金属丰度对球状星团动力学演化的影响

动力学过程和恒星演化及二者的互相影响都会对球状星团的演化产生重要影响.由于金属丰度会影响恒星的演化轨迹,与之相伴随的恒星质量损失率的变化也会对球状星团的动力学过程造成影响.通过一系列N体模拟研究金属丰度对球状星团的质量损失率、半径等的影响,并分析其原因,同时研究了大质量恒星以及星团初始数密度分布的影响.模拟中采用的球状星团模型初始成员星数目N=50000,运行于类银河系的引力势中并考虑成员星的演化.结果显示,由于低金属丰度恒星拥有较快的演化时标,所以贫金属球状星团在早期会拥有较高的质量损失,但与此同时它们的核塌缩时间会比后者显著推迟,因此在核塌缩之后其质量损失会被富金属星团反超.另外由于大质量恒星演化导致的质量损失较大,所以大质量星的存在会使金属丰度更加显著地影响球状星团早期的扩张以及随后的核塌缩过程,同时星团的初始数密度分布也对该效应有着不可忽视的影响.     

嵌埋星团质量分层的数值模拟研究

目前,对大质量恒星形成的初始条件不很清楚,特别是大质量恒星是否形成于星团中心仍有争议.有人从时标上考虑,认为嵌埋星团的质量分层现象意味着大质量恒星只能在星团中心诞生.利用Monte Carlo方法对嵌埋星团的动力学演化进行了数值模拟,并与观测进行了比较.假设初始时刻大质量恒星随机分布,一定比例的嵌埋星团因为大质量恒星的随机运动,在演化的某个时刻会呈现暂态质量分层,其中一部分相当明显.这说明,大质量恒星在中心形成并非嵌埋星团质量分层现象的唯一解释.此外,气体的动力学摩擦能有效地减小动力学质量分层的时标,从而增大暂态质量分层的概率.     

演化的星族合成方法

演化的星族合成方法是在给定恒星形成率和初始质量函数的前提下，利用理论的恒星演化轨迹和恒星光谱库得到的组合特征（光谱，光度），拟合星系、星团等恒星复合天体的观测特征，给出其中星族组成的一种有效方法。对演化的星族合成方法在天体物理研究中的重要意义及其原理和算法以及影响演化星族合成方法结果的最主要的四个输入量：恒星演化轨迹、恒星光谱库、初始质量函数和恒星形成率进行了评述。     

毫秒磁星作为伽玛射线暴中心引擎的研究

正伽玛暴是一种短时标的高能光子爆发现象.通常把持续时间短于～2 s的暴称为短暴,长于～2 s的暴称为长暴.大量观测已经证实,长暴起源于大质量恒星的塌缩,因而与超新星成协.短暴最可能的起源是致密双星并合.目前,伽玛暴研究的一个核心问题是确定其中心引擎究竟为黑洞还是中子星.本文第1章详细阐述了相关进展.数值模拟发现黑洞可产生相对论喷流,因而可作为伽玛暴的中心引擎.然而,有一些观测特征似乎     

不同仪器GRB观测数据的一致性分析

BATSE(Compton Gamma-Ray Observatory/Bursts and Transient SourceExperiment)、Swift(Swift Gamma-ray Burst Explorer)和Fermi卫星(Fermi GammaraySpace Telescope)提供了大量的GRB样本.研究比较3种仪器观测的暴的特征,发现虽然有红移暴的数目、所有暴中长暴的比例以及光子流量分布(lgN-lgP分布)等有显著差异,但是暴的持续时间、伽玛辐射总流量、谱硬度比等均没有显著差异.考虑Swift和Fermi暴的观测能段不同,进行修正以后,发现lgN-lgP分布的差异也基本消除.有红移暴的数目、长暴占总暴数的比例是由仪器本身的灵敏度决定的,即不同仪器决定不同GRB的观测特征,但是它们的本质是一致的.     

