银河系和河外星系的消光规律及尘埃性质

对星际消光的研究,有利于人们还原天体本来的光度与颜色,也有利于人们了解星际尘埃的性质。小麦哲伦云紫外波段的消光曲线比银河系紫外波段的消光曲线更加陡峭,大麦哲伦云的平均消光曲线介于银河系的平均消光曲线与小麦哲伦云的消光曲线之间。M31的消光曲线与银河系的消光曲线类似,都存在2 175?A的驼峰和紫外波段增强的特征。此外,对高红移河外星系尘埃的星光衰减曲线以及M33,M101,NGC 2207,M82,Ia型超新星、星暴星系、类星体和活动星系核消光规律的研究结果表明,它们所处的星际环境不同,消光规律及尘埃特征也不同。     

银河系星际消光空间分布的研究进展

银河系中星际尘埃的存在使观测到的天体的亮度和颜色发生改变。银河系尘埃消光改正不止是河内/外天体观测性质研究的必要步骤,也是银河系自身性质研究的重要课题。从三个方面对银河系消光的研究进展进行了综述,包括银河系的整体消光、银河系内天体消光的测量和银河系消光曲线。最后进行了总结和展望。     

致密介质中柱状喷流的伽玛射线暴余辉

很多伽玛射线暴应当是产生于极端相对论性的喷流.关于喷流,绝大多数的讨论都是围绕锥状喷流展开.然而有观测发现,一些天体中的喷流在很长距离上始终保持着几乎不变的截面积,即表现为柱状喷流.研究致密介质环境中有侧向膨胀的柱状喷流的余辉,描述其动力学演化和辐射过程,分别得到解析解和数值解,并对两者进行了对比.研究的暴周星际介质是光学厚的,在初始的主暴阶段,喷流辐射出高度准直的高能射线,升华了暴周介质,形成一个在光学波段光学薄的柱状通道.余辉阶段,由于喷流是有侧向膨胀的,观测者只能收集到视觉面积占比例越来越小的光学辐射,理论上可得到衰减极为快速的光变曲线,流量随时间的衰减约为Svα t-p-1(p为电子幂律分布的谱指数).如此迅速的衰减使得光学余辉将难以被观测到,提供了一种对暗伽玛射线暴的解释.     

射电星系3C 288中的尘埃环

射电星系根据射电形态可以分为FRⅠ和FRⅡ两类,而根据其光学光谱特征可以分为低激发和高激发射电星系。它们在观测上表现出不同,其原因并不清楚,部分原因是喷流与周围介质的相互作用导致的,也可能是因为FR Ⅰ和FR Ⅱ的中央引擎吸积模式不同导致的。为了检验这些源是否存在尘埃环,用CLOUDY程序模拟3C 288射电源的光致电离模型,发现理论得到的电离光度远大于实际观测到的光学核光度,这说明在3C 288中确实存在遮挡效应。为了进一步检验这个结论,又根据尘埃消光来验证高喷流射电功率星系中的遮挡效应是尘埃环造成的。     

高速Ia型超新星的蓝色超出: 星周介质的尘埃散射

Ia型超新星起源于碳氧白矮星在质量接近钱德拉塞卡极限时的热核爆炸,并被广泛地用作宇宙学距离的标准烛光.然而, Ia型超新星的前身星系统和爆炸机制还存在很多不明确的地方.近几十年来, Ia型超新星的星周环境得到了越来越多的关注.星周介质的空间分布性质为探究Ia型超新星的物理起源提供了重要线索.同时星周尘埃的散射会在Ia型超新星晚期的光变曲线、光谱和偏振等方面产生可观测效应.光谱上正常的Ia型超新星可以分成两类:喷射物速度正常和高速Ia型超新星.对比两者的光变曲线可以发现高速Ia型超新星在光极大后几个月内有明显颜色偏蓝的超出.该蓝色超出可以通过星周介质中的尘埃散射拟合得到.同时, Ia型超新星晚期光谱的拟合可以限制星周尘埃的颗粒大小等性质,晚期的偏振信号可以有效地限制星周尘埃的空间分布.拟合结果表明针对Ia型超新星晚期的多次图像偏振观测是揭示其星周尘埃环境特征的重要手段.     

