磁中子星的一维吸积柱模型

本给出了改进的一维吸积柱模型,探讨了在大吸积率下吸积柱的结构及物理性质,并理论推导吸积柱内下落物质的温度、密度及速度的变化。计算结果表明,它自上而下可以分为几个部分：冲击区（辐射压减速压）、理想气体压减速区、简并气体压减速区、外流区。作为例证,详细计算了吸积率Ｍ≈１０＾１７ｇ／ｓ,极区磁场强度Ｂｍ≈１０＾８Ｔ的中子星吸积柱。探讨了吸积柱内的热核反应,认为它可能与低质量Ｘ射线双星的低频ＱＰＯ（准周     

黑洞吸积盘的演化与黑洞自转：Ⅰ.薄盘的演化

在同时考虑薄盘吸积与ＢｌａｎｄｆｏｒｄＺｎａｊｅｋ过程的条件下讨论了黑洞吸积盘的的演化,并指出黑洞的无量纲角动量ａ是标志黑洞吸积盘演化特征的重要参量．得到描述黑洞自转的３个特征值：（１）ａＩ≈０．９８３标志在薄盘吸积与ＢｌａｎｄｆｏｒｄＺｎａｊｅｋ能量提取机制的作用下,中心黑洞的无量纲角动量ａ演化的上限;（２）ａＩＩ≈０．９４０对应于黑洞薄盘内边缘半径ｒｍｓ演化的极小值;（３）ａＩＩＩ≈０．８２４对应于活动星系核射电噪度（ｒａｄｉｏｌｏｕｄｎｅｓ）的极大值     

钡星重元素丰度的再研究

在包括双星及逃逸物质的系统总角动量守恒模型的基础上,采用星风质量吸积机制,由伴星通过逐次脉冲从主星吸积物质并与其外包层进行混合的模型出发,自洽地计算了钡星的重元素超丰,并给出理论计算结果与观测值的比较．计算结果表明,当取Ｂｏｎｄｉ－Ｈｏｙｌｅ质量吸积率的五分之一作为实际吸积率时,对于轨道周期较长（Ｐ＞１６００天）、相距较远的钡星系统,在误差范围内,理论计算曲线与大多数样品星的重元素丰度观测值相符合;而对于ＨＤ２０４０７５和ＨＤ１６４５８两颗钡星,将质量吸积率增大为Ｂｏｎｄｉ－Ｈｏｙｌｅ质量吸积率的二分之一时,计算结果与观测值符合较好,这表明质量吸积率在Ｂｏｎｄｉ－Ｈｏｙｌｅ质量吸积率的十分之一至二分之一之间．对于具有较短轨道周期（Ｐ＜６００天）的钡星系统,计算结果与丰度观测值偏差较大,表明钡星系统中还有其它的形成机制．     

星风吸积引起的钡星重元素超丰

首先指出目前星风吸积模型中理论上的不自洽，考虑到δｒ≠０，重新推导轨道参量变化方程，消去了理论上的不自洽．提出一个新模型：首次将星风吸积同内禀ＡＧＢ星核合成模型结合起来计算钡（Ｂａ）星的重元素超丰，并将计算结果与观测值进行了比较．各参量按标准情况取值时，计算结果不太理想．取Ｂｏｎｄｉｈｏｙｌｅ吸积率的０．５倍或取较大的星风速率时，对于较长轨道周期（Ｐ＞１０００天）的Ｂａ星，计算结果与观测值基本符合；而较短轨道周期（Ｐ＜６００天）的Ｂａ星，其重元素超丰机制可能是盘吸积     

吸积对超新星SN 1987A的热光度贡献的再探讨

超新星ＳＮ１９８７Ａ中央可能存在的致密天体（如中子星）,通过吸积那些超新星爆发时抛射速度小于逃逸速度的物质而产生的吸积光度,对ＳＮ１９８７Ａ晚期的热光度演化有重要的贡献．本文改进了以前的理论模型,给出了更为合理的定量计算,对所提出的特殊的吸积机制作出了仔细分析,特别强调,这种吸积是膨胀包层中的物质由于引力作用引起的长时期内的连续回落,它有别于其它的吸积模型,并给出直接的观测证据．假设致密天体是１４Ｍ⊙,半径为１０６ｃｍ的中子星,我们的计算表明,它的吸积能提供足够的能量来形成ＳＮ１９８７Ａ热光度曲线在１０５０天左右时的鼓包,并且使热光度曲线从８００天以后下降变缓     

