and high HCN-to-CO ratios suggestive of higher fractions of dense gas than in the surrounding environment. It is suggested that these are natal molecular cloud complexes of massive star formation. The CMZ of NGC 253 is also compared with that of our Galaxy in CO(2–1) at the same 20 pc resolution. Their overall gas distributions are strikingly similar. The five molecular cloud complexes appear to be akin to such molecular complexes as Sgr A, Sgr B2, Sgr C, and the l = 13 cloud in the Galactic center. On the other hand, the starburst CMZ in NGC 253 has higher temperatures and higher surface (and presumably volume) densities than its non-starburst cousin.

It has been suggested that multiple scattering on circumstellar dust could explain the non-standard reddening observed in the line of sight to Type Ia supernovae. In this work, we use Monte Carlo simulations to examine how the scattered light would affect the shape of optical light curves and spectral features. We find that the effects on the light curve widths, apparent time evolution of color excess, and blending of spectral features originating at different photospheric velocities should allow for tests of the circumstellar dust hypothesis on a case by case basis. Our simulations also show that for circumstellar shells with radii r = 10¹⁶–10¹⁹ cm, the light curve modifications are well described by the empirical Δm₁₅ parameter and intrinsic color variations of order σ_BV = 0.05–0.1 arise naturally. For large shell radii an excess light curve tail is expected in B-band, as observed in, e.g., SN2006X.

Accurate photodissociation cross sections have been obtained for the A¹Σ⁺ ← X¹Σ⁺ electronic transition of HeH⁺ using ab initio potential curves and dipole transition moments. Partial cross sections have been evaluated for all rotational transitions from the vibrational levels v'' = 0–11 and over the entire accessible wavelength range λλ100–1129. Assuming a Boltzmann distribution of the rovibrational levels of the X¹Σ⁺ state, photodissociation cross sections are presented for temperatures between 500 and 12,000 K. A similar set of calculations was performed for the pure rovibrational photodissociation in the X¹Σ⁺ electronic ground state, but covering photon wavelengths into the far-infrared. Applications of the cross sections to the destruction of HeH⁺ in the early universe and in UV-irradiated environments such as primordial halos and protoplanetary disks are briefly discussed.

We present deep Hubble Space Telescope WFPC2 optical observations obtained as part of the ACS Nearby Galaxy Survey Treasury as well as early release Wide Field Camera 3 ultraviolet and infrared observations of the nearby dwarf starbursting galaxy NGC 4214. Our data provide a detailed example of how covering such a broad range in wavelength provides a powerful tool for constraining the physical properties of stellar populations. The deepest data reach the ancient red clump at M_F814W ∼ − 0.2. All of the optical data reach the main-sequence turnoff for stars younger than ∼300 Myr and the blue He-burning sequence for stars younger than 500 Myr. The full color–magnitude diagram (CMD) fitting analysis shows that all three fields in our data set are consistent with ∼75% of the stellar mass being older than 8 Gyr, in spite of showing a wide range in star formation rates at present. Thus, our results suggest that the scale length of NGC 4214 has remained relatively constant for many gigayears. As previously noted by others, we also find the galaxy has recently ramped up production consistent with its bright UV luminosity and its population of UV-bright massive stars. In the central field we find UV point sources with F336W magnitudes as bright as −9.9. These are as bright as stars with masses of at least 52–56 M_☉ and ages near 4 Myr in stellar evolution models. Assuming a standard initial mass function, our CMD is well fitted by an increase in star formation rate beginning 100 Myr ago. The stellar populations of this late-type dwarf are compared with those of NGC 404, an early-type dwarf that is also the most massive galaxy in its local environment. The late-type dwarf appears to have a similar high fraction of ancient stars, suggesting that these dominant galaxies may form at early epochs even if they have low total mass and very different present-day morphologies.

We analyze the Swift/BAT sample of short gamma-ray bursts, using an objective Bayesian Block procedure to extract temporal descriptors of the bursts' initial pulse complexes (IPCs). The sample is comprised of 12 and 41 bursts with and without extended emission (EE) components, respectively. IPCs of non-EE bursts are dominated by single pulse structures, while EE bursts tend to have two or more pulse structures. The medians of characteristic timescales—durations, pulse structure widths, and peak intervals—for EE bursts are factors of ∼2–3 longer than for non-EE bursts. A trend previously reported by Hakkila and colleagues unifying long and short bursts—the anti-correlation of pulse intensity and width—continues in the two short burst groups, with non-EE bursts extending to more intense, narrower pulses. In addition, we find that preceding and succeeding pulse intensities are anti-correlated with pulse interval. We also examine the short burst X-ray afterglows as observed by the Swift/X-Ray Telescope (XRT). The median flux of the initial XRT detections for EE bursts (∼6×10⁻¹⁰ erg cm⁻² s⁻¹) is ≳20× brighter than for non-EE bursts, and the median X-ray afterglow duration for EE bursts (∼60,000 s) is ∼30× longer than for non-EE bursts. The tendency for EE bursts toward longer prompt-emission timescales and higher initial X-ray afterglow fluxes implies larger energy injections powering the afterglows. The longer-lasting X-ray afterglows of EE bursts may suggest that a significant fraction explode into denser environments than non-EE bursts, or that the sometimes-dominant EE component efficiently powers the afterglow. Combined, these results favor different progenitors for EE and non-EE short bursts.

We report the discovery of HAT-P-30b, a transiting exoplanet orbiting the V = 10.419 dwarf star GSC 0208-00722. The planet has a period P = 2.810595 ± 0.000005  days, transit epoch T_c = 2455456.46561 ± 0.00037 (BJD), and transit duration 0.0887 ± 0.0015 days. The host star has a mass of 1.24 ± 0.04 M_☉, radius of 1.21 ± 0.05 R_☉, effective temperature of 6304 ± 88 K, and metallicity [Fe/H] = +0.13 ± 0.08. The planetary companion has a mass of 0.711 ± 0.028 M_J and radius of 1.340 ± 0.065 R_J yielding a mean density of 0.37 ± 0.05 g cm⁻³. We also present radial velocity measurements that were obtained throughout a transit that exhibit the Rossiter–McLaughlin effect. By modeling this effect, we measure an angle of λ = 735 ± 90 between the sky projections of the planet's orbit normal and the star's spin axis. HAT-P-30b represents another example of a close-in planet on a highly tilted orbit, and conforms to the previously noted pattern that tilted orbits are more common around stars with T_eff⋆ ≳ 6250 K.

Accretion studies have been focused on the flow around bodies with point mass gravitational potentials, but few general results are available for non-point mass distributions. Here, we study the accretion flow onto non-divergent, core potentials moving through a background medium. We use Plummer and Hernquist potentials as examples to study gas accretion onto star clusters, dwarf and large galaxy halos, and galaxy clusters in a variety of astrophysical environments. The general conditions required for a core potential to collectively accrete large quantities of gas from the external medium are derived using both simulations and analytic results. The consequences of large mass accumulation in galaxy nuclei, dwarf galaxies, and star clusters are twofold. First, if the gas cools effectively star formation can be triggered, generating new stellar members in the system. Second, if the collective potential of the system is able to alter the ambient gas properties before the gas is accreted onto the individual core members, the augmented mass supply rates could significantly alter the state of the various accreting stellar populations and result in an enhanced central black hole accretion luminosity.

We present an analysis of 15 Chandra observations of the nearby spiral galaxy M81 taken over the course of six weeks in 2005 May–July. Each observation reaches a sensitivity of ∼10³⁷ erg s⁻¹. With these observations and one previous deeper Chandra observation, we compile a master source list of 265 point sources, extract and fit their spectra, and differentiate basic populations of sources through their colors. We also carry out variability analyses of individual point sources and of X-ray luminosity functions (XLFs) in multiple regions of M81 on timescales of days, months, and years. We find that, despite measuring significant variability in a considerable fraction of sources, snapshot observations provide a consistent determination of the XLF of M81. We also fit the XLFs for multiple regions of M81 and, using common parameterizations, compare these luminosity functions to those of two other spiral galaxies, M31 and the Milky Way.