主序星的Ⅴ波段经验质光关系

恒星质量是恒星物理以及恒星系统动力学研究中一个不可或缺的参量.双星轨道拟合是获取恒星(动力学)质量的最可靠途径,而绝大部分恒星的质量仍然需要通过恒星质光关系来估计,因此,通过拟合恒星动力学质量和光度数据得到经验质光关系的工作具有重要意义.尽管主序星的Ⅴ波段质光关系由于金属丰度的影响而具有一定的弥散性,但有研究表明这种影响主要限于恒星质量小于 0.6M_⊙的情况.对于较大质量的主序星,近年来的观测拟合研究积累了比较充分的动力学质量和Ⅴ波段光度数据,从而为显著改进上述质光关系提供了可能.利用一个能合理分配两个不同量纲观测量权重的拟合方法,根据 203 颗恒星的动力学质量和光度数据给出了主序星的Ⅴ波段经验质光关系,该结果对此前结果的改进不仅具有统计显著性,而且其对恒星质量估计的相对误差已达到约 5%.因此,该结果不仅可以用于开展有关恒星物理或恒星系统动力学方面的统计性研究,而且对具体实际多星系统的长期动力学研究和短期定位研究等也有应用价值.     

球状星团的金属丰度特征与其恒星形成史

本文根据球状星团所特有的金属丰度特征,利用星族综合方法,探讨了球状星团诸恒星的形成史。研究表明,这些恒星不可能通过恒星形成率和初始质量函数均不随时间变化的单一恒星形成模式产生。原初云通过恒星演化而得到金属丰度污染的过程和多数恒星的形成过程必须分为两个不同的阶段。 球状星团得到金属丰度污染的过程中,若恒星形成具有通常的初始质量函数,则其恒星形成率必须较低,且初始质量函数不能太陡,从而使污染过程中只形成数量较少的低质量恒星,以保证单个球状星团内金属丰度的均匀性。另一种可能性是污染阶段有非常特殊的初始质量函数,只形成大质量的恒星,从而除提供适量污染外不留下任何痕迹。 多数恒星应是在原初云不同部位得到适当污染后通过局域的短暂的爆发性恒星形成(星暴过程)产生。本文进一步探讨了在Fan和Rees球状星团形成模型的框架下,两相介质中温云块相互碰撞造成星暴过程的可能性。     

LAMOST-Kepler视场中富锂巨星表面转动研究

Kepler卫星提供的长时序、高精度的光度观测和郭守敬望远镜(LAMOST)提供的大规模光谱观测为研究恒星表面转动周期与富锂巨星锂丰度关系提供了良好的数据.将LAMOST搜寻到的富锂巨星与Kepler观测交叉,获得了619颗共同源,研究了其中295颗有良好观测数据的富锂巨星的表面转动.在205颗有星震学参数的恒星中提取出14颗恒星的转动周期,其中氦核燃烧星(HeB) 11颗,红巨星支(RGB) 2颗, 1颗演化阶段未确定.本样本中的极富锂巨星(A(Li) 3.3 dex)皆为HeB;对于90颗没有星震学参数的样本因而没有依靠星震学手段确定演化阶段的恒星中,有22颗提取出了自转周期.前者的自转探测率为6.8%,显著高于之前工作中大样本巨星2.08%的探测率.同时,此研究首次从自转周期的角度确认了恒星转动与巨星锂增丰存在相关性,在增丰程度较弱时,自转周期分布比较弥散;强锂增丰的星倾向于快速转动.富锂巨星与极富锂巨星在转动速度随锂丰度的演化上展现了两个序列,在转动-锂丰度图上的A(Li)≈3.3 dex处产生第2个下降序列,或许暗示了两者在形成机制上的不同.极富锂巨星的样本中,随巨星锂增丰程度增强,恒星转速加快.这种相关性为由转动引起的额外混合作为富锂巨星形成的机制提供了支持.     

