Some Bianchi type-II cosmological models in Brans–Dicke theory

In this work, the Bianchi type-II anisotropic cosmological models have been investigated in the context of Brans–Dicke (BD) theory in the presence as well as absence of a magnetic field. The energy conditions of the models have been examined. The physical and kinematical behaviors of the models have also been discussed.     

Precession instability models of SS433

Several models are available to describe precession instability of SS433: 6- and 7-parameter models, sinusoidal and damped sinusoidal models etc. From the available observation data, it appears that 6- and 7-parameter models are most likely to be incorrect. We have analyzed and compared the sinusoidal and damped sinusoidal models in terms of figures of phase residual, and then proposed and improved a damped sinusoidal model. The data from the three models (sinusoidal, damped sinusoidal, and improved damped sinusoidal model) have been subjected to regression analysis for the purpose of determining the differences among the models.     

The rosseland mean opacity of interstellar grains

We have calculated the opacity of interstellar grains in the temperature range 10–1500 K. Two composite grain models have been considered. One of them consists of silicate coated with an ice mantle and the second has a graphite core coated also with an ice mantle. These models are compared with isolated grain models. An exact analytical and computational development of Güttler's formulae for composite grain models has been used to calculate the extinction coefficient.It has been found that the thickness of the mantle affects the opacity of the interstellar grains. The opacity of composite models differs from that of the isolated models. The effect of the different species (ice, silicate, and graphite) is also clear.     

Prolate Jaffe models for galaxies

We introduce a class of prolate Jaffe models for elliptical galaxies, which are a further extension of Jaffe's spherical models of axisymmetric elliptical systems, and study the properties of their densities, circular velocities, velocity dispersions and two-integral even distribution functions. The form of the potential allows the density to be expressed simply as a function of the potential and radial coordinate R . The models have finite total mass and their densities at large distances decay radially as r ⁻⁴, except on the major axis, where the densities decay as r ⁻³. It is known from Hunter's formulae that the velocity dispersions for prolate models can be expressed in terms of elementary functions of R and z , unlike those for the oblate Jaffe models recently given by Jiang, and that the prolate models have anisotropic velocity distributions. Thus the prolate models are easier to study than the oblate models. It is also found that the two-integral even distribution functions on the physical boundary of the galaxies increase monotonically with the relative energy, for the prolate models. Furthermore, numerical calculation shows that the two-integral even distribution functions generated from their densities are non-negative, even for very 'squeezed' prolate Jaffe models. However, the edge-on projected surface densities for these prolate models cannot be expressed as simply as for the oblate models.     

New calculations of interstellar dust (ID) contraction in a non-isothermal regime

We have calculated the opacity as resulting from different interstellar grain models, molecules, atoms, and ions. The resulting opacities have been applied to a numerical code used to follow the thermal evolution of a contracting cloud in one dimension. An exact analytical and computational developments of both Mie theory for isolated grains and Güttler's formulae for composite grain models have been used to calculate the extinction coefficients. We have studied two models of composite grain and three models of isolated grain. The opacity of interstellar grains has been calculated in the temperature range 10–1500 K. The molecular opacity is splitted into continuous and line opacities. The different sources of continuous opacity have been studied. The line opacity has also been included. The atomic opacities are also considered. The hydrodynamical equations are solved explicitly but the energy and Poisson equations are solved implicitly.It has been found that the thermal evolution during contraction of protostellar clouds is sensitive to both: the assumed grain models and the considered chemical composition. A cloud of an initial temperature of 10 K collapsed to a stage in which the temperature increases to 91 000 K and the density reached to 0.16 g cm^–3.     

Chemo-dynamical Evolution of the ISM in Galaxies

Chemo-dynamical models have been introduced in the late eighties andare a generally accepted tool for understanding galaxy evolution. Theyhave been successfully applied to one-dimensional problems, e.g. theevolution of non-rotating galaxies, and two-dimensional problems,e.g. the evolution of disk galaxies. Recently, also three-dimensionalchemo-dynamical models have become available. In these models thedynamics of different components, i.e. dark matter, stars and amulti-phase interstellar medium, are treated in a self-consistent wayand several processes allow for an exchange of matter, energy andmomentum between the components or different gas phases. Some resultsof chemo-dynamical models and their comparison with observations ofchemical abundances or star formation histories will be reviewed.     