In this paper, we describe a uniform analysis of eight transits and eleven secondary eclipses of the extrasolar planet GJ 436b obtained in the 3.6, 4.5, and 8.0 μm bands using the IRAC instrument on the Spitzer Space Telescope between UT 2007 June 29 and UT 2009 February 4. We find that the best-fit transit depths for visits in the same bandpass can vary by as much as 8% of the total (4.7σ significance) from one epoch to the next. Although we cannot entirely rule out residual detector effects or a time-varying, high-altitude cloud layer in the planet's atmosphere as the cause of these variations, we consider the occultation of active regions on the star in a subset of the transit observations to be the most likely explanation. We find that for the deepest 3.6 μm transit the in-transit data have a higher standard deviation than the out-of-transit data, as would be expected if the planet occulted a star spot. We also compare all published transit observations for this object and find that transits observed in the infrared typically have smaller timing offsets than those observed in visible light. In this case, the three deepest Spitzer transits are all measured within a period of five days, consistent with a single epoch of increased stellar activity. We reconcile the presence of magnetically active regions with the lack of significant visible or infrared flux variations from the star by proposing that the star's spin axis is tilted with respect to our line of sight and that the planet's orbit is therefore likely to be misaligned. In contrast to the results reported by Beaulieu et al., we find no convincing evidence for methane absorption in the planet's transmission spectrum. If we exclude the transits that we believe to be most affected by stellar activity, we find that we prefer models with enhanced CO and reduced methane, consistent with GJ 436b's dayside composition from Stevenson et al. It is also possible that all transits are significantly affected by this activity, in which case it may not be feasible to characterize the planet's transmission spectrum using broadband photometry obtained over multiple epochs. These observations serve to illustrate the challenges associated with transmission spectroscopy of planets orbiting late-type stars; we expect that other systems, such as GJ 1214, may display comparably variable transit depths. We compare the limb-darkening coefficients predicted by PHOENIX and ATLAS stellar atmosphere models and discuss the effect that these coefficients have on the measured planet–star radius ratios given GJ 436b's near-grazing transit geometry. Our measured 8 μm secondary eclipse depths are consistent with a constant value, and we place a 1σ upper limit of 17% on changes in the planet's dayside flux in this band. These results are consistent with predictions from general circulation models for this planet, which find that the planet's dayside flux varies by a few percent or less in the 8 μm band. Averaging over the eleven visits gives us an improved estimate of 0.0452% ± 0.0027% for the secondary eclipse depth; we also examine residuals from the eclipse ingress and egress and place an upper limit on deviations caused by a non-uniform surface brightness for GJ 436b. We combine timing information from our observations with previously published data to produce a refined orbital ephemeris and determine that the best-fit transit and eclipse times are consistent with a constant orbital period. We find that the secondary eclipse occurs at a phase of 0.58672 ± 0.00017, corresponding to ecos (ω) = 0.13754 ± 0.00027, where e is the planet's orbital eccentricity and ω is the longitude of pericenter. We also present improved estimates for other system parameters, including the orbital inclination, a/R_⋆, and the planet–star radius ratio.

We use a minimum spanning tree (MST) algorithm to characterize the spatial distribution of Galactic far-IR sources and derive their clustering properties. We aim to reveal the spatial imprint of different types of star-forming processes, e.g., isolated spontaneous fragmentation of dense molecular clouds, or events of triggered star formation around H ii regions, and highlight global properties of star formation in the Galaxy. We plan to exploit the entire Herschel infrared GALactic (Hi-GAL) survey of the inner Galactic plane to gather significant statistics on the clustering properties of star-forming regions and to look for possible correlations with source properties such as mass, temperature, or evolutionary stage. In this paper, we present a pilot study based on the two 2° × 2° fields centered at longitudes l = 30° and l = 59° obtained during the science demonstration phase of the Herschel mission. We find that over half of the clustered sources are associated with H ii regions and infrared dark clouds. Our analysis also reveals a smooth chromatic evolution of the spatial distribution where sources detected at short wavelengths, likely protostars surrounded by warm circumstellar material emitting in the far-infrared, tend to be clustered in dense and compact groups around H ii regions while sources detected at long wavelengths, presumably cold and dusty density enhancements of the ISM emitting in the submillimeter, are distributed in larger and looser groups.

Planetary migration poses a serious challenge to theories of planet formation. In gaseous and planetesimal disks, migration can remove planets as quickly as they form. To explore migration in a planetesimal disk, we combine analytic and numerical approaches. After deriving general analytic migration rates for isolated planets, we use N-body simulations to confirm these results for fast and slow migration modes. Migration rates scale as m⁻¹ (for massive planets) and (1 + (e_H/3)³)⁻¹, where m is the mass of a planet and e_H is the eccentricity of the background planetesimals in Hill units. When multiple planets stir the disk, our simulations yield the new result that large-scale migration ceases. Thus, growing planets do not migrate through planetesimal disks. To extend these results to migration in gaseous disks, we compare physical interactions and rates. Although migration through a gaseous disk is an important issue for the formation of gas giants, we conclude that migration has little impact on the formation of terrestrial planets.

Transit surveys combined with Doppler data have revealed a class of gas giant planets that are massive and highly enriched in heavy-elements (e.g., HD 149026b, GJ436b, and HAT-P-20b). It is tempting to consider these planets as validation of core accretion plus gas capture because it is often assumed that disk instability planets should be of nebular composition. We show in this paper, to the contrary, that gas giants that form by disk instability can have a variety of heavy-element compositions, ranging from sub- to super-nebular values. High levels of enrichment can be achieved through one or multiple mechanisms, including enrichment at birth, planetesimal capture, and differentiation plus tidal stripping. As a result, the metallicity of an individual gas giant cannot be used to discriminate between gas giant formation modes.

We have developed a variational data assimilation technique for the Sun using a toy αΩ dynamo model. The purpose of this work is to apply modern data assimilation techniques to solar data using a physically based model. This work represents the first step toward a complete variational model of solar magnetism. We derive the adjoint αΩ dynamo code and use a minimization procedure to invert the spatial dependence of key physical ingredients of the model. We find that the variational technique is very powerful and leads to encouraging results that will be applied to a more realistic model of the solar dynamo.

The matter density field exhibits a nearly lognormal probability density distribution after entering into the nonlinear regime. Recently, it has been shown that the shape of the power spectrum of a logarithmically transformed density field is very close to the linear density power spectrum, motivating an analytic study of it. In this paper, we develop cosmological perturbation theory for the power spectrum of this field. Our formalism is developed in the context of renormalized perturbation theory, which helps to regulate the convergence behavior of the perturbation series, and of the Taylor series expansion we use for the logarithmic mapping. This approach allows us to handle the critical issue of density smoothing in a straightforward way. We also compare our perturbative results with simulation measurements.

Chandra ACIS observed the field of the extended TeV source HESS J1834–087 for 47 ks. A previous XMM-Newton EPIC observation of the same field revealed a point-like source (XMMU J183435.3–084443) and an offset region of faint extended emission. In the low-resolution, binned EPIC images the two appear to be connected. However, the high-resolution Chandra ACIS images do not support the alleged connection. In these images, XMMU J183435.3–084443 is resolved into a point source, CXOU J183434.9–084443 (L_{0.5–8 keV} ≃ 2.3 × 10³³ erg s⁻¹, for a distance of 4 kpc; photon index Γ ≃ 1.1), and a compact (≲ 20'') nebula with an isotropic morphology and a softer spectrum (L_{0.5–8 keV} ≃ 4.1 × 10³³ erg s⁻¹, Γ ≃ 2.7). The nature of the nebula is uncertain. We discuss a dust scattering halo and a pulsar-wind nebula as possible interpretations. Based on our analysis of the X-ray data, we re-evaluate the previously suggested interpretations of HESS J1834–087 and discuss a possible connection to the Fermi Large Area Telescope source 1FGL J1834.3–0842c. We also obtained an upper limit of 3 × 10⁻¹⁴ erg cm⁻² s⁻¹ on the unabsorbed flux of the SGR J1833–0832 (in quiescence), which happened to be in the ACIS field of view.