伽玛暴宇宙学的研究

伽玛射线暴(简称伽玛暴,gamma-ray burst (GRB))是一种来自宇宙空间中的伽玛射线波段流量突然增亮的现象,最早由Vela卫星在1967年发现.1997年人们通过余辉测得了伽玛暴的红移,从而确定了其宇宙学的起源.伽玛暴宇宙学包括用长暴的标准烛光关系限制暗能量和宇宙学参数,用长暴研究高红移的恒星形成率,研究金属丰度的演化、尘埃及量子引力等.     

γ射线暴X射线耀发的研究进展

γ射线暴是宇宙中恒星尺度的最剧烈爆发现象。γ射线暴瞬时辐射结束后,进入余辉辐射阶段。X射线耀发是γ射线暴X射线辐射衰减过程中出现的短时标闪耀现象。X射线耀发的脉冲轮廓具有不对称性,其上升时标小于下降时标。在部分γ射线暴中,X射线耀发的亮度达到瞬时辐射的亮度。X射线耀发的持续时间与峰值时间具有线性关系。X射线耀发的光谱比X射线余辉的光谱硬。早期X射线耀发与晚期X射线耀发相比,其脉冲轮廓较窄,光谱较硬。X射线耀发产生的物理过程类似于γ射线暴瞬时辐射的物理过程。在火球(fireball)模型中,内部壳层之间发生碰撞,产生的内激波加速电子,电子的同步辐射产生X射线耀发。当火球扫过星际介质,外激波加速电子时,电子的同步辐射也可产生X射线耀发。在光球(photospere)模型中,能量耗散发生在光学厚的区域,热辐射的光谱峰值落在X射线能段附近,γ射线暴的喷流在光球半径处会产生X射线耀发。如果射线暴喷流由坡印亭能流主导,喷流就会与星际介质相互作用,磁场的不稳定性使磁场发生耗散,产生的能量形成X射线耀发。γ射线暴的喷流具有几何效应。一部分同步辐射可能发生在喷流辐射面的高纬度处。由于曲率效应(curvature effect),各向异性辐射与各向同性辐射相比,X射线耀发的峰值出现较晚。此外,在γ射线暴发生后,黑洞会间歇性地吸积外部介质。在吸积过程中,黑洞周围的磁场会调节吸积的速率和喷流中的能量,这是出现多个X射线耀发的原因。     

Ia型超新星的消光改正

Ia型超新星因其在宇宙学上起着标准烛光的作用而备受关注,正确的消光改正才能还原其本征亮度和颜色,然后才能在宇宙学中恰当地应用。综述了不同环境中的尘埃对Ia型超新星消光的影响,并讨论Ia型超新星绝对星等的校正和Rv值偏小的原因。Ia型超新星绝对星等的弥散主要受两个方面的影响,一是由前身星状态所导致的固有弥散,二是寄主星系内尘埃的消光。利用与距离无关的参数对绝对星等校正后,可以将弥散降低到0.1 mag以下。Ia型超新星的内禀色指数呈现出随时间演化的一致性,使得其红化比较容易计算。大量的观测和分析表明,Ia型超新星在寄主星系中的Rv值在低红化时,其情形比较接近银河系;而在红化严重时偏小,多在2.0以下,低于银河系的平均值3.1。一种可能的解释是Ia型超新星的星周尘埃对星光的多重散射,另一种可能是Ia型超新星寄主星系的尘埃与典型的银河系内星际尘埃的差异。     

类星体SDSS J0916+2921的类银河系2175 ?尘埃消光特征

光谱观测显示类星体SDSS J091613.60+292106.1 (简称J0916+2921,系统红移zem=1.1418±0.0018)中有特殊的2175?尘埃消光特征,其强度显著大于银河系平均强度.光谱同时探测到与尘埃成协的丰富气体吸收线,确定吸收线系统红移为zabs=1.1413±0.0002,和类星体红移一致.气态金属离子柱密度相对太阳丰度的比例为[Al/Zn]=-1.68±0.10,[Cr/Zn]=-0.49±0.10,[Fe/Zn]=-0.81±0.18.尘埃耗散作用显著,说明该系统中尘埃十分丰富,与观测的强尘埃消光特征吻合.类银河系的2175?尘埃消光特征在类星体光谱中多见于中间插入吸收线系统,至今未明确认证内禀吸收线系统出现该特征,类星体SDSS J0916+2921是目前仅有的数个候选者之一.该类星体X射线辐射相较一般类星体更强,后续可用作研究2175?尘埃在强高能射线照射条件下的形成与离解平衡,以揭示2175?尘埃的化学成分、物理性质和起源.     

