球状星团的天体测量

球状星团是银河系中最年老的天体之一，是储存着银河系早期演化珍贵信息的“化石”。球状星团的天体测量，主要包括球状星团天区内恒星相对自行的测定，并由这些相对自行数据采用适当方法定出星团的绝对自行，或者直接测定绝对自行。利用这些自行数据，或者进一步与测光和视向速度数据结合，可以开展与球状星团的距离、运动、动力学状况、质量、年龄、演化等等以及银河系的结构和演化等有关的一系列重要的研究。在本文中对本世纪７０     

球状星团的天体测量(英)

球状星团是银河系中最年老的天体之一，是储存着银河系早期演化珍贵信息的“化石”．球状星团的天体测量，主要包括球状星因天区内恒星相对自行的测定，并由这些相对自行数据采用适当方法定出星团的绝对自行，或者直接测定绝对自行．利用这些自行数据，或者进一步与测光和视向速度数据结合，可以开展与球状星团的距离、运动、动力学状况、质量、年龄、演化等等以及银河系的结构和演化等有关的一系列重要的研究．在本文中对本世纪70年代中期以来在球状星团相对自行测定和成员概率估计、内部运动检测、绝对自行测定和空间运动研究这三方面取得的成果和进展以及存在的问题作了评述．     

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。     

白矮星将再度辉煌

白矮星将再度辉煌瑞林自从类星体、脉冲星和黑洞等奇异天体脍炙人口以来，在天文科普刊物中便很少提到白矮星了。但实际上，从１９世纪６０年代发现第一颗白矮星（天狼Ｂ星）至今，天文学家对它们的搜寻和研究一直没有停止过，而近年来的两项发现和宇宙年龄危机问题有可能...     

恒星形成的辐射流体(磁流体)数值模拟研究进展

恒星是宇宙中最重要的天体之一,恒星形成是天体物理学研究的重要课题。天文学家正致力于理解与恒星形成相关的一系列问题,如:大质量恒星和星团是如何形成的?什么样的物理过程决定了恒星形成区域的物理性质?初始质量函数由什么确定?恒星形成率由什么决定?近年来,恒星形成的数值模拟研究不断发展。主要介绍了辐射流体(磁流体)的数值研究在原恒星核坍缩、大质量恒星形成、星云破碎和坍缩、星团形成和初始质量函数等方面的模拟研究进展。     

稳定物质转移通道形成的极小质量白矮星

极小质量白矮星双星对于双星演化、公共包层、AM CVn双星、星震学研究都十分重要。考虑到它们的周期比较短,它们还是重要的引力波源,然而它们的形成和演化仍然不清楚。通过利用最新的一种磁滞模型,建立了极小质量白矮星双星的形成和演化模型。研究给出它们形成的初始参数空间,并发现通过稳定物质转移形成的极小质量白矮星的质量范围为(0.11～0.21) M_⊙。此外,研究发现部分极小质量白矮星双星能在宇宙年龄内演化成为AM CVn双星,这些极小质量白矮星的质量范围为0.14～0.16 M_⊙。研究还发现它们的引力波信号能被LISA、天琴、太极探测器探测到。最后还讨论了不同的物质积累效率对于同一双星系统演化结果的影响,发现它除了明显影响主星最终质量外,对于伴星和双星演化过程并无太大影响。     

银河系核纪年研究进展

确定银河系的年龄是现代天体物理学的一项基本任务。其方法之一是核纪年法，即通过恒星中某一长寿命放射性元素的丰度随时间的变化来确定恒星的年龄，并以此作为银河系年龄的下限，其中目前的观测丰度来自恒星的光谱分析，恒星形成时的初始丰度来自理论模型的预言。这种方法最初是利用元素对Th／Nd来确定G矮星的年龄，近年来开始利用元素对Th／Eu和U／Th来确定晕族场星和球状星团内恒星的年龄。简要介绍了核纪年法确定银河系年龄的原理，回顾了恒星中Th和U的观测研究，其中着重介绍了极贫金属星的研究。详细讨论了用核纪年法估计银河系年龄的不确定性。作为与核纪年法的比较，简单介绍了确定银河系年龄的其他方法。提出了今后需要进一步研究的几项工作。     