A 30 day contiguous photometric run with the Microvariability and Oscillations of STars (MOST) satellite on the WN5-6b star WR 110 (HD 165688) reveals a fundamental periodicity of P = 4.08 ± 0.55 days along with a number of harmonics at periods P/n, with n ≈ 2, 3, 4, 5, and 6, and a few other possible stray periodicities and/or stochastic variability on timescales longer than about a day. Spectroscopic radial velocity studies fail to reveal any plausible companion with a period in this range. Therefore, we conjecture that the observed light-curve cusps of amplitude ∼0.01 mag that recur at a 4.08 day timescale may arise in the inner parts, or at the base, of a corotating interaction region (CIR) seen in emission as it rotates around with the star at constant angular velocity. The hard X-ray component seen in WR 110 could then be a result of a high velocity component of the CIR shock interacting with the ambient wind at several stellar radii. Given that most hot, luminous stars showing CIRs have two CIR arms, it is possible that either the fundamental period is 8.2 days or, more likely in the case of WR 110, there is indeed a second weaker CIR arm for P = 4.08 days, that occurs ∼two-thirds of a rotation period after the main CIR. If this interpretation is correct, WR 110 therefore joins the ranks with three other single WR stars, all WN, with confirmed CIR rotation periods (WR 1, WR 6, and WR 134), albeit with WR 110 having by far the lowest amplitude photometric modulation. This illustrates the power of being able to secure intense, continuous high-precision photometry from space-based platforms such as MOST. It also opens the door to revealing low-amplitude photometric variations in other WN stars, where previous attempts have failed. If all WN stars have CIRs at some level, this could be important for revealing sources of magnetism or pulsation in addition to rotation periods.

We present new multi-configuration Very Large Array H i spectral line observations of the M81 group dwarf irregular post-starburst galaxy DDO 165. The H i morphology is complex, with multiple column density peaks surrounding a large region of very low H i surface density that is offset from the center of the stellar distribution. The bulk of the neutral gas is associated with the southern section of the galaxy; a secondary peak in the north contains ∼15% of the total H i mass. These components appear to be kinematically distinct, suggesting that either tidal processes or large-scale blowout have recently shaped the interstellar medium (ISM) of DDO 165. Using spatially resolved position–velocity maps, we find multiple localized high-velocity gas features. Cross-correlating with radius–velocity analyses, we identify eight shell/hole structures in the ISM with a range of sizes (∼400–900 pc) and expansion velocities (∼7–11 km s⁻¹). These structures are compared with narrow- and broadband imaging from the Kitt Peak National Observatory and the Hubble Space Telescope (HST). Using the latter data, recent works have shown that DDO 165's previous "burst" phase was extended temporally (≳1 Gyr). We thus interpret the high-velocity gas features, H i holes, and kinematically distinct components of the galaxy in the context of the immediate effects of "feedback" from recent star formation (SF). In addition to creating H i holes and shells, extended SF events are capable of creating localized high-velocity motion of the surrounding interstellar material. A companion paper connects the energetics from the H i and HST data.

We compare the stellar populations and complex neutral gas dynamics of the M81 group dIrr galaxy DDO 165 using data from the Hubble Space Telescope and the Very Large Array. Cannon et al. in Paper I of this series identified two kinematically distinct H i components, multiple localized high velocity gas features, and eight H i holes and shells (the largest of which spans ∼2.2 × 1.1 kpc). Using the spatial and temporal information from the stellar populations in DDO 165, we compare the patterns of star formation (SF) over the past 500 Myr with the H i dynamics. We extract localized star formation histories within 6 of the 8 H i holes identified in Paper I, as well as 23 other regions that sample a range of stellar densities and neutral gas properties. From population synthesis modeling, we derive the energy outputs (from stellar winds and supernovae) of the stellar populations within these regions over the last 100 Myr, and compare with refined estimates of the energies required to create the H i holes. In all cases, we find that "feedback" is energetically capable of creating the observed structures in the interstellar medium (ISM). Numerous regions with significant energy inputs from feedback lack coherent H i structures but show prominent localized high velocity gas features; this feedback signature is a natural product of temporally and spatially distributed SF. In DDO 165, the extended period of heightened SF activity (lasting more than 1 Gyr) is energetically capable of creating the observed holes and high velocity gas features in the neutral ISM.

We report the first detections of Blue Straggler Stars (BSS) in the bulge of the Milky Way. Proper motions from extensive space-based observations along a single sight line allow us to separate a sufficiently clean and well-characterized bulge sample such that we are able to detect a small population of bulge objects in the region of the color–magnitude diagram commonly occupied by young objects and blue stragglers. Variability measurements of these objects clearly establish that a fraction of them are blue stragglers. Out of the 42 objects found in this region of the color–magnitude diagram, we estimate that at least 18 are genuine BSS. We normalize the BSS population by our estimate of the number of horizontal branch stars in the bulge in order to compare the bulge to other stellar systems. The BSS fraction is clearly discrepant from that found in stellar clusters. The blue straggler population of dwarf spheroidals remains a subject of debate; some authors claim an anticorrelation between the normalized blue straggler fraction and integrated light. If this trend is real, then the bulge may extend it by three orders of magnitude in mass. Conversely, we find that the genuinely young (<5 Gyr) population in the bulge, must be at most 3.4% under the most conservative scenario for the BSS population.

We expect a detectable correlation between two seemingly unrelated quantities: the four-point function of the cosmic microwave background (CMB) and the amplitude of flux decrements in quasar (QSO) spectra. The amplitude of CMB convergence in a given direction measures the projected surface density of matter. Measurements of QSO flux decrements trace the small-scale distribution of gas along a given line of sight. While the cross-correlation between these two measurements is small for a single line of sight, upcoming large surveys should enable its detection. This paper presents analytical estimates for the signal-to-noise ratio (S/N) for measurements of the cross-correlation between the flux decrement and the convergence, $\langle \delta \mathcal {F}\kappa \rangle$, and for measurements of the cross-correlation between the variance in flux decrement and the convergence, $\langle (\delta \mathcal {F})^2 \kappa \rangle$. For the ongoing BOSS (SDSS-III) and Planck surveys, we estimate an S/N of 30 and 9.6 for these two correlations. For the proposed BigBOSS and ACTPOL surveys, we estimate an S/N of 130 and 50, respectively. Since $\langle (\delta \mathcal {F})^2 \kappa \rangle \propto \sigma _8^4$, the amplitude of these cross-correlations can potentially be used to measure the amplitude of σ₈ at z ∼ 2%–2.5% with BOSS and Planck and even better with future data sets. These measurements have the potential to test alternative theories for dark energy and to constrain the mass of the neutrino. The large potential signal estimated in our analytical calculations motivates tests with nonlinear hydrodynamic simulations and analyses of upcoming data sets.