大质量恒星演化的若干问题(英文)

大质量恒星由于其高光度、短寿命和质量损失 ,对星系的积分光谱能量分布和重元素增丰起主导作用 ,从而在研究星系的形成和演化上具有特殊的意义。特别是随着天文设备的长足进展 ,我们可以回溯宇宙演化的历史 ,得到形成初期时星系的观测性质。那时 ,大质量恒星主导星系的辐射性质 ,这更加突出了对大质量恒星进一步了解的迫切性。但是大质量恒星的演化性质相对中小质量恒星而言 ,有很多不确定性。本文通过对比现有恒星模型与实测结果 ,对现有大质量恒星演化理论中存在的几个与对流和质量损失相关的问题进行了评述 ,并对从理论上 ,特别是通过数字模拟方法对这些问题进行诊断提出了展望。     

银河系中探测第一代恒星的几率

第一代恒星是形成于宇宙大爆炸后的原始气体中的、不含重于碳元素的、寿命大于14Gyr的、迄今尚未演化的最古老的恒星.长期的观测结果表明银河系中尚未发现金属丰度([Fe/H])为零、甚至金属丰度[Fe/H]≤-6的恒星.为解释这一观察现象,将以Tsuiimoto等人提出的银晕的化学演化模型为基础,假设形成第一代恒星的初始质量函数具有Miller-Scalo的形式,从理论上预言和讨论探测第一代恒星的可能性.利用已有的晕星的观测资料限定模型的参数.如果形成恒星的云的质量为106M-107M,模型结果预言探测到第一代恒星的几率为6.14×10-4-6.14×10-5.     

The Effect of the Initial Mass Function of Stars on the Burst Rate of GRBs

For the mechanism of production of γ-ray bursts (GRBs) it is rather generally recognized that the long-term γ-ray burst (LGRB) originates from the deaths of massive stars while the short-term γ-ray burst (SGRB) originates from the merging of close binaries. Therefore the speculation naturally follows that the number of LGRBs is directly proportional to the star formation rate (SFR). However, it is indicated from recent data analyses that this speculation does not fit the observations very well. It is considered that only massive stars with masses greater than a certain critical mass can produce the LGRB, so the initial mass function (IMF) of stars can significantly affect the production rate of LGRBs. In this paper it is considered that the IMF of stars can be used to explain the observed number distribution of the LGRBs with the redshift, and this has led to some good results.     

A quark nova in the wake of a core-collapse supernova:a unifying model for long duration gamma-ray bursts and fast radio bursts

By appealing to a quark nova(QN;the explosive transition of a neutron star to a quark star) in the wake of a core-collapse supernova(CCSN) explosion of a massive star,we develop a unified model for long duration gamma-ray bursts(LGRBs) and fast radio bursts(FRBs).The time delay(years to decades)between the SN and the QN,and the fragmented nature(i.e.,millions of chunks) of the relativistic QN ejecta are key to yielding a robust LGRB engine.In our model,an LGRB light curve exhibits the interaction of the fragmented QN ejecta with turbulent(i.e.,filamentary and magnetically saturated) SN ejecta which is shaped by its interaction with an underlying pulsar wind nebula(PWN).The afterglow is due to the interaction of the QN chunks,exiting the SN ejecta,with the surrounding medium.Our model can fit BAT/XRT prompt and afterglow light curves simultaneously with their spectra,thus yielding the observed properties of LGRBs(e.g.,the Band function and the X-ray flares).We find that the peak luminositypeak photon energy relationship(i.e.,the Yonetoku law),and the isotropic energy-peak photon energy relationship(i.e.,the Amati law) are not fundamental but phenomenological.FRB-like emission in our model results from coherent synchrotron emission(CSE) when the QN chunks interact with non-turbulent weakly magnetized PWN-SN ejecta,where conditions are prone to the Weibel instability.Magnetic field amplification induced by the Weibel instability in the shocked chunk frame sets the bunching length for electrons and pairs to radiate coherently.The resulting emission frequency,luminosity and duration in our model are consistent with FRB data.We find a natural unification of high-energy burst phenomena from FRBs(i.e.,those connected to CCSNe) to LGRBs including X-ray flashes(XRFs) and X-ray rich GRBs(XRR-GRBs) as well as superluminous SNe(SLSNe).We find a possible connection between ultra-high energy cosmic rays and FRBs and propose that a QN following a binary neutron star merger can yield a short duration GRB(SGRB) with fits to BAT/XRT light curves.     