疏散星团NGC 6866视场中5颗类太阳振动红巨星的星震学研究

由于星团中所有成员星是在近似相同的时间里由相同的气体星云形成,所有成员星具有相同的年龄、距离和化学元素,所以星团中的所有成员星可作为一个整体来研究.此时自由参量较少,这为天体物理的研究提供了一个独特的机会.因此,对星团年龄的确定比对单颗恒星年龄的确定更准确.以耶鲁-伯明翰(Yale-Brimingham)Pipeline技术为核心结合星震学方法,分析并推导了星团NGC 6866视场中5颗被观测到具有类太阳振动红巨星的年龄、质量等恒星参数.发现该星团视场中这5颗类太阳振动恒星中有4颗恒星的年龄不完全一致且大于1 Gyr,远大于利用其他方法得到的星团NGC 6866的年龄(约0.65 Gyr).这意味着这4颗类太阳振动恒星可能不是该星团的成员星.同时利用星震学方法获得KIC 8329894恒星的年龄为0.60976_(-0.085175)~(+0.057327)Gyr,与星团的年龄基本一致;另外,把这5颗被Kepler所观测到的红巨星与LAMOST(大天区面积多目标光纤光谱天文望远镜)的数据做了交叉证认,从而更加确定该红巨星为星团成员星.因此该恒星的星震学年龄也可以作为疏散星团年龄的进一步验证和限制.     

银河系球状星团射电脉冲星的统计研究

球状星团是银河系中最古老的天体系统之一,其恒星密度极端高的核心有利于创造双星之间进行物质交换的环境,从而形成毫秒脉冲星双星、掩食脉冲双星、主序-毫秒脉冲双星、高轨道偏心率双星等双星系统,通过对这些系统进行研究有助于进一步认识球状星团的动力学、双星系统的演化和星际介质等相关问题。自30年前在球状星团中发现第一颗射电脉冲星至今,随着较高灵敏度射电望远镜的不断建成和使用,以及数据数字化处理能力的提高,天文学家在球状星团射电脉冲星的观测和理论研究方面取得很大进展。收集并分析了最新的球状星团脉冲星的数据,研究了球状星团射电脉冲星的自转周期和轨道周期的基本性质,讨论了球状星团脉冲星的搜寻,最后统计分析了双星系统,包括不同伴星类型的脉冲星的分布以及掩食双星系统的性质。     

星系中球状星团的形成与演化

球状星团与星系的形成和演化过程密切相关,研究球状星团是研究星系形成和演化的重要途径之一。综述了球状星团在观测上的统计结论,观测对理论模型的限制,利用数值模拟和半解析模型研究球状星团形成和演化等阶段的进展。理论研究主要包括球状星团的形成、密度轮廓、初始质量函数,球状星团在质量、大小、空间上的演化,星系并合对球状星团形成和演化的影响等。通过介绍当前研究的主要模型、相关的研究结果以及该领域尚未解决的一些难题,阐明球状星团与星系的关系,深化对球状星团和星系形成和演化的认识。     

Galactic models and white dwarf populations

We make use of a previous well-tested Galactic model, but describing the observational behaviour of the various stellar components in terms of suitable assumptions on their evolutionary status. In this way we are able to predict the expected distribution of Galactic white dwarfs (WDs), with results which appear in rather good agreement with recent estimates of the local WD luminosity function. The predicted occurrence of WDs in deep observations of selected Galactic fields is presented, and we discuss the role played by WDs in star counts. The effects on the theoretical predictions of different white dwarf evolutionary models, ages, initial mass functions and relations between progenitor mass and WD mass are also discussed.     

The white dwarf luminosity function – II. The effect of the measurement errors and other biases

The disc white dwarf luminosity function is an important tool for studying the solar neighbourhood, since it allows the determination of several Galactic parameters, the most important one being the age of the Galactic disc. However, only the     method has been employed so far for observationally determining the white dwarf luminosity function, whereas for other kind of luminosity functions several other methods have been frequently used. Moreover, the procedures to determine the white dwarf luminosity function are not free of biases. These biases have two different origins: they can either be of statistical nature or a consequence of the measurement errors. In a previous paper we carried out an in-depth study of the first category of biases for several luminosity function estimators. In this paper we focus on the biases introduced by the measurement errors and on the effects of the degree of contamination of the input sample used to build the disc white dwarf luminosity function by different kinematical populations. To assess the extent of these biases we use a Monte Carlo simulator to generate a controlled synthetic population and analyse the behaviour of the disc white dwarf luminosity function for several assumptions about the magnitude of the measurement errors and for several degrees of contamination, comparing the performances of the most robust luminosity function estimators under such conditions.     