A detailed study of the blue supergiant UIT 005 (B2-2.5Ia⁺) in M 33 is presented. The results of our quantitative spectral analysis indicate that the star is a very luminous (log L/L_☉ ∼ 5.9 dex) and massive (M ∼ 50 M_☉) object, showing a very high nitrogen-to-oxygen ratio in its surface (N/O∼8, by mass). Based on the derived Mg and Si abundances, we argue that this high N/O ratio cannot be the result of an initial low O content due to its location on the disk of M 33, a galaxy known to present a steep metallicity gradient. In combination with the He abundance, the most plausible interpretation is that UIT 005 is in an advanced stage of evolution, showing in its surface N enrichment and O depletion resulting from mixing with CNO processed material from the stellar interior. A comparison with the predictions of current stellar evolutionary models indicates that there are significant discrepancies, in particular with regard to the degree of chemical processing, with the models predicting a much lower degree of O depletion than observed. At the same time, the mass-loss rate derived in our analysis is an order of magnitude lower than the values considered in the evolutionary calculations. Based on a study of the surrounding stellar population and the nearby cluster, NGC 588, using Hubble Space Telescope/WFPC2 photometry, we suggest that UIT 005 could be in fact a runaway star from this cluster. Regardless of its origin, the derived parameters place the star in a region of the Hertzsprung–Russell diagram where luminous blue variables (LBVs) are usually found, but we find no evidence supporting photometric or spectroscopic variability, except for small Hα changes, otherwise observed in Galactic B-type supergiants. Whether UIT 005 is an LBV in a dormant state or a regular blue supergiant could not be discerned in this study. Subsequent monitoring would help us to improve our knowledge of the more massive stars, bridging the gap between regular and more exotic blue supergiants.

We present a Monte Carlo calculation of the astrophysical rate of the ¹⁵O(α, γ)¹⁹Ne reaction based on an evaluation of published experimental data. By considering the likelihood distributions of individual resonance parameters derived from measurements, estimates of upper and lower limits on the reaction rate at the 99.73% confidence level are derived in addition to the recommended, median value. These three reaction rates are used as input for three separate calculations of Type I X-ray bursts (XRBs) using spherically symmetric, hydrodynamic simulations of an accreting neutron star. In this way the influence of the ¹⁵O(α, γ)¹⁹Ne reaction rate on the peak luminosity, recurrence time, and associated nucleosynthesis in models of Type I XRBs is studied. Contrary to previous findings, no substantial effect on any of these quantities is observed in a sequence of four bursts when varying the reaction rate between its lower and upper limits. Rather, the differences in these quantities are comparable to the burst-to-burst variations with a fixed reaction rate, indicating that uncertainties in the ¹⁵O(α, γ)¹⁹Ne reaction rate do not strongly affect the predictions of this Type I XRB model.

We present carbon and oxygen abundances for 941 FGK stars—the largest such catalog to date. We find that planet-bearing systems are enriched in these elements. We self-consistently measure N_C/N_O, which is thought to play a key role in planet formation. We identify 46 stars with N_C/N_O ⩾ 1.00 as potential hosts of carbon-dominated exoplanets. We measure a downward trend in [O/Fe] versus [Fe/H] and find distinct trends in the thin and thick disks, supporting the work of Bensby et al. Finally, we measure sub-solar N_C/N_O = 0.40^+0.11_{− 0.07}, for WASP-12, a surprising result as this star is host to a transiting hot Jupiter whose dayside atmosphere was recently reported to have N_C/N_O ⩾ 1 by Madhusudhan et al. Our measurements are based on 15,000 high signal-to-noise spectra taken with the Keck 1 telescope as part of the California Planet Search. We derive abundances from the [O i] and C i absorption lines at λ = 6300 and 6587 Å using the SME spectral synthesizer.

The hard X-ray (HXR) emission in solar flares comes almost exclusively from a very small part of the flaring region, the footpoints of magnetic loops. Using RHESSI observations of solar flare footpoints, we determine the radial positions and sizes of footpoints as a function of energy in six near-limb events to investigate the transport of flare accelerated electrons and the properties of the chromosphere. HXR visibility forward fitting allows us to find the positions/heights and the sizes of HXR footpoints along and perpendicular to the magnetic field of the flaring loop at different energies in the HXR range. We show that in half of the analyzed events, a clear trend of decreasing height of the sources with energy is found. Assuming collisional thick-target transport, HXR sources are located between 600 and 1200 km above the photosphere for photon energies between 120 and 25 keV, respectively. In the other events, the position as a function of energy is constant within the uncertainties. The vertical sizes (along the path of electron propagation) range from 1.3 to 8 arcsec which is up to a factor four larger than predicted by the thick-target model even in events where the positions/heights of HXR sources are consistent with the collisional thick-target model. Magnetic mirroring, collisional pitch-angle scattering, and X-ray albedo are discussed as potential explanations of the findings.

We investigate the acceleration source of the impulsive solar energetic particle (SEP) events on 2007 January 24. Combining the in situ electron measurements and remote-sensing solar observations, as well as the calculated magnetic fields obtained from a potential-field source-surface model, we demonstrate that the jets associated with the hard X-ray flares and type-III radio bursts, rather than the slow and partial coronal mass ejections, are closely related to the production of interplanetary electron streams. The jets, originated from the well-connected active region (AR 10939) whose magnetic polarity structure favors the eruption, are observed to be forming in a coronal site, extending to a few solar radii, and having a good temporal correlation with the electron solar release. The open-field lines near the jet site are rooted in a negative polarity, along which energetic particles escape from the flaring AR to the near-Earth space, consistent with the in situ electron pitch angle distribution. The analysis enables us to propose a coronal magnetic topology relating the impulsive SEP events to their solar source.

We develop one-zone galaxy formation models in the early universe, taking into account dust formation and evolution by supernova (SN) explosions. We focus on the time evolution of dust size distribution, because H₂ formation on the dust surface plays a critical role in the star formation process in the early universe. In the model, we assume that star formation rate (SFR) is proportional to the total amount of H₂. We consistently treat (1) the formation and size evolution of dust, (2) the chemical reaction networks including H₂ formation both on the surface of dust and in gas phase, and (3) the SFR in the model. First, we find that, because of dust destruction due to both reverse and forward shocks driven by SNe, H₂ formation is more suppressed than in situations without such dust destruction. At the galaxy age of ∼0.8 Gyr, for galaxy models with virial mass M_vir = 10⁹ M_☉ and formation redshift z_vir = 10, the molecular fraction is 2.5 orders of magnitude less in the model with dust destruction by both shocks than that in the model without dust destruction. Second, we show that the H₂ formation rate strongly depends on the interstellar medium (ISM) density around SN progenitors. The SFR in higher ISM density is lower, since dust destruction by reverse shocks is more effective in higher ISM density. We conclude that not only the amount but also the size distribution of dust related to star formation activity strongly affects the evolution of galaxies in the early universe.

A magnetohydrodynamic model that includes a complete electrical conductivity tensor is used to estimate conditions for photospherically driven, linear, non-plane Alfvénic oscillations extending from the photosphere to the lower corona to drive a chromospheric heating rate due to Pedersen current dissipation that is comparable to the observed net chromospheric radiative loss of ∼10⁷ erg cm⁻² s⁻¹. The heating rates due to electron current dissipation in the photosphere and corona are also computed. The wave amplitudes are computed self-consistently as functions of an inhomogeneous background (BG) atmosphere. The effects of the conductivity tensor are resolved numerically using a resolution of 3.33 m. The oscillations drive a chromospheric heating flux F_Ch ∼ 10⁷–10⁸ erg cm⁻² s⁻¹ at frequencies ν ∼ 10²–10³ mHz for BG magnetic field strengths B ≳ 700 G and magnetic field perturbation amplitudes ∼0.01–0.1 B. The total resistive heating flux increases with ν. Most heating occurs in the photosphere. Thermalization of Poynting flux in the photosphere due to electron current dissipation regulates the Poynting flux into the chromosphere, limiting F_Ch. F_Ch initially increases with ν, reaches a maximum, and then decreases with increasing ν due to increasing electron current dissipation in the photosphere. The resolution needed to resolve the oscillations increases from ∼10 m in the photosphere to ∼10 km in the upper chromosphere and is ∝ν^−1/2. Estimates suggest that these oscillations are normal modes of photospheric flux tubes with diameters ∼10–20 km, excited by magnetic reconnection in current sheets with thicknesses ∼0.1 km.

