河外星系中的厚盘

自1979年以来,越来越多的观测证据表明,在许多盘状星系中普遍存在厚盘成分,它们在结构、化学组成和运动学等方面与薄盘有着明显的不同.一般而言,星系中厚盘的标高比薄盘大得多,厚盘恒星与薄盘恒星相比,金属丰度较低而年龄较老,绕星系中心的转动速度较慢而速度弥散度较大,上述观测证据表明厚盘确实是星系长期演化后的产物,对于研究河外星系的形成和演化史有着重要的天体物理意义.该文在简单回顾银河系和河外星系中厚盘发现和确认史的基础上.介绍了对星系厚盘结构参数等性质的探测途径和目前取得的一些主要认识,并与薄盘恒星的相应特征作了比较,结构参数主要包括厚盘和薄盘的标长、标高和轴比,其他观测性质有两类盘的颜色、年龄、金属度和运动学特性等.最后,结合上述观测性质,对厚盘形成的几种可能的机制及未来可以开展的工作做了简要的说明和讨论.     

银盘恒星的年龄-金属丰度关系研究

该文回顾了恒星的AMR(年龄-金属丰度关系)研究的历史,评述了研究的现状;介绍和比较了确定恒星年龄和金属丰度的有关方法;分析和讨论了最近有关AMR研究的4个大样本工作,分别利用每两样本之间的共同样本星,详细比较了它们分别给出的恒星年龄、金属丰度和AMR;结果表明样本的选择效应以及确定恒星年龄和金属丰度的方法或采用参数的不同都会影响AMR.通过比较选取了恒星年龄比较一致且金属丰度精度相对较高的2个样本,分别包括4 007和1 042颗恒星,用纯运动学标准确定了各自的星族成分,分别讨论了薄盘和厚盘恒星的AMR,结果显示厚盘恒星的存在明显的AMR,而薄盘恒星的A:MR不如厚盘那么明显,也提出了进一步研究AMR需要开展的几项工作.     

银河系厚盘恒星元素丰度研究

自20世纪80年代开始的观测表明,银河系存在厚盘结构,但其形成机制至今还是一个没有解决的问题。近年来很多工作发现,厚盘恒星的α元素丰度随金属丰度的变化趋势与薄盘不同, 这种不同还表现在Al、Mn、Zn、中子俘获元素等其他元素上。结合年龄、运动学参数研究, 恒星元素丰度可以对银河系厚盘形成机制提供观测依据。     

银河系厚盘的本地空间密度

本文利用北银极天区的现代恒星计数资料，采用标准的银河系模型，通过恒星统计积分方程对参数值组合模型的亮度函数进行计数分析，证明了银河系厚盘恒星成份存在的必要性，并进一步得出太阳附近厚盘成份的空间密度的最佳期望值为薄盘密度的０．０４１．     

银河系的厚盘成分和现代恒星计数分析

在本文中介绍有关银河系成分的认识及星族概念的新进展，对近年新发现的一个恒星成分、厚盘”的主要特性以及利用现代恒星计数分析测定厚盘大尺度结构参数的方法作了较详细的论述。     

观测结果对三成分多分量星系化学演化模型的约束

本文是在银河系化学演化的基础上，利用银河系的三成分(threezone)(即晕、厚盘和薄盘)多相(multi-phase)(气体，分子云，大、小质量恒星以及剩余物质)的化学演化的理论模型，讨论了以下观测约束：1、质量面密度、恒星形成率，各分区质量比；2、场星的年龄-金属丰度关系；3、α元素化学演化；4、太阳附近G矮星金属丰度分布；5、三成分金属丰度特征量；6、超新星爆发率；7、内落速率。结果表明，三成分多分量模型能够较好地满足观测约束，比较真实地反映星系演化过程。可以用该模型计算元素的星系化学演化。     