Multiwavelength study of X-ray selected star-forming galaxies within the Chandra Deep Field-South

We have combined multiwavelength observations of a selected sample of star-forming galaxies with galaxy evolution models in order to compare the results obtained for different star formation rate (SFR) tracers and to study the effect that the evolution of the star-forming regions has on them. We also aimed at obtaining a better understanding of the corrections due to extinction and nuclear activity on the derivation of the SFR. We selected the sample from Chandra data for the well studied region Chandra Deep Field -South (CDFS) and chose the objects that also have ultraviolet (UV) and infrared (IR) data from Galaxy Evolution Explorer ( GALEX ) and Great Observatories Origins Deep Survey (GOODS) Spitzer , respectively.
Our main finding is that there is good agreement between the extinction corrected SFR(UV) and the SFR(X), and we confirm the use of X-ray luminosities as a trustful tracer of recent star formation activity. Nevertheless, at SFR(UV) larger than about  5 M_⊙ yr⁻¹  there are several galaxies with an excess of SFR(X) suggesting the presence of an obscured active galactic nucleus (AGN) not detected in the optical spectra. We conclude that the IR luminosity is driven by recent star formation even in those galaxies where the SFR(X) is an order of magnitude higher than the SFR(UV) and therefore may harbour an AGN. One object shows SFR(X) much lower than expected based on the SFR(UV); this SFR(X) 'deficit' may be due to an early transient phase before most of the massive X-ray binaries were formed. An X-ray deficit could be used to select extremely young bursts in an early phase just after the explosion of the first supernovae associated with massive stars and before the onset of massive X-ray binaries.     

Bimodal distribution of short gamma-ray bursts: Evidence for two distinct types of short gamma-ray bursts

GRB 170817A was confirmed to be associated with GW170817, which was produced by a neutron star - neutron star merger. It indicates that at least some short gamma-ray bursts come from binary neutron star mergers. Theoretically, it is widely accepted that short gamma-ray bursts can be produced by two distinctly different mechanisms, binary neutron star mergers and neutron star - black hole mergers. These two kinds of bursts should be different observationally due to their different trigger mechanisms. Motivated by this idea, we collect a universal data set constituted of 51 short gamma-ray bursts observed by Swift/BAT, among which 14 events have extended emission component. We study the observational features of these 51 events statistically. It is found that our samples consist of two distinct groups. They clearly show a bimodal distribution when their peak photon fluxes at 15–150 keV band are plotted against the corresponding fluences. Most interestingly, all the 14 short bursts with extended emission lie in a particular region of this plot. When the fluences are plotted against the burst durations, short bursts with extended emission again tend to concentrate in the long duration segment. These features strongly indicate that short gamma-ray bursts really may come from two distinct types of progenitors. We argue that those short gamma-ray bursts with extended emission come from the coalescence of neutron stars, while the short gamma-ray bursts without extended emission come from neutron star - black hole mergers.     

Star formation histories of local group dwarf galaxies

We present the study of the star formation histories (SFHs) of a sample of Local Group dwarf galaxies (LGDGs), via the analysis and modelling, with the means of an evolutionary stellar population synthesis, of their colour-magnitude (CM) diagrams. It appears that important parameters to describe the SFHs are star formation rate (SFR) and duration of star formation. We find a possible correlation between the mass and the SFRs. The correlation might be the origin of the observed luminosity-metallicity relation in the LGDGs. A well-defined correlation between the durations of star formation and the distance from M31 or the Galaxy indicates that the current early-type dwarf galaxies should have been transformed from late-type by strong tidal force of the massive galaxies. This revised version was published online in August 2006 with corrections to the Cover Date.     