In this paper, we examine the mode of dynamo action in the implicit large-eddy magnetohydrodynamical simulation of solar convection reported upon in Ghizaru et al. Motivated by the presence of a strong and well-defined large-scale axisymmetric magnetic component undergoing regular polarity reversals, we define the fluctuating component of the magnetic field as the difference between the total field and its zonal average. The subsequent analysis follows the physical logic and mathematical formulation of mean-field electrodynamics, whereby a turbulent electromotive force (EMF) is computed by the suitable averaging of cross-correlations between fluctuating flow and field components and expressed in terms of the mean field via a linear truncated tensorial expansion. We use singular value decomposition to perform a linear least-squares fit of the temporal variation of the EMF to that of the large-scale magnetic component, which yields the components of the full α-tensor. Its antisymmetric component, describing general turbulent pumping, is also extracted. The α-tensor so calculated reproduces a number of features already identified in local, Cartesian simulations of magnetohydrodynamical rotating convection, including an α_ϕϕ component positive in the northern solar hemisphere, peaking at high latitudes, and reversing sign near the bottom of the convection zone; downward turbulent pumping throughout the convecting layer; and significant equatorward turbulent pumping at mid latitudes, and poleward at high latitudes in subsurface layers. We also find that the EMF contributes significantly to the regeneration of the large-scale toroidal magnetic component, which from the point of view of mean-field dynamo models would imply that the simulation operates as an α²Ω dynamo. We find little significant evidence of α-quenching by the large-scale magnetic field. The amplitude of the magnetic cycle appears instead to be regulated primarily by a magnetically driven reduction of the differential rotation.

Observations of 108 coronal holes (CHs) from 1998–2008 were used to investigate the correlation between fast solar wind (SW) and several parameters of CHs. Our main goal was to establish the association between coronal bright points (CBPs; as sites of magnetic reconnection) and fast SW. Using in situ measurements of the SW, we have connected streams of the fast SW at 1 AU with their source regions, CHs. We studied a correlation between the SW speed and selected parameters of CHs: total area of the CH, total intensity inside the CH, fraction of area of the CH associated with CBPs, and their integrated brightness inside each CH. In agreement with previous studies, we found that the SW speed most strongly correlates with the total area of the CHs. The correlation is stronger for the non (de)projected areas of CHs (which are measured in image plane) suggesting that the near-equatorial parts of CHs make a larger contribution to the SW measured at near Earth orbit. This correlation varies with solar activity. It peaks for periods of moderate activity, but decreases slightly for higher or lower levels of activity. A weaker correlation between the SW speed and other studied parameters was found, but it can be explained by correlating these parameters with the CH's area. We also studied the spatial distribution of CBPs inside 10 CHs. We found that the density of CBPs is higher in the inner part of CHs. As such, results suggest that although the reconnection processes occurring in CBPs may contribute to the fast SW, they do not serve as the main mechanism of wind acceleration.

Approximately 1% of low-redshift (z ≲ 0.3) optically selected type 2 active galactic nuclei (AGNs) show a double-peaked [O iii] narrow emission line profile in their spatially integrated spectra. Such features are usually interpreted as either due to kinematics, such as biconical outflows and/or disk rotation of the narrow line region (NLR) around single black holes, or due to the relative motion of two distinct NLRs in a merging pair of AGNs. Here, we report follow-up near-infrared (NIR) imaging and optical slit spectroscopy of 31 double-peaked [O iii] type 2 AGNs drawn from the Sloan Digital Sky Survey (SDSS) parent sample presented in Liu et al. The NIR imaging traces the old stellar population in each galaxy, while the optical slit spectroscopy traces the NLR gas. These data reveal a mixture of origins for the double-peaked feature. Roughly 10% of our objects are best explained by binary AGNs at (projected) kpc-scale separations, where two stellar components with spatially coincident NLRs are seen. ∼50% of our objects have [O iii] emission offset by a few kpc, corresponding to the two velocity components seen in the SDSS spectra, but there are no spatially coincident double stellar components seen in the NIR imaging. For those objects with sufficiently high-quality slit spectra, we see velocity and/or velocity dispersion gradients in [O iii] emission, suggestive of the kinematic signatures of a single NLR. The remaining ∼40% of our objects are ambiguous and will need higher spatial resolution observations to distinguish between the two scenarios. Our observations therefore favor the kinematics scenario with a single AGN for the majority of these double-peaked [O iii] type 2 AGNs. We emphasize the importance of combining imaging and slit spectroscopy in identifying kpc-scale binary AGNs, i.e., in no cases does one of these alone allow an unambiguous identification. We estimate that ∼0.5%–2.5% of the z ≲ 0.3 type 2 AGNs are kpc-scale binary AGNs of comparable luminosities, with a relative orbital velocity ≳ 150 km s⁻¹.

The stellar initial mass function (IMF), along with the star formation rate, is one of the fundamental properties that any theory of star formation must explain. An interesting feature of the IMF is that it appears to be remarkably universal across a wide range of environments. Particularly, there appears to be little variation in either the characteristic mass of the IMF or its high-mass tail between clusters with different metallicities. Previous attempts to understand this apparent independence of metallicity have not accounted for radiation feedback from high-mass protostars, which can dominate the energy balance of the gas in star-forming regions. We extend this work, showing that the fragmentation of molecular gas should depend only weakly on the amount of dust present, even when the primary heating source is radiation from massive protostars. First, we report a series of core collapse simulations using the ORION AMR code that systematically vary the dust opacity and show explicitly that this has little effect on the temperature or fragmentation of the gas. Then, we provide an analytic argument for why the IMF varies so little in observed star clusters, even as the metallicity varies by a factor of 100.

We investigate the correlation between 151 MHz radio luminosity, L_151 MHz, and jet power, P_jet, for a sample of low-power radio galaxies, of which the jet power is estimated from X-ray cavities. The jet power for a sample of Fanaroff–Riley I radio galaxies (FR Is) is estimated with the derived empirical correlation. We find that P_jet/L_Edd is positively correlated with $L_{\rm X}^{2\hbox{--}10\rm \ {keV}}/\it L_{\rm Edd}$ for FR Is, where L_Edd is the Eddington luminosity and L^2–10 keV_X is the 2–10 keV X-ray luminosity. We calculate the jet power of a hybrid model, as a variant of a Blandford–Znajek model proposed by Meier, based on the global solution of the advection-dominated accretion flow (ADAF) surrounding a Kerr black hole (BH). Our model calculations suggest that the maximal jet power is a function of the mass accretion rate and the BH spin parameter j. The hard X-ray emission is believed to be mainly from the ADAFs in FR Is, and the mass accretion rate is therefore constrained with the X-ray emission in our ADAF model calculations. We find that the dimensionless angular momentum of BH j ≳ 0.9 is required in order to reproduce the observed relation of $P_{\rm jet}/L_{\rm Edd}\hbox{--}L_{\rm X}^{2\hbox{--}10\rm \ {keV}}/\it L_{\rm Edd}$ for FR Is. Our conclusion will be strengthened if part of the X-ray emission is contributed by the jets. Our results suggest that BHs in FR Is are rapidly spinning, which are almost not affected by the uncertainty of the BH mass estimates.