银河系铝元素丰度研究

恒星的Al元素丰度可以为探索星团和星系的化学演化提供重要线索.通过系统分析银河系薄盘、厚盘、核球、银晕以及M4、M5等球状星团中恒星的[Al/Fe]随恒星金属丰度[Fe/H]的变化趋势,得出银河系薄盘、厚盘和核球恒星的[Al/Fe]随着[Fe/H]的增加而缓慢下降,而球状星团M4和M5恒星的[Al/Fe]随[Fe/H]增加没有下降趋势,这暗示Ia超新星对M4和M5恒星元素丰度的贡献比较小.详细研究了银河系恒星[Al/Fe]与[Mg/Fe]、[Na/Fe]的相关性,结果表明银河系场星的[Al/Fe]与[Mg/Fe]正相关,但在球状星团M4和M5恒星中未见此相关性;银河系盘星及M4和M5等球状星团恒星的[Al/Fe]与[Na/Fe]都存在正相关.     

行星状星云的形状与环绕的伴星有关

有迹象表明,实际上所有的中、高金属度恒星都环绕着某种东西。天文学家是通过观察恒星膨胀的气体遗迹得出这一推论的。以色列海法大学索科尔认为一颗濒死恒星在经历红巨星阶段后抛射的行星状星云的形状揭示了它的轨道伴星的大小和本原。 行星状星云是宇宙中最漂亮的     

观测结果对三成分多分量星系化学演化模型的约束

本文是在银河系化学演化的基础上,利用银河系的三成分(threezone)(即晕、厚盘和薄盘)多相(multi phase)(气体,分子云,大、小质量恒星以及剩余物质)的化学演化的理论模型,讨论了以下观测约束:1、质量面密度、恒星形成率,各分区质量比;2、场星的年龄-金属丰度关系;3、α元素化学演化;4、太阳附近G矮星金属丰度分布;5、三成分金属丰度特征量;6、超新星爆发率;7、内落速率。结果表明,三成分多分量模型能够较好地满足观测约束,比较真实地反映星系演化过程。可以用该模型计算元素的星系化学演化。     

银河系厚盘及其形成机制

银河系厚盘的发现，对于研究银河系以至星系的结构和演化具有重要意义。在简单回顾银河系结构研究史和厚盘发现过程的基础上，综合介绍了人们对银河系厚盘各方面性质认识的现状，并对迄今为止所提出的几种厚盘形成机制作了比较详细的说明和讨论。就目前来看，与伴星系的并合可能是形成厚盘最为可能的机制。     

Information from the Kinematics of F and G Stars in the Solar Neighborhood

We have calculated the orbital parameters for 90 stars in Chen et al. and updated the kinematic data for stars in Edvardsson et al. by using the accurate Hipparcos parallaxes and proper motions, and recalculated the \\\\\\\\\\\\-element abundances in Edvardsson et al. in a way consistent with Chen et al. The two sets of data are combined in a study of stellar populations and characteristics of F & G stars in the solar neighborhood. We confirm the result of Chen et al. that a distinguishable group of stars may belong to the thick disk rather than the thin disk. The ages for the stars are determined using the theoretical isochrones of VandenBerg et al. The age-metallicity relation is investigated for different subgroups according to distance from the sun and galactic orbital parameters. It is found that a mixing of stars with different orbital parameters significantly affect the age-metallicity relation for the disk. Stars with orbits confined to the solar circle all have metallicities [Fe/H] > -0.3 irresp     

Formation of galactic subsystems in light of the magnesium abundance in field stars: The thick disk