The impact of a merger episode in the galactic disc white dwarf population

In this paper we analyse the consequences in the white dwarf population of a hypothetical merger episode in our Galactic disc. We have studied several different merging scenarios with our Monte Carlo simulator. For each one of these scenarios we have derived the main characteristics of the resulting white dwarf population and we have compared them with the available observational data, namely the white dwarf luminosity function and the kinematic properties of the white dwarf population. Our results indicate that very recent (less than ∼6 Gyr ago) and massive (∼16 per cent of the mass of our Galaxy) merger episodes are quite unlikely in view of the available kinematical properties of the disc white dwarf population. Smaller merger episodes (of the order of ∼4 per cent of the mass of our Galaxy) are, however, compatible with our current knowledge of those kinematical properties. Finally, we prove that the white dwarf luminosity function is quite insensitive to such a merger episode.     

白矮星研究的某些进展

综合叙述了白矮星诞生率研究的近况，详细介绍了ＤＡ，非ＤＡ型白矮星质量，质量分布以及确定质量的方法，对白矮星光度函数作了较为全面的回顾，指出了一些目前白矮星研究工作中仍存在的问题。     

Simulating Gaia performances on white dwarfs

One of the most promising space missions of the European Space Agency is the astrometric satellite Gaia , which will provide very precise astrometry and multicolour photometry, for all 1.3 billion objects to   V ∼ 20  , and radial velocities with accuracies of a few km s⁻¹ for most stars brighter than   V ∼ 17  . Consequently, full homogeneous six-dimensional phase-space information for a huge number of stars will become available. Our Monte Carlo simulator has been used to estimate the number of white dwarfs potentially observable by Gaia . From this we assess the white dwarf luminosity functions that Gaia will obtain and discuss in depth the scientific returns of Gaia in the specific field of white dwarf populations. Scientifically attainable goals include, among others, a reliable determination of the age of the Galactic disc, a better knowledge of the halo of the Milky Way and the reconstruction of the star formation history of the Galactic disc. Our results also demonstrate the potential impact of a mission such as Gaia within the context of current understanding of white dwarf cooling theory.     

The initial–final mass relationship of white dwarfs revisited: effect on the luminosity function and mass distribution

The initial–final mass relationship connects the mass of a white dwarf with the mass of its progenitor in the main sequence. Although this function is of fundamental importance to several fields in modern astrophysics, it is not well constrained either from the theoretical or from the observational points of view. In this work, we revise the present semi-empirical initial–final mass relationship by re-evaluating the available data. The distribution obtained from grouping all our results presents a considerable dispersion, which is larger than the uncertainties. We have carried out a weighted least-squares linear fit of these data and a careful analysis to give some clues on the dependence of this relationship on some parameters such as metallicity or rotation. The semi-empirical initial–final mass relationship arising from our study covers the range of initial masses from 1.0 to  6.5 M_⊙  , including in this way the low-mass domain, poorly studied until recently. Finally, we have also performed a test of the initial–final mass relationship by studying its effect on the luminosity function and on the mass distribution of white dwarfs. This was done by using different initial–final mass relationships from the literature, including the expression derived in this work, and comparing the results obtained with the observational data from the Palomar Green Survey and the Sloan Digital Sky Survey. We find that the semi-empirical initial–final mass relationship derived here gives results in good agreement with the observational data, especially in the case of the white dwarf mass distribution.     