We investigate the relation of the stellar initial mass function and the dense core mass function (CMF), using stellar masses and positions in 14 well-studied young groups. Initial column density maps are computed by replacing each star with a model initial core having the same star formation efficiency (SFE). For each group the SFE, core model, and observational resolution are varied to produce a realistic range of initial maps. A clump-finding algorithm parses each initial map into derived cores, derived core masses, and a derived CMF. The main result is that projected blending of initial cores causes derived cores to be too few and too massive. The number of derived cores is fewer than the number of initial cores by a mean factor of 1.4 in sparse groups and 5 in crowded groups. The mass at the peak of the derived CMF exceeds the mass at the peak of the initial CMF by a mean factor of 1.0 in sparse groups and 12.1 in crowded groups. These results imply that in crowded young groups and clusters, the mass distribution of observed cores may not reliably predict the mass distribution of protostars that will form in those cores.

For the first time we present a new data set of emission line widths for 118 star-forming regions in H ii galaxies (HiiGs). This homogeneous set is used to investigate the L–σ relation in conjunction with optical spectrophotometric observations. We were able to classify their nebular emission line profiles due to our high-resolution spectra. Peculiarities in the line profiles such as sharp lines, wings, asymmetries, and in some cases more than one component in emission were verified. From a new independent homogeneous set of spectrophotometric data, we derived physical condition parameters and performed statistical principal component analysis. We have investigated the potential role of metallicity (O/H), Hβ equivalent width (W_Hβ), and ionization ratio [O iii]/[O ii] to account for the observational scatter of the L–σ relation. Our results indicate that the L–σ relation for HiiGs is more sensitive to the evolution of the current starburst event (short-term evolution) and dated by W_Hβ or even the [O iii]/[O ii] ratio. The long-term evolution measured by O/H also plays a potential role in determining the luminosity of the current burst for a given velocity dispersion and age as previously suggested. Additionally, galaxies showing Gaussian line profiles present tighter correlations indicating that they are the best targets for the application of the parametric relations as an extragalactic cosmological distance indicator. Best fits for a restricted homogeneous sample of 45 HiiGs provide us with a set of new extragalactic distance indicators with an rms scatter compatible with observational errors of δlog L_Hα = 0.2 dex or 0.5 mag. Improvements may still come from future optimized observational programs to reduce the observational uncertainties on the predicted luminosities of HiiGs in order to achieve the precision required for the application of these relations as tests of cosmological models.

We study the star formation rates (SFRs) of galaxies as a function of local galaxy density at 0.6 < z < 0.9. We used a low-dispersion prism in IMACS on the 6.5 m Baade (Magellan I) telescope to obtain spectra and measured redshifts to a precision of σ_z/(1 + z) ∼ 1% for galaxies with z_AB < 23.3 mag. We utilized a stellar mass-limited sample of 977 galaxies above M > 1.8 × 10¹⁰ M_☉ (log M/M_☉ >10.25) to conduct our main analysis. With three different SFR indicators, (1) Spitzer MIPS 24 μm imaging, (2) spectral energy distribution (SED) fitting, and (3) [O ii]λ3727 emission, we find the median specific SFR (SSFR) and SFR to decline from the low-density field to the cores of groups and a rich cluster. For the SED- and [O ii]-based SFRs, the decline in SSFR is roughly an order of magnitude while for the MIPS-based SFRs, the decline is a factor of ∼4. We find approximately the same magnitude of decline in SSFR even after removing the sample of galaxies near the cluster. Galaxies in groups and a cluster at these redshifts therefore have lower star formation (SF) activity than galaxies in the field, as is the case at z ∼ 0. We investigated whether the decline in SFR with increasing density is caused by a change in the proportion of quiescent and star-forming galaxies (SFGs) or by a decline in the SFRs of SFGs. Using the rest-frame U − V and V − J colors to distinguish quiescent galaxies from SFGs (including both unattenuated blue galaxies and reddened ones), we find that the fraction of quiescent galaxies increases from ∼32% to 79% from low to high density. In addition, we find the SSFRs of SFGs, selected based on U − V and V − J colors, to decline with increasing density by factors of ∼5–6 for the SED- and [O ii]-based SFRs. The MIPS-based SSFRs for SFGs decline with a shallower slope. The declining SFRs of SFGs with density are paralleled by a decline in the median A_V, providing indirect evidence that the cold gas content that fuels future SF is diminished in higher density environments. The order of magnitude decline in the SSFR–density relation at 0.6 < z < 0.9 is therefore driven by both a combination of declining SFRs of SFGs as well as a changing mix of SFGs and quiescent galaxies.

Serendipitous observations of a pair z = 0.37 interacting galaxies (one hosting a quasar) show a massive gaseous bridge of material connecting the two objects. This bridge is photoionized by the quasar (QSO), revealing gas along the entire projected 38 h⁻¹₇₀ kpc sightline connecting the two galaxies. The emission lines that result give an unprecedented opportunity to study the merger process at this redshift. We determine the kinematics, ionization parameter (log U ≈ −2.5 ± 0.03), column density (N_{H, ⊥} ≈ 10²¹ cm⁻²), metallicity ([M/H] ≈ − 0.20 ± 0.15), and mass (≈10⁸ M_☉) of the gaseous bridge. We simultaneously constrain properties of the QSO host (M_DM > 8.8 × 10¹¹ M_☉) and its companion galaxy (M_DM > 2.1 × 10¹¹ M_☉; M_⋆ ∼ 2 × 10¹⁰ M_☉; stellar burst age = 300–800 Myr; SFR ∼6 M_☉ yr⁻¹; and metallicity 12 + log (O/H) = 8.64  ±  0.2). The general properties of this system match the standard paradigm of a galaxy–galaxy merger caught between first and second passages while one of the galaxies hosts an active quasar. The companion galaxy lies in the so-called green valley, with a stellar population consistent with a recent starburst triggered during the first passage of the merger and has no discernible active galactic nucleus activity. In addition to providing case studies of quasars associated with galaxy mergers, quasar/galaxy pairs with QSO-photoionized tidal bridges such as this one offer unique insights into the galaxy properties while also distinguishing an important and inadequately understood phase of galaxy evolution.

In this paper, we refine our method for the abundance analysis of high-resolution spectroscopy of the integrated light of unresolved globular clusters (GCs). This method was previously demonstrated for the analysis of old (>10 Gyr) Milky Way (MW) GCs. Here, we extend the technique to young clusters using a training set of nine GCs in the Large Magellanic Cloud. Depending on the signal-to-noise ratio of the data, we use 20–100 Fe lines per cluster to successfully constrain the ages of old clusters to within a ∼5 Gyr range, the ages of ∼2 Gyr clusters to a 1–2 Gyr range, and the ages of the youngest clusters (0.05–1 Gyr) to a ∼200 Myr range. We also demonstrate that we can measure [Fe/H] in clusters with any age less than 12 Gyr with similar or only slightly larger uncertainties (0.1–0.25 dex) than those obtained for old MW GCs (0.1 dex); the slightly larger uncertainties are due to the rapid evolution in stellar populations at these ages. In this paper, we present only Fe abundances and ages. In the next paper in this series, we present our complete analysis of ∼20 elements for which we are able to measure abundances. For several of the clusters in this sample, there are no high-resolution abundances in the literature from individual member stars; our results are the first detailed chemical abundances available. The spectra used in this paper were obtained at Las Campanas with the echelle on the du Pont Telescope and with the MIKE spectrograph on the Magellan Clay Telescope.

We study the galactic-scale triggering of star formation. We find that the largest mass scale not stabilized by rotation, a well-defined quantity in a rotating system and with clear dynamical meaning, strongly correlates with the star formation rate in a wide range of galaxies. We find that this relation can be understood in terms of self-regulation toward marginal Toomre stability and the amount of turbulence allowed to sustain the system in this self-regulated quasi-stationary state. We test such an interpretation by computing the predicted star formation rates for a galactic interstellar medium characterized by a lognormal probability distribution function and find good agreement with the observed relation.