The space velocities and Galactic orbital elements of stars calculated from the currently available high-accuracy observations in our compiled catalog of spectroscopic magnesium abundances in dwarfs and subgiants in the solar neighborhood are used to identify thick-disk objects. We analyze the relations between chemical, spatial, and kinematic parameters of F–G stars in the identified subsystem. The relative magnesium abundances in thick-disk stars are shown to lie within the range 0.0 < [Mg/Fe] < 0.5 and to decrease with increasingmetallicity starting from [Fe/H] ≈ ?1.0. This is interpreted as evidence for a longer duration of the star formation process in the thick disk. We have found vertical gradients in metallicity (grad_Z[Fe/H] = ?0.13 ± 0.04 kpc^?1) and relative magnesium abundance (grad_Z[Mg/Fe] = 0.06 ± 0.02 kpc^?1), which can be present in the subsystem only in the case of its formation in a slowly collapsing protogalaxy. However, the gradients in the thick disk disappear if the stars whose orbits lie in the Galactic plane, but have high eccentricities and low azimuthal space velocities atypical of the thin-disk stars are excluded from the sample. The large spread in relative magnesium abundance (?0.3 < [Mg/Fe] < 0.5) in the stars of the metal-poor “tail” of the thick disk, which constitute ≈8% of the subsystem, can be explained in terms of their formation inside isolated interstellar clouds that interacted weakly with the matter of a single protogalactic cloud. We have found a statistically significant negative radial gradient in relative magnesium abundance in the thick disk (grad_R[Mg/Fe] = ?0.03 ± 0.01 kpc^{? 1}) instead of the expected positive gradient. The smaller perigalactic orbital radii and the higher eccentricities for magnesium-richer stars, which, among other stars, are currently located in a small volume of the Galactic space near the Sun, are assumed to be responsible for the gradient inversion. A similar, but statistically less significant inversion is also observed in the subsystem for the radial metallicity gradient.     

The local oxygen abundance evolution in models with galactic dust expulsion

The abundance evolution of oxygen in the local galactic disk is discussed. The age-metallicity relation of nearby stars is confronted with predictions from simple evolution models where infall of primordial gas and outflow of disk gas and dust are allowed. Gas infall and expulsion of dust grains have considerable effects on the age-metallicity relation. Dust outflow may be important in order to solve the discrepancy between the observed rate of infall of primordial gas and rates predicted by simple evolution models.     

A Study of the Scale Height of the Thin Galactic Disk in the Solar Neighborhood

An attempt is made to determine the scale height of the thin Galactic disk in the solar neighborhood using various stellar samples from the accurate astrometric data of the Hipparcos satellite. Using the Tycho color indices and considering the completeness of the samples, several samples were selected from the main sequence and the horizontal branch. Based on two rather complete samples, it is found that the scale height of the Galactic disk defined by the O-B type main sequence stars is 103.1 ± 3.0 pc and the Sun is located at height of 15.2 ± 7.3 pc above its mean plane; while the scale height of the Galactic disk defined by the horizontal branch stars is 144.0 the ±10.0 pc and the Sun is located at height 3.5 ± 5.4 pc above the mean plane. Owing to the incompleteness of observational samples, it is not possible to use the main sequence stars of spectral types A, F, G, K and M to make a reliable determination of the scale height.     

Formation of galactic subsystems in light of the magnesium abundance in field stars: The thin disk

Data from our compiled catalog of spectroscopically determined magnesium abundances in stars with accurate parallaxes are used to select thin-disk dwarfs and subgiants according to kinematic criteria. We analyze the relations between the relative magnesium abundances in stars, [Mg/Fe], and their metallicities, Galactic orbital elements, and ages. The [Mg/Fe] ratios in the thin disk at any metallicity in the range ?1.0 dex <[Fe/H] < ?0.4 dex are shown to be smaller than those in the thick disk, implying that the thin-disk stars are, on average, younger than the thick-disk stars. The relative magnesium abundances in such metal-poor thin-disk stars have been found to systematically decrease with increasing stellar orbital radii in such a way that magnesium overabundances ([Mg/Fe] > 0.2 dex) are essentially observed only in the stars whose orbits lie almost entirely within the solar circle. At the same time, the range of metallicities in magnesium-poor stars is displaced from ?0.5 dex < [Fe/H] < +0.3 dex to ?0.7 dex < [Fe/H] < +0.2 dex as their orbital radii increase. This behavior suggests that, first, the star formation rate decreases with increasing Galactocentric distance and, second, there was no star formation for some time outside the solar circle, while this process was continuous within the solar circle. The decrease in the star formation rate with increasing Galactocentric distance is responsible for the existence of a negative radial metallicity gradient (grad _R[Fe/H] = ?0.05 ± 0.01 kpc^?1) in the disk, which shows a tendency to increase with decreasing age. At the same time, the relative magnesium abundance exhibits no radial gradient. We have confirmed the existence of a steep negative vertical metallicity gradient (grad _Z[Fe/H] = ?0.29 ± 0.06 kpc^?1) and detected a significant positive vertical gradient in relative magnesium abundance (grad _Z[Mg/Fe] = 0.13 ± 0.02 kpc^?1); both gradients increase appreciably in absolute value with decreasing age. We have found that there is not only an age-metallicity relation, but also an age-magnesium abundance relation, in the thin disk. We surmise that the thin disk has a multicomponent structure, but the existence of a negative trend in the star formation rate along the Galactocentric radius does not allow the stars of its various components to be identified in the immediate solar neighborhood.     

