解决G矮星问题的银河系化学演化三成分模型

为解释名的Ｇ矮星问题,提出银河系化学演化的三成分模型,即由银晕、厚盘和薄盘所构成的演化模型。相邻演化阶段间隔着一个快速坍缩过程,对不同星族成分的演化过程分别进行模拟,并在总体上得到一个太阳附近区域的Ｇ矮星丰度分布函数,检验了三种不同的模型：初始富化模型、比例生成模型和坍缩模型,利用最小二乘拟合得到最佳模型的参数。结果表明,太阳附近区域的化学演化受物质交换的影响较小,至少在银河系演化的晚期,可将太     

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。     

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。     

太阳附近G矮星金属含量分布和化学演化

利用新的太阳附近２５ｐｃ内的Ｇ矮星金属含量分布函数,本文在假定星际介质丰度非均匀情况下,讨论了四种化学演化模型的合理性．结果表明,简单模型和坍缩模型与实际情况相差较大,而ＰＩＥ模型和ＰＰＹ模型不仅对金属含量分布函数拟合较好,而且传统的Ｇ矮星问题也可以得到解释．这表明太阳附近可能受银晕影响较小．     

太阳附近G矮星金属含量分布和化学演化

利用新的太阳附近２５ｐｃ内的Ｇ矮星金属含量分布函数，本在假定星系介质丰度非均匀情况下，讨论了四种化学演化模型的合理性，结果表明，简单模型和坍缩模型与实际情况相差较大，而ＰＩＥ模型和ＰＰＹ模型不仅对金属含量分布函数拟合较好，而且传统的Ｇ矮星问题也可以得到解释，这表明太阳附近可能受银晕影响较小。     

太阳附近F和G矮星的金属含量统计

收集了一个新的较为完备的太阳附近25pc内的F,G矮星样本,利用最新的窄带测光资料和定标关系导出了148颗F矮星和382颗G矮星的金属含量的分布,讨论了对银河系化学演化的意义。     

红亚矮星研究进展

红亚矮星是甚小质量恒星中的贫金属成员,质量从约0.5M⊙(M⊙为太阳质量)到H燃烧的最小质量(0:075M⊙0:085M⊙,取决于金属丰度),其寿命长于哈勃年龄,是银河系结构和化学增丰史的重要示踪体。与银盘上数量最多的恒星成员红矮星不同,红亚矮星在太阳附近非常稀少,并且其运动学特征与盘矮星有较大差异,属于年老银河系星族,即为年老盘星族、厚盘星族或晕星族。观测上,红亚矮星可以根据其不同于红矮星的自行、测光和光谱特征被识别和证认。由于其恒星表面大气温度很低,并且颜色比同质量的矮星更蓝,因此红亚矮星在赫罗图上位于主序带末端的下方,介于矮星与白矮星之间。红亚矮星的光学波段光谱由金属氧化物(如TiO和VO)和氢化物(如CaH和H2O)的分子吸收带占主导。红亚矮星可按其光谱形态和分子带特征分成不同的光谱型和金属丰度等级,其中晚M型到早L型的亚矮星既可能是小质量的恒星,也可能是较大质量的年轻褐矮星。介绍了对红亚矮星研究的历史背景和前沿动态,详细阐述了光谱分析方法在研究亚矮星中的重要性,以及根据光谱特征对亚矮星进行分类的方法。最后,总结了甚小质量恒星大气模型的发展过程,并探讨了如何利用模型对亚矮星的大气参数进行估算等热点问题。     

银河系化学演化及球状星团多星族问题研究

<正>恒星的化学元素丰度特征能够反映其形成和演化历史.以化学元素丰度为手段,研究了银河系中恒星的径向迁移对银河系化学演化的影响,以及球状星团中渐进巨星支(AGB)恒星的多星族问题.近年的观测和理论研究表明:恒星在银盘里有径向迁移.基于详细的银河系化学演化模型,再采用分布函数模拟恒星径向迁移过程,研究了恒星的径向迁移对银盘径向元素丰度梯度的影响.结果显示:     

星族合成

以简单星族为例介绍了演化星族合成的算法 ,并总结了在演化星族合成中常用的恒星演化库 ,光谱库 ,初始质量函数和合成判据 ,最后简要讨论了目前星族合成中仍存在的问题。     