With the advent of the Event Horizon Telescope (EHT), a millimeter/submillimeter very long baseline interferometer (VLBI), it has become possible to image a handful of black holes with sub-horizon resolutions. However, these images do not translate into microarcsecond absolute positions due to the lack of absolute phase information when an external phase reference is not used. Due to the short atmospheric coherence time at these wavelengths, nodding between the source and phase reference is impractical. However, here we suggest an alternative scheme which makes use of the fact that many of the VLBI stations within the EHT are arrays in their own right. With this we show that it should be possible to absolutely position the supermassive black holes at the centers of the Milky Way (Sgr A*) and M87 relative to nearby objects with precisions of roughly 1 μas. This is sufficient to detect the perturbations to Sgr A*'s position resulting from interactions with the stars and stellar-mass black holes in the Galactic cusp on year timescales, and severely constrain the astrophysically relevant parameter space for an orbiting intermediate-mass black hole, implicated in some mechanisms for producing the young massive stars in the Galactic center. For M87, it allows the registering of millimeter images, in which the black hole may be identified by its silhouette against nearby emission, and existing larger-scale radio images, eliminating present ambiguities in the nature of the radio core and inclination, opening angle, and source of the radio jet.

LS 5039 is a high-mass binary with a period of 4 days, containing a compact object and an O-star, one of the few high-mass binaries detected in γ-rays. Our Chandra Advanced CCD Imaging Spectrometer observation of LS 5039 provided a high-significance (≈10σ) detection of extended emission clearly visible for up to 1' from the point source. The spectrum of this emission can be described by an absorbed power-law model with photon index Γ = 1.9 ± 0.3, somewhat softer than the point-source spectrum Γ = 1.44 ± 0.07, with the same absorption, N_H = (6.4  ±  0.6) × 10²¹ cm⁻². The observed 0.5–8 keV flux of the extended emission is ≃ 8.8 × 10⁻¹⁴ erg s⁻¹cm⁻² or 5% of the point-source flux; the latter is a factor of ∼2 lower than the lowest flux detected so far. Fainter extended emission with comparable flux and a softer (Γ ≈ 3) spectrum is detected at even greater radii (up to 2'). Two possible interpretations of the extended emission are a dust scattering halo and a synchrotron nebula powered by energetic particles escaping the binary. We discuss both of these scenarios and favor the nebula interpretation, although some dust contribution is possible. We have also found transient sources located within a narrow stripe south of LS 5039. We discuss the likelihood of these sources to be related to LS 5039.

The activity levels of stars are influenced by several stellar properties, such as stellar rotation, spectral type, and the presence of stellar companions. Analogous to binaries, planetary companions are also thought to be able to cause higher activity levels in their host stars, although at lower levels. Especially in X-rays, such influences are hard to detect because coronae of cool stars exhibit a considerable amount of intrinsic variability. Recently, a correlation between the mass of close-in exoplanets and their host star's X-ray luminosity has been detected, based on archival X-ray data from the ROSAT All-Sky Survey. This finding has been interpreted as evidence for star–planet interactions. We show in our analysis that this correlation is caused by selection effects due to the flux limit of the X-ray data used and due to the intrinsic planet detectability of the radial velocity method, and thus does not trace possible planet-induced effects. We also show that the correlation is not present in a corresponding complete sample derived from combined XMM-Newton and ROSAT data.

We present multiwavelength observations of the ultraluminous blazar-type radio loud quasar PKS 0528+134 in quiescence during the period 2009 July–December. Four Target-of-Opportunity observations with the XMM-Newton satellite in the 0.2–10 keV range were supplemented with optical observations at the MDM Observatory, radio and optical data from the GLAST-AGILE Support Program of the Whole Earth Blazar Telescope and the Very Long Baseline Array, additional X-ray data from the Rossi X-ray Timing Explorer (2–10 keV) and from Suzaku (0.5–10 keV) as well as γ-ray data from the Fermi Large Area Telescope in the 100 MeV–200 GeV range. In addition, publicly available data from the SMARTS blazar monitoring program and the University of Arizona/Steward Observatory Fermi Support program were included in our analysis. We found no evidence of significant flux or spectral variability in γ-rays and most radio bands. However, significant flux variability on a timescale of several hours was found in the optical regime, accompanied by a weak trend of spectral softening with increasing flux. We suggest that this might be the signature of a contribution of unbeamed emission, possibly from the accretion disk, at the blue end of the optical spectrum. The optical flux is weakly polarized with rapid variations of the degree and direction of polarization, while the polarization of the 43 GHz radio core remains steady, perpendicular to the jet direction. Optical spectropolarimetry of the object in the quiescent state suggests a trend of increasing degree of polarization with increasing wavelength, providing additional evidence for an unpolarized emission component, possibly thermal emission from the accretion disk, contributing toward the blue end of the optical spectrum. Over an extended period of several months, PKS 0528+134 shows moderate (amplitude ≲ 50%) flux variability in the X-rays and most radio frequencies on ∼1–2 week timescales. We constructed four spectral energy distributions (SEDs) corresponding to the times of the XMM-Newton observations. We find that even in the quiescent state, the bolometric luminosity of PKS 0528+134 is dominated by its γ-ray emission. A leptonic single-zone jet model produced acceptable fits to the SEDs with contributions to the high-energy emission from both synchrotron self-Compton radiation and Comptonization of direct accretion disk emission. Fit parameters close to equipartition between the energy densities of the magnetic field and the relativistic electron population were obtained. The moderate variability on long timescales, compared to expected radiative cooling timescales, implies the existence of ongoing particle acceleration, while the observed optical polarization variability seems to point toward a turbulent acceleration process. Turbulent particle acceleration at stationary features along the jet therefore appears to be a viable possibility for the quiescent state of PKS 0528+134.

Radio and optical images of the M 87 jet show bright filaments, twisted into an apparent double helix, extending from HST-1 to knot A. Proper motions within the jet suggest a decelerating jet flow passing through a slower, accelerating wave pattern. We use these observations to develop a mass and energy flux-conserving model describing the jet flow and conditions along the jet. Our model requires the jet to be an internally hot, but subrelativistic plasma, from HST-1 to knot A. Subsequently, we assume that the jet is in pressure balance with an external cocoon and we determine the cocoon conditions required if the twisted filaments are the result of the Kelvin–Helmholtz (KH) unstable elliptical mode. We find that the cocoon must be cooler than the jet at HST-1 but must be about as hot as the jet at knot A. Under these conditions, we find that the observed filament wavelength is near the elliptical mode maximum growth rate and growth is rapid enough for the filaments to develop and saturate well before HST-1. We generate a pseudo-synchrotron image of a model jet carrying a combination of normal modes of the KH instability. The pseudo-synchrotron image of the jet reveals that (1) a slow decline in the model jet's surface brightness is still about five times faster than the real jet, (2) KH-produced dual helically twisted filaments can appear qualitatively similar to those on the real jet if any helical perturbation to the jet is very small or nonexistent inside knot A, and (3) the knots in the real jet cannot be associated with the twisted filamentary features and are unlikely to be the result of a KH instability. The existence of the knots in the real jet, the limb brightening of the real jet in the radio, and the slower decline of the surface brightness of the real jet indicate that additional processes—such as unsteady jet flow and internal particle acceleration—are occurring within the jet. Disruption of the real jet beyond knot A by KH instability is consistent with the jet and cocoon conditions we find at knot A.