Generalized polytropic models in Finch-Skea spacetime

In present article, we have formulated the new physical generalized polytropic models with anisotropic matter distribution in the absence of electromagnetic field. We have assumed the Finch-Skea spacetime to develop the models, which fulfill all the criteria for the physical adequacy. Distinct physical features of the obtained models have been examined and evaluated. Furthermore, the physical quantities are presented graphically to meet the physical conditions of viable astronomical models. These results can be reduced to the different cases of uncharged anisotropic fluid such as linear, quadratic and polytropic equations of state.     

Bulk viscous fluid hypersurface–homogeneous cosmological models with time varying G and Λ

Hypersurface–homogeneous cosmological models containing a bulk viscous fluid with time varying G and Λ have been presented. We have shown that the field equations are solvable for any arbitrary cosmic scale function. The viscosity coefficient of bulk viscous fluid is assumed to be a power function of the energy density. Exact solutions of Einstein’s field equations are obtained which represent an expanding, shearing and accelerating/decelerating models of the universe. The physical and kinematical behaviours of the models are also discussed.     

Some magnetofluid cosmological models of plane symmetry

Considering the cylindrically-symmetric metric of Marder, some plane-symmetric magnetofluid cosmological models have been derived. Various physical and geometrical properties of the models have been discussed.     

我们可以否定非标准太阳模型了吗?

虽然标准太阳模型取得了比较引人注目的成功,但是无论是只由太阳中微子流量的观测资料进行直接分析,还是从太阳振荡的角度讨论问题,目前都还不能完全否定非标准太阳模型。由近期的Ｓｕｐｅｒ Ｋａｎｉｏｋａｎｄｅ实验结果还无法解释太阳中微子问题也说明由天体物理研究对太阳中微子问题解释的可能性依然存在,而满足观测结果的非标准太阳模型则可以对太阳中微子问题的解决提供很大帮助。     

Bianchi type-I models in self-creation cosmology with constant deceleration parameter

We have studied the evolution of homogeneous and anisotropic Bianchi type-I cosmological models filled with perfect fluid in Barber second self-creation theory by assuming a special law of variation for Hubble’s parameter that yield a constant value of deceleration parameter. Some physical consequences of the models have been discussed in case of Zel’dovich fluid and radiation dominated fluid.     

A complete reference of the analytical synchrotron external shock models of gamma-ray bursts

Gamma-ray bursts are most luminous explosions in the universe. Their ejecta are believed to move towards Earth with a relativistic speed. The interaction between this “relativistic jet” and a circumburst medium drives a pair of (forward and reverse) shocks. The electrons accelerated in these shocks radiate synchrotron emission to power the broad-band afterglow of GRBs. The external shock theory is an elegant theory, since it invokes a limit number of model parameters, and has well predicted spectral and temporal properties. On the other hand, depending on many factors (e.g. the energy content, ambient density profile, collimation of the ejecta, forward vs. reverse shock dynamics, and synchrotron spectral regimes), there is a wide variety of the models. These models have distinct predictions on the afterglow decaying indices, the spectral indices, and the relations between them (the so-called “closure relations”), which have been widely used to interpret the rich multi-wavelength afterglow observations. This review article provides a complete reference of all the analytical synchrotron external shock afterglow models by deriving the temporal and spectral indices of all the models in all spectral regimes, including some regimes that have not been published before. The review article is designated to serve as a useful tool for afterglow observers to quickly identify relevant models to interpret their data. The limitations of the analytical models are reviewed, with a list of situations summarized when numerical treatments are needed.     

Helical fields and filamentary molecular clouds – I

We study the equilibrium of pressure truncated, filamentary molecular clouds that are threaded by rather general helical magnetic fields. We first apply the virial theorem to filamentary molecular clouds, including the effects of non-thermal motions and the turbulent pressure of the surrounding ISM. When compared with the data, we find that many filamentary clouds have a mass per unit length that is significantly reduced by the effects of external pressure, and that toroidal fields play a significant role in squeezing such clouds.
We also develop exact numerical MHD models of filamentary molecular clouds with more general helical field configurations than have previously been considered. We examine the effects of the equation of state by comparing 'isothermal' filaments, with constant total (thermal plus turbulent) velocity dispersion, with equilibria constructed using a logatropic equation of state.
Our theoretical models involve three parameters: two to describe the mass loading of the toroidal and poloidal fields, and a third that describes the radial concentration of the filament. We thoroughly explore our parameter space to determine which choices of parameters result in models that agree with the available observational constraints. We find that both equations of state result in equilibria that agree with the observational results. Moreover, we find that models with helical fields have more realistic density profiles than either unmagnetized models or those with purely poloidal fields; we find that most isothermal models have density distributions that fall off as r ^−1.8 to r ⁻², while logatropes have density profiles that range from r ⁻¹ to r ^−1.8. We find that purely poloidal fields produce filaments with steep radial density gradients that are not allowed by the observations.     

