Table of contents

We study the evolution of the dark energy parameter within the scope of a spatially non-flat and isotropic Friedmann-Robertson-Walker model filled with barotropic fluid and bulk viscous stresses. We have obtained cosmological solutions that do not have a Big Rip singularity, and concluded that in both non-interacting and interacting cases the non-flat open Universe crosses the phantom region. We find that during the evolution of the Universe, the equation of state for dark energy ω_D changes from ω^eff_D > −1 to ω^eff_D < −1, which is consistent with recent observations.

Motivated by the increasing evidence for the need of a geometry that resembles Bianchi morphology to explain the observed anisotropy in the WMAP data, we have discussed some features of Bianchi type VIo universes in the presence of a fluid that has an anisotropic equation of state (EoS) parameter in general relativity. We present two accelerating dark energy (DE) models with an anisotropic fluid in Bianchi type VI₀ space-time. To ensure a deterministic solution, we choose the scale factor , which yields a time-dependent deceleration parameter, representing a class of models which generate a transition of the universe from the early decelerating phase to the recent accelerating phase. Under suitable conditions, the anisotropic models approach an isotropic scenario. The EoS for DE ω is found to be time-dependent and its existing range for derived models is in good agreement with data from recent observations of type Ia supernovae (SNe Ia) (Knop et al. 2003), SNe Ia data combined with cosmic microwave background (CMB) anisotropy and galaxy clustering statistics (Tegmark et al. 2004a), as well as the latest combination of cosmological datasets coming from CMB anisotropies, luminosity distances of high redshift SNe Ia and galaxy clustering. For different values of n, we can generate a class of physically viable DE models. The cosmological constant Λ is found to be a positive decreasing function of time and it approaches a small positive value at late time (i.e. the present epoch), which is corroborated by results from recent SN Ia observations. We also observe that our solutions are stable. The physical and geometric aspects of both models are also discussed in detail.

We use the latest data to investigate observational constraints on the new generalized Chaplygin gas (NGCG) model. Using the Markov Chain Monte Carlo method, we constrain the NGCG model with type Ia supernovae from the Union2 set (557 data), the usual baryonic acoustic oscillation (BAO) observation from the spectroscopic Sloan Digital Sky Survey data release 7 galaxy sample, the cosmic microwave background observation from the 7-year Wilkinson Microwave Anisotropy Probe results, newly revised data on H(z), as well as a value of θ_BAO (z = 0.55) = (3.90° ± 0.38°) for the angular BAO scale. The constraint results for the NGCG model are ω_X = −1.0510^+0.1563_−0.1685(1σ)^+0.2226_−0.2398(2σ), η=1.0117^+0.0469_−0.0502(1σ)^+0.0693_−0.0716(2σ) and Ω_X=0.7297^+0.0229_−0.0276(1σ)^+0.0329_−0.0402(2σ), which give a rather stringent constraint. From the results, we can see that a phantom model is slightly favored and the probability that energy transfers from dark matter to dark energy is a little larger than the inverse.

We investigate the effects of the cooling function in the formation of clumps of protoplanetary disks using two-dimensional smoothed particle hydrodynamic simulations. We use a simple prescription for the cooling rate of the flow, du/dt = −u/τ_cool, where u and τ_cool are the internal energy and cooling timescale, respectively. We assume the ratio of local cooling to dynamical timescale, Ωτ_cool = β, to be a constant and also a function of the local temperature. We found that for the constant β and γ = 5/3, fragmentation occurs only for β ≲ 7. However, in the case of β having temperature dependence and γ = 5/3, fragmentation can also occur for largervalues of β. By increasing the temperature dependence of the cooling timescale, the mass accretion rate decreases, the population of clumps/fragments increases, and the clumps/fragments can also form in the smaller radii. Moreover, we found that the clumps can form even in a low mass accretion rate, ≲ 10⁻⁷M_⊙ yr⁻¹, in the case of temperature-dependent β. However, clumps form with a larger mass accretion rate, > 10⁻⁷M_⊙ yr⁻¹, in the case of constant β.

Using the Herschel ATLAS science demonstration phase data cross-identified with SDSS DR7 spectra, we select 297 galaxies with F_250μm > 5σ. The sample galaxies are classified into five morphological types, and more than 40% of the galaxies are peculiar/compact galaxies. The peculiar galaxies show higher far-infrared/submillimeter luminosity-to-mass ratios than the other types. We perform and analyze the correlations of far-infrared/submillimeter and Hα luminosities for different morphological types and different spectral types. The Spearman rank coefficient decreases and the scatter increases with the wavelength increasing from 100 μm to 500 μm. We conclude that a single Herschel SPIRE band is not good for tracing star formation activities in galaxies. AGNs contribute less to the far-infrared/submillimeter luminosities and do not show a difference from star-forming galaxies. However, the earlier type galaxies present significant deviations from the best fit of star-forming galaxies.