银河系化学演化研究进展

银河系的形成与演化是天体物理学研究的重大前沿课题,银河系的化学演化在其中更具有极其重要的地位。随着观测资料的不断积累和理论工作的不断深入,银河系化学演化的研究取得了一系列进展。在观测方面,从太阳附近区域,整个银盘,银晕和核球等方面简要回顾了银河系化学演化模型主要观测约束的近期结果;在化学演化模型方面,回顾了银河系化学演化研究的发展历程和近期进展,并对未来的研究进行了展望。     

Solving the G-dwarf problem with a three-component model of the chemical evolution of the Galaxy

A three-component chemical evolution model of the Galaxy is presented, which we believe will cast a new light on the G-dwarf problem. The model is based on a scenario of the Galaxy consisting of three major evolutionary phases: halo, thick disk and thin disk, separated by two short interludes of rapid collapse. The evolution of different stellar populations are treated separately, the combination of which yields an overall metallicity distribution function for the solar neighbourhood. We tested three different models using the same set of basic equations: the “prompt initial enrichment” (PIE) model, the “proportional yield” (PPY) model and the “collapse” (CLP) model. Best-fit parameters are derived. The results show that the different populations have remarkably different IMFs, while mass exchange has only minimally affected the chemical evolution in the solar vicinity, so that the solar vicinity can be regarded as a closed system, at least in the late stage of the Galactic evolution.     

The G-dwarf problem in the solar neighbourhood: a Lagrangian picture

Recent data on the empirical metallicity distribution of G dwarfs in the disk solar neighbourhood are fitted in two different ways. We use an extended Poisson distribution in the limit where the probability of star formation is small, and a Gauss distribution in the limit where a large number of physical variables is required to determine stellar metal abundance. Both are found to reproduce the data at the same (acceptable) extent, with a slight preference for the former. The emprirical, differential metallicity distribution of G dwarfs in the disk solar neighbourhood is compared with its theoretical counterpart, in the picture of a closed, comoving model of chemical evolution. The limits of the currently used infall models are discussed and a scenario of galactic formation and evolution is presented. The Galactic history is thought as made of two main phases: contraction (which produces the extended component) and equilibrium (which gives the disk). In this view, the stars observed within the solar cylinder did not necessarily arise from the primordial gas which later collapsed into the disk solar neighbourhood. It is found that the G-dwarf problem is strongly alleviated, with the possible exception of the low-metallicity and high-metallicity tail of the distribution. The best choice of parameters implies: (i) a metal yield in the contraction phase which is larger by a factor of about 5 with respect to the equilibrium phase; (ii) a model halo mass fraction of about 0.3; (iii) a model disk mass fraction of about 0.6. It provides additional support to the idea of a generalized Schmidt star formation law, which is different in different phases of evolution. The model, cumulative, G-dwarf metallicity distribution in the disk solar neighbourhood is found to predict too may low-metallicity stars with respect to its empirical counterpart, related to a Poissonian or Gaussian fit. The main resons for the occurrence of a G-dwarf problem are discussed. Finally, a stochastic process of star formation, related to a Poisson distribution, is briefly outlined.     

History of Star Formation and Chemical Enrichment in the Milky Way Disk

Based on a physical treatment of the star formation law similar to that given by Efstathiou, we have improved our two-component chemical evolution model for the Milky Way disk. Two gas infall rates are compared, one exponential, one Gaussian. It is shown that the star formation law adopted in this paper depends more strongly on the gas surface density than that in Chang et al. It has large effects on the history of star formation and gas evolution of the whole disk. In the solar neighborhood, the history of chemical evolution and star formation is not sensitive to whether the infall rate is Gaussian or exponential. For the same infall time scale, both forms predict the same behavior for the current properties of the Galactic disk. The model predictions do depend on whether or not the infall time scale varies with the radius, but current available observations cannot decide which case is the more realistic. Our results also show that it would be inadequate to describe the gradient evolution along the Gala     