Galaxy populations in the Fornax and Virgo clusters

In these first results from a photographic survey of nearby clusters and groups of galaxies, we compare the luminosity functions of galaxies of various Hubble types in the core of the Formax cluster to luminosity functions in the core of the Virgo cluster from Sandageet al. (1985). The galaxy classifications for the two clusters are based on identical plate material, and are hence directly comparable. The properties (galaxy density, velocity dispersion, X-ray luminosity) of the two clusters are quite different, yet we find few significant differences in the luminosity functions, or in the morphological mix of galaxies between the two clusters. In particular, while there is some indication that the ratios of giants to dwarfs and early to late-type galaxies in the two clusters differ, we cannot exclude the possibility that the ratios are identical. We discuss the selection limits of our survey and the completeness corrections that must be applied to the faint end of the luminosity function. The effective surface brightness of early-type galaxies in our sample decreases with decreasing luminosity. We show that this correlation is not an artifact of our selection criteria for dwarf spheroidal galaxies and use it to determine the relative distances to the Fornax and Virgo clusters. We also present evidence for a radial variation in the luminosity function of dwarf ellipticals within the two clusters in the sense that the faint end of the luminosity distribution is enhanced at the centre of each cluster.     

A white dwarf in the state of an ejector

We analyze the possibility of observational identification of white dwarfs in the state of an ejector. Among the distinctive features of this class of objects are a high rate of rotational energy loss comparable to or higher than the observed luminosity of the object and nonthermal gamma-ray and/or radio spectra. The manifestations of the white dwarf in the close binary system AE Aquarii closely match these criteria. We show that most of the peculiar manifestations of this object in the hard spectral range and the observed pattern of mass transfer in the system can be explained in terms of a model in which the state of the white dwarf is classified as an ejector.     

Evolutionary and pulsational properties of white dwarf stars

White dwarf stars are the final evolutionary stage of the vast majority of stars, including our Sun. Since the coolest white dwarfs are very old objects, the present population of white dwarfs contains a wealth of information on the evolution of stars from birth to death, and on the star formation rate throughout the history of our Galaxy. Thus, the study of white dwarfs has potential applications in different fields of astrophysics. In particular, white dwarfs can be used as independent reliable cosmic clocks, and can also provide valuable information about the fundamental parameters of a wide variety of stellar populations, such as our Galaxy and open and globular clusters. In addition, the high densities and temperatures characterizing white dwarfs allow these stars to be used as cosmic laboratories for studying physical processes under extreme conditions that cannot be achieved in terrestrial laboratories. Last but not least, since many white dwarf stars undergo pulsational instabilities, the study of their properties constitutes a powerful tool for applications beyond stellar astrophysics. In particular, white dwarfs can be used to constrain fundamental properties of elementary particles such as axions and neutrinos and to study problems related to the variation of fundamental constants. These potential applications of white dwarfs have led to renewed interest in the calculation of very detailed evolutionary and pulsational models for these stars. In this work, we review the essentials of the physics of white dwarf stars. We enumerate the reasons that make these stars excellent chronometers, and we describe why white dwarfs provide tools for a wide variety of applications. Special emphasis is placed on the physical processes that lead to the formation of white dwarfs as well as on the different energy sources and processes responsible for chemical abundance changes that occur along their evolution. Moreover, in the course of their lives, white dwarfs cross different pulsational instability strips. The existence of these instability strips provides astronomers with a unique opportunity to peer into their internal structure that would otherwise remain hidden from observers. We will show that this allows one to measure stellar masses with unprecedented precision and to infer their envelope thicknesses, to probe the core chemical stratification, and to detect rotation rates and magnetic fields. Consequently, in this work, we also review the pulsational properties of white dwarfs and the most recent applications of white dwarf asteroseismology.     

Detached white-dwarf close-binary stars – CV's extended family

I review detached binaries consisting of white dwarfs with either other white dwarfs or low mass main-sequence stars in tight orbits around them. Orbital periods have been measured for 15 white dwarf/white dwarf systems and 22 white dwarf/M dwarf systems. While small compared to the number of periods known for CVs (>300), I argue that each variety of detached system has a space density an order of magnitude higher that of CVs. While theory matches the observed distribution of orbital periods of the white dwarf/white dwarf binaries, it predicts white dwarfs of much lower mass than observed. Amongst both types of binary are clear examples of helium core white dwarfs, as opposed to the usual CO composition; similar systems must exist amongst the CVs. White dwarf/M dwarf binaries suffer from selection effects which diminish the numbers seen at long and short periods. They are useful for the study of irradiation; I discuss evidence to suggest that Balmer emission is broadened by optical depth effects to an extent which limits its usefulness for imaging the secondary stars in CVs.