Ellipsoids, material points and material segments

Simple models of potential based on material points and material segments are confronted with a homogeneous ellipsoid potential. A spheroid is approximated with a pair of material points or with one material segment. The segment model proves to be more accurate. For a triaxial ellipsoid, two models are considered: one with five material points and one with two material segments and a point. When their parameters are determined with the fourth sectorial harmonic ignored, both simple models have a similar approximation error. Numerical tests indicate that the approximate models of a triaxial ellipsoid are 5 to 10 times faster than exact formulation, whereas for a spheroid the approximate models are at most twice as fast as the exact formulae.     

Some cosmological models with heat and null radiation flow

Some New Bianchi type-V cosmological models, incorporating heat flow and null radiation flow, along with perfect fluid distribution have been discussed. The physical and kinematical behaviors of the models have been analyzed.     

Anisotropic Bianchi type-I models in string cosmology

The present study deals with a spatially homogeneous and anisotropic Bianchi-I cosmological models representing massive strings. The energy-momentum tensor, as formulated by Letelier (1983), has been used to construct massive string cosmological models for which we assume the expansion scalar in the models is proportional to one of the components of shear tensor. The Einstein’s field equations have been solved by applying a variation law for generalized Hubble’s parameter in Bianchi-I space-time. We have analysed a comparative study of accelerating and decelerating models in the presence of string scenario. The study reveals that massive strings dominate in the decelerating universe whereas strings dominate in the accelerating universe. The strings eventually disappear from the universe for sufficiently large times, which is in agreement with current astronomical observations.     

Two-fluid models of superfluid neutron star cores

Both relativistic and non-relativistic two-fluid models of neutron star cores are constructed, using the constrained variational formalism developed by Brandon Carter and co-workers. We consider a mixture of superfluid neutrons and superconducting protons at zero temperature, taking into account mutual entrainment effects. Leptons, which affect the interior composition of the neutron star and contribute to the pressure, are also included. We provide the analytic expression of the Lagrangian density of the system, the so-called master function, from which the dynamical equations can be obtained. All the microscopic parameters of the models are calculated consistently using the non-relativistic nuclear energy density functional theory. For comparison, we have also considered relativistic mean field models. The correspondence between relativistic and non-relativistic hydrodynamical models is discussed in the framework of the recently developed 4D covariant formalism of Newtonian multifluid hydrodynamics. We have shown that entrainment effects can be interpreted in terms of dynamical effective masses that are larger in the relativistic case than in the Newtonian case. With the nuclear models considered in this work, we have found that the neutron relativistic effective mass is even greater than the bare neutron mass in the liquid core of neutron stars.     

尘埃辐射转移模型

尘埃辐射转移模型对解释和探索宇宙中众多的多尘埃天体的观测现象可发挥重要的作用。目前所见的四种球对称系统中的尘埃辐射转移方法可被总结为：二流Ｅｄｄｉｎｇｔｏｎ近似模型方法,Ｅｄｄｉｎｇｔｏｎ因子迭代模型方法、射线跟踪法和Ｍｏｎｔｅ－Ｃａｒｌｏ模型方法,除了第一种方法外,其余方法在原理上都没有近似性。现在使用最多的是后两种方法。Ｍｏｎｔｅ－Ｃａｒｌｏ方法是其中最灵活的一种,它可被用于对非球对称系统的辐     

Radially anisotropic models of collisionless spherical stellar systems without central singularity

A new class of the simplest equilibrium two-parameter distribution functions for spherical stellar systems with a radially anisotropic stellar velocity distribution is investigated. The models under consideration are a less singular counterpart of the so-called generalized polytropes, which in the past were among the most popular models in works on the equilibrium and stability of gravitating systems. In contrast to the well-known generalized polytropes, the proposed models have finite density and potential at the center. The absence of a singularity is necessary for a proper consideration of the radial orbit instability, which is the most important instability of spherical stellar systems. The main observed parameters of the proposed models (potential, density, anisotropy) are compared with those in well-known equilibrium models.     

Exact Kantowski–Sachs cosmological models in general theory of relativity

We have studied cosmological model generated by perfect fluid coupled with mass less scalar field for Kantowski–Sachs space–time in general theory of relativity. Two different physically viable models of the universe are obtained by using a special law of variation for Hubble’s parameter that yields a constant value of deceleration parameter. Some physical consequences of the models have been discussed in case of Zel’dovich fluid.