黑洞吸积盘内区温度的分布特征和演化特征

对黑洞吸积盘内区温度的径向分布特征和演化特征作了详细的讨论．结果表明：（１）盘内区的温度并非随径向坐标ｒ单调减少,在接近盘的内边缘处有一个盘温的峰值环．在盘温的峰值环和盘的内边缘之间形成一个温度梯度很大的冷却区;（２）在盘吸积的过程中,盘内区温度的峰值和冷却区的平均温度梯度均随中心黑洞的无量纲角动量ａ的增加而单调增加,而金温峰值环半径和冷却区的径向宽度均随ａ的增加而单调减小;（３）盘的热辐射光度随ａ的增加而单调增加．     

外赋AGB星星风吸积的角动量守恒模型

用整个系统的角动量守恒条件代替切向动量守恒条件,推导出星风质量吸积及轨道参量变化方程．在新的轨道参量变化方程的基础上,计算了外赋AGB星系统的星风质量吸积及轨道参量的变化．将星风吸积模型同内禀AGB星核合成模型结合起来,通过逐次脉冲吸积质量和混合,自洽地计算外赋AGB星的重元素超丰,并给出计算结果与观测值的比较．对初始质量较大的Ba星（M2．0＝2．5M⊙）,当系统轨道周期大于1300天时,属于星风吸积,小于600天成为共同包层双星或灾变双星．对初始质量较小的Ba星（M2．0＝13M⊙）,当系统轨道周期大于1600天时,属于星风吸积,小于600天时成为灾变双星,由此可以解释Ba星的重元素超丰和轨道参量的观测事实,并有利于解释观测到的外赋S星轨道周期的600天下限．随着星风吸积过程的进行,轨道偏心率逐渐增大,这对解释Ba星轨道偏心率平均值大于外赋S星和CH星平均值的观测事实有利．     

吸积盘和冕的研究与应用

本文主要研究有热冕存在的吸积盘的特征。首先，我们采用最新的不透明度表和态方程表对经典薄盘的结构进行了探讨。然后考虑吸积盘上下方存在的热冕，分析了冕中的物理过程并研究了冕的结构。在此基础上，我们计算了有冕盘的结构并与无冕盘比较，讨论了盘冕共存系统中物质的蒸发。采用内区径流（亦译为平流）ADAF主导吸积与外区有冕盘吸积方案，研究了黑洞双星系统中光谱态之间转换的作用，提出了黑洞双星系统中光谱态转换的新机制，主要研究成果如下：1、不透明度是吸积盘不稳定性的重要因素。不透明度表和相应的态方程表的改进对吸积盘结构的影响不大，相对于粘滞系数和混合程的不确定性而言是可以忽略的。2、系统的冕结构研究表明，冕的存在对盘的结构有重要影响。吸积物质从冷盘蒸发到热冕中会使内盘物质耗空，在内区只有冕存在。这样的结构将对吸积盘的辐射产生重要作用。3、数值计算了冕的演化，发现物质蒸发使吸积盘理论能自然地诠释如下观测特征：矮新星爆发中出现的紫外边延滞、X－射线后爆发，以及爆发温度。然后，我们提出了新的关于WZ Sge型矮新星和X－射线新星在沉寂阶段的演化模型。在此基础上，我们数值模拟了WZ Sge的长周期演化。4、提出了冕盘的蒸发是吸积从外区薄盘向内区ADAF转化的原因。利用蒸发模型计算了X－射线瞬变源的转换半径，发现理论值与观测值基本一致。5、用有冕盘蒸发模型探讨了X－射线双星中（如Cyg X-1,LMC X-1,LMC X-3)光谱态、从硬到软的变化，发现吸积率的变化导致了ADAF和薄盘之间转换半径的变化，从而使光谱态发生了变化。     

吸积盘整体结构的统一描述

在考虑径向对流和不同光学厚度的基础上，我们采用一般的辐射致冷假设；在统一的框架内考察了吸积盘的整体结构．对α模型，我们发现在不同的盘区存在不同的整体结构．在吸积率较低时，存在三种类型解：光学厚的局部辐射致冷解；光学薄的局部致冷解；光学薄的对流致冷解．这些解在盘内较大范围内都存在且互不交叉．但在吸积率较高和粘滞系数较大时，两种局部致冷解会相互交叉，而对流为主的解在所有盘区都稳定存在．另一方面，在吸积率较高和粘滞系数较小时，两种光学薄解会相互交叉，而光学厚和局部致冷为主的解在所有盘区都存在．     

黑洞X射线双星的类氢类氦铁Kα发射线

用径移主导吸积流模型（ADAF）不仅可以成功解释很多低吸积率天体的连续谱辐射，也可以说明X射线波段的谱线发射，而后者目前尚少有讨论。以黑洞X射线双星GX339－4处在低吸积率的宁静态情况为例，计算了它的铁线发射，表明在ADAF情况下的确可以产生足够强的可以在观测上检测的类氢和类氦铁Kα线。     