We investigate the effects of strong magnetic fields upon the large-scale properties of neutron and protoneutron stars. In our calculations, the neutron star matter was approximated by pure neutron matter. Using the lowest order constrained variational approach at zero and finite temperatures, and employing AV₁₈ potential, we present the effects of strong magnetic fields on the gravitational mass, radius, and gravitational redshift of neutron and protoneutron stars. It is found that the equation of state for a neutron star becomes stiffer with an increase of magnetic field and temperature. This leads to larger values of the maximum mass and radius for the neutron stars.

Solving Newtonian steady-state wind equations while considering the accurate weak interaction rates and magnetic fields (MFs) of young neutron stars, we study the dynamics and nucleosynthesis of neutrino-driven winds (NDWs) from proto neutron stars (PNSs). For a typical 1.4 M_⊙ PNS model, we find that the nucleosynthesis products are closely related to the luminosity of neutrinos and anti-neutrinos. The lower the luminosity is, the larger is the effect on the NDWs caused by weak interactions and MFs. At a high anti-neutrino luminosity of typically 8 × 10⁵¹ erg s⁻¹, neutrinos and anti-neutrinos dominate the processes in an NDW and the MFs hardly change the wind's properties. But at a low anti-neutrino luminosity of 10⁵¹ erg s⁻¹ at the late stage of an NDW the mass of the product and process of nucleosynthesis are changed significantly in strong MFs. Therefore, in most of the models considered for the NDWs from PNSs, based on our calculations, the influences of MFs and the net weak interactions on the nucleosynthesis are not significant.

Based on Dulk and Marsh's approximate theory about nonthermal gyrosynchrotron radiation, one simple impulsive microwave burst with a loop-like structure is selected for radio diagnostics of the coronal magnetic field and column density of nonthermal electrons, which are calculated from the brightness temperature, polarization degree, and spectral index, as well as the turnover frequency, observed by using the Nobeyama Radioheliograph and the Nobeyama Radio Polarimeters, respectively. Very strong variations (up to one or two orders of magnitude) of the calculated transverse and longitudinal magnetic fields with respect to the line-of-sight, as well as the calculated electron column density, appear in the looptop and footpoint sources during the burst. The absolute magnitude and varied range of the transverse magnetic field are evidently larger than those of the longitudinal magnetic field. The time evolution of the transverse magnetic field is always anti-correlated with that of the longitudinal magnetic field, but positively correlated with that of the electron column density. These results strongly support the idea that quantifying the energy released in a flare depends on a reconstruction of the coronal magnetic field, especially for the transverse magnetic field, and they are basically consistent with the recent theoretical and observational studies on the photospheric magnetic field in solar flares.

Magnetic non-potentiality is important for understanding flares and other solar activities in active regions (ARs). Five non-potential parameters, i.e. electric current, current helicity, source field, photospheric free energy, and angular shear, are calculated to quantify the non-potentiality of NOAA AR 11158. Benefitting from the high spatial resolution, high cadence and continuous temporal coverage of vector magnetograms from the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory, both the long-term evolution of the AR and the rapid change during flares are studied. We confirm that, compared with the magnetic flux, the magnetic non-potentiality has a closer connection with the flare, and the emerging flux regions are important for understanding the magnetic non-potentiality and flares. The main results are as follows. (1) The vortex in the source field directly displays the deflection of the horizontal magnetic field. The deflection corresponds to the fast rotating sunspot with a time delay, which suggests that the sunspot rotation leads to an increase in the non-potentiality. (2) Two areas that have evident changes in the azimuth of the vector magnetic field are found near the magnetic polarity inversion line. The change rates of the azimuth are about 1.3° h⁻¹ and 3.6° h⁻¹, respectively. (3) Rapid and prominent increases are found in the variation of helicity during four flares in the regions where their initial brightening occurs. The recovery of the increases takes 3–4 h for the two biggest flares (X2.2 and M6.6), but only takes about 2 h for the two other smaller flares (M2.2 and M1.6).

We describe the design and construction of a new rapid 3-channel CCD photometer, dedicated to simultaneous multicolor photometric observations of rapidly variable objects. This photometer is equipped on the 1-meter telescope at the Xinglong Observatory. It allows simultaneous imaging within fields of view of 18.8' × 18.8', 18.2' × 17.6' and 9.2' × 9.2' in the Sloan Digital Sky Survey's g', r' and i' bands, respectively. The results of its calibration and performance are reported.