Gamma暴余辉的相对论流体动力学演化和光变曲线

本文首先对Gamma暴的观测特性和物理过程作了简要的介绍 ,而后 ,对火球模型的相对论流体动力学机制和同步加速辐射机制作了论述。主要工作是 :具体研究火球所抛出壳层的相对论流体动力学演化 ,应用同步加速辐射机制 ,通过由共动坐标系到实验室坐标系的相对论变换 ,得到Gamma暴余辉的光变曲线。对于火球壳层的不同的动力学演化规律 ,各向同性或各向异性的壳层抛出形式 ,以及不同的外部介质环境 ,所得到的光变曲线都各不相同。通过对这些不同的光变曲线的比较 ,明确了Gamma暴余辉的整体的物理演化图象以及各种物理过程在Gamma暴余辉演化过程中所起的作用 ,并从余辉演化的方面进一步理解了Gamma暴的物理本质     

Effect of Dust Extinction on Gamma-ray Burst Afterglows

In order to study the effect of dust extinction on the afterglow of gamma-ray bursts (GRBs), we carry out numerical calculations with high precision based on the rigorous Mie theory and the latest optical properties of interstellar dust grains, and analyze the different extinction curves produced by dust grains with different physical parameters. Our results indicate that the absolute extinction quantity is substantially determined by the medium density and metallicity. However, the shape of the extinction curve is mainly determined by the size distribution of the dust grains. If the dust grains aggregate to form larger ones, they will cause a flatter or grayer extinction curve with lower extinction quantity. On the contrary, if the dust grains are disassociated to smaller ones due to some uncertain processes, they will cause a steeper extinction curve with larger amount of extinction. These results might provide an important insight into understanding the origin of the optically dark GRBs.     

Circumstellar dust and circumstellar extinction curves

We have calculated the circumstellar extinction curves produced by dust grains which absorb and scatter the stellar radiation in the shells of pre-main-sequence stars. A Monte Carlo method was used to model the radiative transfer in non-spherical shells. The dependence on the particle size distribution and the dust shell parameters has been examined.The application of the theoretical results to explain the extinction and polarization of the Herbig Be star HD 45677 shows that the dust shell is not disk-like and that very small grains are absent in it.     

Extinction curves flattened by reverse shocks in supernovae

We investigate the extinction curves of young galaxies in which dust is supplied from Type II supernovae (SNe II) and/or pair instability supernovae (PISNe). Since at high redshift ( z > 5), low-mass stars cannot be dominant sources for dust grains, SNe II and PISNe, whose progenitors are massive stars with short lifetimes, should govern the dust production. Here, we theoretically investigate the extinction curves of dust produced by SNe II and PISNe, taking into account reverse shock destruction induced by collision with ambient interstellar medium. We find that the extinction curve is sensitive to the ambient gas density around a SN, since the efficiency of reverse shock destruction strongly depends on it. The destruction is particularly efficient for small-sized grains, leading to a flat extinction curve in the optical and ultraviolet wavelengths. Such a large ambient density as   n _H≳ 1 cm⁻³  produces too flat an extinction curve to be consistent with the observed extinction curve for SDSS J1048+4637 at z = 6.2. Although the extinction curve is highly sensitive to the ambient density, the hypothesis that the dust is predominantly formed by SNe at z ∼ 6 is still allowed by the current observational constraints. For further quantification, the ambient density should be obtained by some other methods. Finally, we also discuss the importance of our results for observations of high- z galaxies, stressing a possibility of flat extinction curves.     

Three-component dust models for interstellar extinction

Interstellar extinction curves obtained from the ‘extinction without standard’ method were used to constrain the dust characteristics in the mean ISM (R _V = 3.1), along the lines of sight through a high latitude diffuse molecular cloud towards HD 210121 (R _V = 2.1) and in a dense interstellar environment towards the cluster NGC 1977 (R _V = 6.42). We have used three-component dust models comprising silicate, graphite and very small carbonaceous grains (polycyclic aromatic hydrocarbons) following the grain size distributions introduced by Li & Draine in 2001. It is shown that oxygen, carbon and silicon abundances derived from our models are closer with the available elemental abundances for the dust grains in the ISM if F & G type stars atmospheric abundances are taken for the ISM than the solar. The importance of very small grains in modelling the variation of interstellar extinction curves has been investigated. Grain size distributions and elemental abundances locked up in dust are studied and compared at different interstellar environments using these three extinction curves. We present the albedo and the scattering asymmetry parameter evaluated from optical to extreme-UV wavelengths for the proposed dust models.     