A one-dimensional accretion column model for magnetized neutron stars

The infalling movement of the matter accreted onto a magnetized neutron star is discussed. A one-dimensional accretion column model is presented to describe the variations of the infalling velocity, density and temperature of the infalling plasma. The column can be divided from top down into four zones, impact, deceleration of ideal gas, deceleration of degenerate gas and outflow. As an example, the accretion column for an accretion rate of 10¹⁷ g/s and a polar magnetic field of ≈ 10⁸ T was calculated. We discuss thermonuclear reaction inside the column, and consider that it may be related to the quasi-periodic oscillation (QPO) of the X-ray flux in low-mass close binaries.     

A model for the rapid X-ray Burster

A model for the Rapid X-ray Burster (MXB 1730-333) based on an accreting rotating magnetized neutron star in a binary system is proposed. The bursts are attributed to instabilities produced at an equilibrium surface above the poles of the neutron star, which is created by the infalling gas supported by a combination of radiation and relativistic gas pressures. The special feature of the proposed model is that, when accretion onto the poles is prevented by radiation pressure, relativistic gas streams out of the polar region.     

On the formation of protostellar disks

An analysis is presented of the hydrodynamic aspects of the growth of protostellar disks from the accretion (or collapse) of a rotating gas cloud. The size, mass, and radiative properties of protostellar disks are determined by the distribution of mass and angular momentum in the clouds from which they are formed, as well as from the dissipative processes within the disks themselves. The angular momentum of the infalling cloud is redistributed by the action of turbulent viscosity on a shear layer near the surface of the disk (downstream of the accretion shock) and on the radial shear across cylindrical surfaces parallel to the rotation axis. The fraction of gas that is fed into a central core (protostar) during accretion depends on the ratio of the rate of viscous diffusion of angular momentum to the accretion rate; rapid viscous diffusion (or a low accretion rate) promotes a large core-to-disk mass ratio. The continuum radiation spectrum of a highly viscous disk is similar to that of a steady-state accretion disk without mass addition. It is possible to construct models of the primitive solar nebula as an accretion disk, formed by the collapse of a slowly rotating protostellar cloud, and containing the minimum mass required to account for the planets. Other models with more massive disks are also possible.     

Evolution of the star formation rate in galaxies with increasing densities

Some bursts of star formation are thought to be associated with situations in which a galaxy's density is increasing. Examples include protogalaxy collapse, mergers, inflow of gas into a galactic nucleus, or accretion of intergalactic gas. We have examined the evolution of the star formation rate (SFR) and other properties of galaxies with increasing density using one-zone cloud fluid equations describing an extension of the Oort cycle, for which the equilibrium state would give an SFR which increases monotonically with density. However, the calculations show that the energy input associated with the density increase generally dominates the evolution, and forces the system far from its normal equilibrium to a state in which cloud collisions are disruptive rather than coalescent. The calculations predict that starbursts associated with collapse, accretion, or inflow events should be preceded by a long incubation period with a very small SFR. For example, the initial star formation burst in a protogalaxy may be delayed for several billion years until nearly all the infalling material has been accreted onto the growing central object.     

The Environment of YSO Jets

It is commonly accepted that stars form in molecular clouds by the gravitational collapse of dense gas. However, it is precisely not the infalling but the outflowing material that is primarily observed. Outflow motions prevail around both low and high mass young stellar objects. We present here results from a family of self-similar models that could possibly help to understand this paradox. The models take into account the heating of the central protostar for the deflection and acceleration of the gas. The models make room for all the ingredients observed around the central objects, i.e. molecular outflows, fast jets, accretion disks and infalling envelopes. We suggest that radiative heating and magnetic field may ultimately be the main energy sources driving outflows for both low and high mass stars. The models show that the ambient medium surrounding the jet is unhomogeneous in density, velocity, magnetic field. Consequently, we suggest that jets and outflows have a prehistory that is inprinted in their environment, and that this should have direct consequences on the setting of jet numerical simulations.     