We present the results of a search for cool white dwarfs in the United Kingdom InfraRed Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS). The UKIDSS LAS photometry was paired with the Sloan Digital Sky Survey to identify cool hydrogen-rich white dwarf candidates by their neutral optical colors and blue near-infrared colors, as well as faint reduced proper motion magnitudes. Optical spectroscopy was obtained at Gemini Observatory and showed the majority of the candidates to be newly identified cool degenerates, with a small number of G- to K-type (sub)dwarf contaminants. Our initial search of 280 deg² of sky resulted in seven new white dwarfs with effective temperature T_eff ≈ 6000 K. The current follow-up of 1400 deg² of sky has produced 13 new white dwarfs. Model fits to the photometry show that seven of the newly identified white dwarfs have 4120 K ⩽T_eff ⩽ 4480 K, and cooling ages between 7.3 Gyr and 8.7 Gyr; they have 40 km s⁻¹ ⩽ v_tan ⩽ 85 km s⁻¹ and are likely to be thick disk 10–11 Gyr-old objects. The other half of the sample has 4610 K ⩽T_eff ⩽ 5260 K, cooling ages between 4.3 Gyr and 6.9 Gyr, and 60 km s⁻¹ ⩽ v_tan ⩽ 100 km s⁻¹. These are either thin disk remnants with unusually high velocities, or lower-mass remnants of thick disk or halo late-F or G stars.

We have conducted interferometric observations with the Combined Array for Research in Millimeter Astronomy (CARMA) and an on-the-fly mapping with the 45 m telescope at Nobeyama Radio Observatory (NRO45) in the CO (J = 1–0) emission line of the nearby spiral galaxy NGC 3521. Using the new combined CARMA + NRO45 data of NGC 3521, together with similar data for NGC 5194 (M51a) and archival SINGS Hα, 24 μm THINGS H i, and Galaxy Evolution Explorer/Far-UV (FUV) data for these two galaxies, we investigate the empirical scaling law that connects the surface density of star formation rate (SFR) and cold gas (known as the Schmidt–Kennicutt law or S-K law) on a spatially resolved basis and find a super-linear slope for the S-K law when carefully subtracting the background emissions in the SFR image. We argue that plausibly deriving SFR maps of nearby galaxies requires the diffuse stellar and dust background emission to be subtracted carefully (especially in the mid-infrared and to a lesser extent in the FUV). Applying this approach, we perform a pixel-by-pixel analysis on both galaxies and quantitatively show that the controversial result whether the molecular S-K law (expressed as $\Sigma _{\rm SFR}\propto \Sigma _{\rm H_2}^{\gamma _{\rm H_2}}$) is super-linear or basically linear is a result of removing or preserving the local background. In both galaxies, the power index of the molecular S-K law is super-linear ($\gamma _{\rm H_2}\gtrsim 1.5$) at the highest available resolution (∼230 pc) and decreases monotonically for decreasing resolution. We also find in both galaxies that the scatter of the molecular S-K law ($\sigma _{\rm H_2}$) monotonically increases as the resolution becomes higher, indicating a trend for which the S-K law breaks down below some scale. Both $\gamma _{\rm H_2}$ and $\sigma _{\rm H_2}$ are systematically larger in M51a than in NGC 3521, but when plotted against the de-projected scale (δ_dp), both quantities become highly consistent for the two galaxies, tentatively suggesting that the sub-kpc molecular S-K law in spiral galaxies depends only on the scale being considered, without varying among spiral galaxies. A logarithmic function $\gamma _{\rm H_2}=-1.1 \log [\delta _{\rm dp}/{\rm kpc}]+1.4$ and a linear relation $\sigma _{\rm H_2}=-0.2 [\delta _{\rm dp}/{\rm kpc}]+0.7$ are obtained through fitting to the M51a data, which describes both galaxies impressively well on sub-kpc scales. A larger sample of galaxies with better sensitivity, resolution, and broader field of view are required to test the general applicability of these relations.

We present Submillimeter Array (SMA) λ = 0.88 mm observations of an infrared dark cloud G28.34+0.06. Located in the quiescent southern part of the G28.34 cloud, the region of interest is a massive (>10³ M_☉) molecular clump P1 with a luminosity of ∼10³L_☉, where our previous SMA observations at 1.3 mm have revealed a string of five dust cores of 22–64 M_☉ along the 1 pc IR-dark filament. The cores are well aligned at a position angle (P.A.) of 48° and regularly spaced at an average projected separation of 0.16 pc. The new high-resolution, high-sensitivity 0.88 mm image further resolves the five cores into 10 compact condensations of 1.4–10.6 M_☉, with sizes of a few thousand AU. The spatial structure at clump (∼1 pc) and core (∼0.1 pc) scales indicates a hierarchical fragmentation. While the clump fragmentation is consistent with a cylindrical collapse, the observed fragment masses are much larger than the expected thermal Jeans masses. All the cores are driving CO (3–2) outflows up to 38 km s⁻¹, the majority of which are bipolar, jet-like outflows. The moderate luminosity of the P1 clump sets a limit on the mass of protostars of 3–7 M_☉. Because of the large reservoir of dense molecular gas in the immediate medium and ongoing accretion as evident by the jet-like outflows, we speculate that P1 will grow and eventually form a massive star cluster. This study provides a first glimpse of massive, clustered star formation that currently undergoes through an intermediate-mass stage.

We aim at reproducing the height dependence of sunspot wave signatures obtained from spectropolarimetric observations through three-dimensional MHD numerical simulations. A magnetostatic sunspot model based on the properties of the observed sunspot is constructed and perturbed at the photosphere, introducing the fluctuations measured with the Si i λ10827 line. The results of the simulations are compared with the oscillations observed simultaneously at different heights from the He i λ10830 line, the Ca ii H core, and the Fe i blends in the wings of the Ca ii H line. The simulations show a remarkable agreement with the observations. They reproduce the velocity maps and power spectra at the formation heights of the observed lines, as well as the phase and amplification spectra between several pairs of lines. We find that the stronger shocks at the chromosphere are accompanied with a delay between the observed signal and the simulated one at the corresponding height, indicating that shocks shift the formation height of the chromospheric lines to higher layers. Since the simulated wave propagation matches very well the properties of the observed one, we are able to use the numerical calculations to quantify the energy contribution of the magnetoacoustic waves to the chromospheric heating in sunspots. Our findings indicate that the energy supplied by these waves is too low to balance the chromospheric radiative losses. The energy contained at the formation height of the lowermost Si i λ10827 line in the form of slow magnetoacoustic waves is already insufficient to heat the higher layers, and the acoustic energy which reaches the chromosphere is around 3–9 times lower than the required amount of energy. The contribution of the magnetic energy is even lower.

Galactic outflows of cool (∼10⁴ K) gas are ubiquitous in local starburst galaxies and in most high-redshift galaxies. Hot gas from supernovae has long been suspected as the primary driver, but this mechanism suffers from its tendency to destroy the cool gas. We propose a modification of the supernova scenario that overcomes this difficulty. Star formation is observed to take place in clusters. We show that, for L^⋆ galaxies, the radiation pressure from clusters with M_cl ≳ 10⁶ M_☉ is able to expel the surrounding gas at velocities in excess of the circular velocity v_c of the disk galaxy. This cool gas travels above the galactic disk before supernovae erupt in the driving cluster. Once above the disk, the cool outflowing gas is exposed to radiation and hot gas outflows from the galactic disk, which in combination drive it to distances of ∼50 kpc. Because the radiatively driven clouds grow in size as they travel, and because the hot gas is more dilute at large distance, the clouds are less subject to destruction. Therefore, unlike wind-driven clouds, radiatively driven clouds can give rise to the metal absorbers seen in quasar spectra. We identify these cluster-driven winds with large-scale galactic outflows. The maximum cluster mass in a galaxy is an increasing function of the galaxy's gas surface density, so only starburst galaxies are able to drive cold outflows. We find the critical star formation rate for launching large-scale cool outflows to be $\dot{\Sigma }^{{\rm crit}}_*\approx 0.05\, M_\odot \;{\rm yr }^{-1}\;{\rm kpc }^{-2}$, in good agreement with observations.