Interstellar Dust in the Solar System

We deduce the mass distribution and total mass density of interstellar dust streaming into the solar system and compare the results to the conditions of the very local interstellar medium (VLISM). The mass distribution derived from in situ measurements shows a gentler slope and includes larger grains, compared to a model distribution proposed for the wavelength dependence of the interstellar extinction. The mass density of grains in the solar system is consistent with that expected from measurements of the visible interstellar extinction and the abundance constraints of elements in the diffuse interstellar medium (ISM), instead of those in the VLISM. This may imply that interstellar dust grains are not associated with the VLISM and that the conditions of the grains are better represented by the ones expected in the diffuse ISM. If this is the case, then the flatter slope in the mass distribution and the detection of larger interstellar grains in the solar system may even indicate that coagulation growth of dust in the diffuse ISM is more effective than previously inferred. This revised version was published online in July 2006 with corrections to the Cover Date.     

Extinction curves expected in young galaxies

We investigate the extinction curves of young galaxies in which dust is supplied from Type II supernovae (SNe II) and/or pair instability supernovae (PISNe). We adopt Nozawa et al. (2003) for compositions and size distribution of grains formed in SNe II and PISNe. We find that the extinction curve is quite sensitive to internal metal mixing in supernovae (SNe). The extinction curves predicted from the mixed SNe are dominated by SiO₂ and are characterized by a steep rise from infrared to ultraviolet (UV). The dust from unmixed SNe shows a shallower extinction curve, because of the contribution from large-sized (∼0.1 μm) Si grains. However, the progenitor mass is important in unmixed SNe II: if the progenitor mass is smaller than  ∼20 M_⊙  , the extinction curve is flat in UV; otherwise, the extinction curve rises towards the short wavelength. The extinction curve observed in a high-redshift quasar  ( z = 6.2)  favours the dust production by unmixed SNe II. We also provide some useful observational quantities, so that our model might be compared with future high- z extinction curves.     

Dust-induced chemical differentiation in dense regions

Dust grains respond to the physical and chemical conditions of the interstellar region in which they are embedded. The interaction produces an extinction curve which depends on the local environment and on the past history of the dust grains. In this work we present a theoretical study of the effects of radial variations of dust extinction properties on gas-phase chemistry in spherical core–halo clouds. We use observational constraints on the variation range of the extinction curve, and we analyse if the degree of dust environmental processing could be reflected by chemical signatures in the gas-phase molecular concentrations. The results of this work show that significant variations in the photodestruction rates and in the thermal profile of the cloud might induce chemical patterns otherwise excluded in the standard dense-cloud chemistry. Some discrepancies between observations and theoretical provisions are discussed in the light of the present results.     

星周尘埃壳层的消光值

本文首次提出了从观测得到的具有星周尘埃壳层的恒星的能谱分布求取星周尘埃云的消光,并由此可通过改正星周消光改正后的星际消光法求得恒星距离的方法。     

Large Grains in our and External Galaxies

We review some of the evidences that dust properties are dominated by large grains in some astronomical objects. We emphasize the effects of large grains on the extinction and emissivity, but we also discuss some dynamical implications. We also propose a density sequence, where objects characterized by denser environments are also associated with average larger grains. This revised version was published online in September 2006 with corrections to the Cover Date.     

Extinction of light by randomly-oriented non-spherical grains

Waterman'sT-matrix approach is used to derive a simple analytical expression for the extinction cross-section for randomly-oriented non-spherical grains. Numerical results are presented for randomly-oriented oblate and prolate spheroids and Chebyshev particles composed of astronomical silicate. These results are compared with those for spherical grains, and possible influence of the shape of dust grains on the value of interstellar extinction is considered. The range of validity of the Rayleigh approximation for computing extinction efficiency factors for randomly-oriented non-spherical grains is discussed.