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 correlation between black hole mass and bulge velocity dispersion in hierarchical galaxy formation models

Popular models for the origin of gamma-ray bursts (GRBs) include short-lived massive stars as the progenitors of the fireballs. Hence the redshift distribution of GRBs should track the cosmic star formation rate of massive stars accurately. A significant proportion of high-mass star formation activity appears to occur in regions that are obscured from view in the optical waveband by interstellar dust. The amount of dust-enshrouded star formation activity taking place has been estimated by observing the thermal radiation from the dust that has been heated by young stars in the far-infrared and submillimetre wavebands. Here we discuss an alternative probe – the redshift distribution of GRBs. GRBs are detectable at the highest redshifts, and because gamma-rays are not absorbed by dust, the redshift distribution of GRBs should therefore be unaffected by dust extinction. At present the redshifts of GRBs can only be determined from the associated optical transient emission; however, useful information about the prevalence of dust-obscured star formation can also be obtained from the ratio of GRBs with and without an associated optical transient. Eight GRBs currently have spectroscopic redshifts. Once about a hundred redshifts are known, the population of GRBs will provide an important test of different models of the star formation history of the Universe.     

Optical and submillimetre observations of Bok globules – tracing the magnetic field from low to high density

Although the stellar initial mass function (IMF) has only been directly determined in star clusters, it has been manifoldly applied on galaxy-wide scales. But taking the clustered nature of star formation into account the galaxy-wide IMF is constructed by adding all IMFs of all young star clusters leading to an integrated galactic initial mass function (IGIMF). The IGIMF is top-light compared to the canonical IMF in star clusters and steepens with decreasing total star formation rate (SFR). This discrepancy is marginal for large disc galaxies but becomes significant for Small Magellanic Cloud type galaxies and less massive ones. We here construct IGIMF-based relations between the total far- and near-ultraviolet luminosities of galaxies and the underlying SFR. We make the prediction that the Hα luminosity of star-forming dwarf galaxies decreases faster with decreasing SFR than the ultraviolet (UV) luminosity. This turn-down of the Hα/UV-flux ratio should be evident below total SFRs of  10⁻² M_⊙ yr⁻¹  .     

Long gamma-ray burst progenitors: boundary conditions and binary models

The observed association of Long Gamma-Ray Bursts (LGRBs) with peculiar Type Ic supernovae gives support to Woosley‘s collapsar/hypernova model, in which the GRB is produced by the collapse of the rapidly rotating core of a massive star to a black hole. The association of LGRBs with small star-forming galaxies suggests low-metallicity to be a condition for a massive star to evolve to the collapsar stage. Both completely-mixed single star models and binary star models are possible. In binary models the progenitor of the GRB is a massive helium star with a close companion. We find that tidal synchronization during core-helium burning is reached on a short timescale (less than a few millennia). However, the strong core-envelope coupling in the subsequent evolutionary stages is likely to rule out helium stars with main-sequence companions as progenitors of hypernovae/GRBs. On the other hand, helium stars in close binaries with a neutron-star or black-hole companion can, despite the strong core-envelope coupling in the post-helium burning phase, retain sufficient core angular momentum to produce a hypernova/GRB.     

Dense star clusters as the sources of gamma-ray-burst progenitors

Two independent sets of arguments lead us to conclude that the progenitors of superintense bursts (with an energy yield larger than that for ordinary supernovae by one or two orders of magnitude) are born in massive dense star clusters, but generally flare up only after they have left the cluster; these are the same objects that are the progenitors of gamma-ray bursts (GRBs). Each of the giant stellar arcs which are grouped into multiple systems of stellar complexes in the LMC and NGC 6946 could only be produced by a single powerful energy release near its center. The progenitors of these systems of arc-shaped stellar complexes must have had a common source nearby, and it could only be a massive star cluster. Such clusters are actually known near both systems. On the other hand, calculations of the dynamical evolution of star clusters show that close binary systems of compact objects are formed in the dense central parts of the clusters and are then ejected from them during triple encounters. Mergers of the components of such systems are believed to be responsible for GRBs. Since their progenitors are ejected from the cluster before merging, the arc-shaped stellar complexes produced by GRBs are observed near (but not around) the parent clusters. If a considerable fraction of the GRB progenitors are formed as a result star encounters in massive star clusters, and if the GRBs themselves trigger star formation near the parent clusters, then observations of GRBs in star-forming regions are consistent with their origin during mergers of pairs of compact objects.