银河系金属丰度及其演化研究新进展

银河系的银盘、银晕、核球的平均金属丰度为－０．２,－１．６,－０．２。年龄－金属丰度关系给出了银河系形成和演化的线索。为了解释观测的年龄－金属丰度关系及其弥散,银河系化学演化模型必须考虑恒星轨道运动、非瞬时混合等机制。     

The G-dwarf problem in the Galactic spheroid

This paper has two parts: one about observational constraints, and the other about chemical evolution models. In the first part, the empirical differential metallicity distribution (EDMD) is deduced from two different samples involving (i) 268 K-giant bulge stars [Sadler, E.M., Rich, R.M., Terndrup, D.M., 1996. AJ 112, 171], and (ii) 149 globular clusters [Mackey, A.D., van den Bergh, S., 2005. MNRAS 360, 631], in addition to previous results (Caimmi, R., 2001b, AN 322, 241 (C01)) related to (iii) 372 solar neighbourhood halo subdwarfs [Ryan, S.G., Norris, J.E., 1991. AJ 101, 1865]. Under the assumption that each distribution is typical for the corresponding subsystem, the EDMD of the Galactic spheroid is determined by weighting the mass. The empirical age-metallicity relation (EAMR) involving absolute ages is deduced from recent results related to a homogeneous sample of globular clusters [De Angeli, F., Piotto, G., Cassisi, S., et al., 2005. AJ 130, 116]. In the second part, models of chemical evolution for the Galactic halo and bulge are computed, assuming the instantaneous recycling approximation. The EDMD data are fitted, to an acceptable extent, by simple models of chemical evolution implying both homogeneous and inhomogeneous star formation, provided that star formation is inhibited during halo formation and enhanced during bulge formation, with respect to the disk solar neighbourhood, taken to be representative of the whole disk. The initial mass function (IMF) is assumed to be a universal power law, which implies the same value of the true yield in different subsystems. The theoretical differential metallicity distribution (TDMD) is first determined for the halo and the bulge separately, and then for the Galactic spheroid by weighting the mass. The EAMR cannot be fitted into the Simple model that implies homogeneous star formation, but shows a non-monotonic trend characterized by large dispersion. On the other hand, simple models involving inhomogeneous star formation yield a theoretical age-metallicity relation (TAMR) which reproduces the data to an acceptable extent. For gas ouflow from the proto-halo, acceptable models give rise to different predictions in different alternatives. If the Galactic spheroid and disk underwent decoupled chemical evolution, i.e. no gas exchange between the related reservoirs, less than one third of the bulge mass outflowed from the proto-halo. If the Galactic spheroid and disk underwent coupled chemical evolution, i.e. some gas exchange between the related reservoirs, the existence of an unseen baryonic halo (or equivalent amount of gas lost by the Galaxy) with mass comparable to bulge mass, is necessarily needed. In this view, the outflowing proto-halo gas which remains bound to the Galaxy, produces both the bulge and the disk.     

Radial velocities andDDO, BV photometry of Henry Draper G5-M stars near the North Galactic Pole

Radial velocities are given for some 900 stars within 15‡ of the North Galactic Pole, including almost all such stars classified G5 or later in theHenry Draper Catalogue. Luminosities, two-dimensional spectral classes, composition indices, and distances are derived for the majority of the sample throughDDO andBV photometry. More than half of the stars are classified as G5-K5 giants: they show a clear relationship between composition and velocity dispersion for the two Galactic componentsV andW, and a less well-defined trend forU. Four abundance groups exhibit characteristics which imply association with, respectively, the thick disk, old thin disk, young thin disk, and Roman’s “4150” group. The sample is contained within l kpc of the Galactic plane, and no trends with distance are evident.