Radiative heating of interstellar grains falling toward the solar nebula: 1-D diffusion calculations

As the dense molecular cloud that was the precursor of our Solar System was collapsing to form a protosun and the surrounding solar-nebula accretion disk, infalling interstellar grains were heated much more effectively by radiation from the forming protosun than by radiation from the disk's accretion shock. Accordingly, we have estimated the temperatures experienced by these infalling grains using radiative diffusion calculations whose sole energy source is radiation from the protosun. Although the calculations are 1-dimensional, they make use of 2-D, cylindrically symmetric models of the density structure of a collapsing, rotating cloud. The temperature calculations also utilize recent models for the composition and radiative properties of interstellar grains (Pollack et al. 1994. Astrophys. J. 421, 615-639), thereby allowing us to estimate which grain species might have survived, intact, to the disk accretion shock and what accretion rates and molecular-cloud rotation rates aid that survival. Not surprisingly, we find that the large uncertainties in the free parameter values allow a wide range of grain-survival results: (1) For physically plausible high accretion rates or low rotation rates (which produce small accretion disks), all of the infalling grain species, even the refractory silicates and iron, will vaporize in the protosun's radiation field before reaching the disk accretion shock. (2) For equally plausible low accretion rates or high rotation rates (which produce large accretion disks), all non-ice species, even volatile organics, will survive intact to the disk accretion shock. These grain-survival conclusions are subject to several limitations which need to be addressed by future, more sophisticated radiative-transfer models. Nevertheless, our results can serve as useful inputs to models of the processing that interstellar grains undergo at the solar nebula's accretion shock, and thus help address the broader question of interstellar inheritance in the solar nebula and present Solar System. These results may also help constrain the size of the accretion disk: for example, if we require that the calculations produce partial survival of organic grains into the solar nebula, we infer that some material entered the disk intact at distances comparable to or greater than a few AU. Intriguingly, this is comparable to the heliocentric distance that separates the C-rich outer parts of the current Solar System from the C-poor inner regions.     

On the formation of superdense gaseous cores in the nuclei of disk galaxies — I

A model for the formation of superdense gaseous cores by accretion in the nuclei of disk galaxies has been proposed. Equations for radial flow of gas into the nucleus in the presence of aweak galactic magnetic field have been solved, and time scales for the accretion of an exploding mass in the nucleus (10⁹ M ) have been obtained under several different situations in the absence of any rotation. The time scales are found to lie in the range between a few times 10⁷ yr and 10⁸ yr. Such time scales have been proposed by some authors for repeated explosions in the nuclei of galaxies; they have also proposed that spiral arms in disk galaxies are repeatedly formed and destroyed over such time scales. It is shown that the presence of rotational velocities in the infalling gas practically destroys the efficiency of the accretion process unless such velocities are dissipated by frictional forces within the system. Viscosity of gas is the most obvious dissipative agent. The problem of accretion of a rotating viscous gas will be discussed in a subsequent paper.     

Matter infall in collapsing molecular cloud cores with an axial magnetic field

The magnetic fields affect collapse of molecular cloud cores. Here, we consider a collapsing core with an axial magnetic field and investigate its effect on infall of matter and formation of accretion disk. For this purpose, the equations of motion of ions and neutral infalling particles are numerically solved to obtain the streamlines of trajectories. The results show that in a non-steady state of ionization and ion–neutral coupling, which is not unexpected in the case of infall, the radius of accretion disk will be larger as a consequence of axial magnetic field.     

Black hole spin dependence of general relativistic multi-transonic accretion close to the horizon

We introduce a novel formalism to investigate the role of the spin angular momentum of astrophysical black holes in influencing the behavior of low angular momentum general relativistic accretion. We propose a metric independent analysis of axisymmetric general relativistic flow, and consequently formulate the space and time dependent equations describing the general relativistic hydrodynamic accretion flow in the Kerr metric. The associated stationary critical solutions for such flow equations are provided and the stability of the stationary transonic configuration is examined using an elegant linear perturbation technique. We examine the properties of infalling material for both prograde and retrograde accretion as a function of the Kerr parameter at extremely close proximity to the event horizon. Our formalism can be used to identify a new spectral signature of black hole spin, and has the potential of performing the black hole shadow imaging corresponding to the low angular momentum accretion flow.     

Relaxation of protostellar accretion shocks using the smoothed particle hydrodynamics

It is believed that protostellar accretion disks are formed from nearly ballistic infall of molecular matter in rotating core collapse. Collisions of this infalling matter leads to formation of strong supersonic shocks, which if they cool rapidly, result in accumulation of that material in a thin structure in the equatorial plane. Here, we investigate the relaxation time of the protostellar accretion post‐shock gas using the smoothed particle hydrodynamics (SPH). For this purpose, a one‐dimensional head‐on collision of two molecular sheets is considered, and the time evolution of the temperature and density of the post‐shock region simulated. The results show that in strong supersonic shocks, the temperature of the post‐shock gas quickly increases proportional to square of the Mach number, and then gradually decreases according to the cooling processes. Using a suitable cooling function shows that in appropriate time‐scale, the center of the collision, which is at the equatorial plane of the core, is converted to a thin dense molecular disk, together with atomic and ionized gases around it. This structure for accretion disks may justify the suitable conditions for grain growth and formation of proto‐planetary entities (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)