Toward a more consistent evolution model for the local galactic disk

Infall models for the evolution of the local galactic disk were studied and confronted with a large number of observational constraints from the solar vicinity, inclusive of the white dwarf luminosity function. The models are characterized as follows: 1. The key-functions (SFR, IMF, gas infall rate) are not prescribed by simple laws, but are directly derived from observational constraints. 2. A scatter in the metallicity at fixed age is considered which partly reflects inhomogeous chemical evolution. 3. Special attention is drawn to the internal consistency of the models. 4. In addition to infall of low-metallicity gas, metal-enriched outflows are allowed. The “best” model is characterized by a disk age of ≈︁ 12 Gyr, a SFR which is decreasing over the first half and is nearly constant over the second half of the disk evolution, and by a similar temporal run of the gas infall rate. Moderate metal-enriched outflow can not be excluded.     

The G‐dwarf problem in the solar neighbourhood: II. Halo subdwarfs

The oxygen abundance distribution in solar neighbourhood halo subdwarfs is deduced, using two alternative, known empirical relationships, involving the presence or the absence of [O/Fe] plateau for low [Fe/H] values, from a sample of 372 kinematically selected halo stars, for which the iron abundance distribution has been determined by Ryan & Norris (1991). The data are interpreted by a simple, either homogeneous or inhomogeneous model of chemical evolution, using an updated value of the solar oxygen abundance. The effect of changing the solar oxygen abundance, the power‐law exponent in the initial mass function, and the rate of oxygen nucleosyntesis, keeping the remaining input parameters unchanged, is investigated, and a theorem is stated. In all cases, part of the gas must necessarily be inhibited from forming stars, and no disk contamination has to be advocated for fitting the empirical oxygen abundance distribution in halo subdwarfs of the solar neighbourhood (EGD). Then a theorem is stated, which allows a one‐to‐one correspondence between simple, homogeneous models with and without inhibited gas, related to same independent parameters of chemical evolution, except lower stellar mass limit, real yield, and inhibition parameter. The mutual correlations between the latter parameters are also specified. In addition the starting point, and the point related to the first step, of the theoretical distribution of oxygen abundance (TGD) predicted by simple, inhomogeneous models, is calculated analytically. The mean oxygen abundance of the total and only inhibited gas, respectively, are also determined. Following the idea of a universal, initial mass function (IMF), a power‐law with both an exponent p = 2.9, which is acceptably close to Scalo IMF for m ≳ m_⊙, and an exponent p = 2.35, i.e. Salpeter IMF, have been considered. In general, both the age‐metallicity relationship and the empirical distribution of oxygen abundance in G dwarfs of the disk solar neighbourhood, are fitted by power‐law IMF exponents in the range 2.35 ≤ p ≤ 2.9. Acceptable models predict about 15% of the total mass in form of long‐lived stars and remnants, at the end of halo evolution, with a mean gas oxygen abundance which is substantially lower than the mean bulge and initial disk oxygen abundance. To avoid this discrepancy, either the existence of a still undetected, baryonic dark halo with about 15% of the total mass, or an equal amount of gas loss during bulge and disk formation, is necessary. The latter alternative implies a lower stellar mass limit close to 0.2 m_⊙, which is related to a power‐law IMF exponent close to 2.77. Acceptable models also imply a rapid halo formation, mainly during the first step, Δt = 0.5 Gyr, followed by a period (three steps) where small changes occur. Accordingly, statistical fluctuations are found to produce only minor effects on the evolution.     

The evolution of the water distribution in a viscous protoplanetary disk

Astronomical observations have shown that protoplanetary disks are dynamic objects through which mass is transported and accreted by the central star. This transport causes the disks to decrease in mass and cool over time, and such evolution is expected to have occurred in our own solar nebula. Age dating of meteorite constituents shows that their creation, evolution, and accumulation occupied several Myr, and over this time disk properties would evolve significantly. Moreover, on this timescale, solid particles decouple from the gas in the disk and their evolution follows a different path. It is in this context that we must understand how our own solar nebula evolved and what effects this evolution had on the primitive materials contained within it. Here we present a model which tracks how the distribution of water changes in an evolving disk as the water-bearing species experience condensation, accretion, transport, collisional destruction, and vaporization. Because solids are transported in a disk at different rates depending on their sizes, the motions will lead to water being concentrated in some regions of a disk and depleted in others. These enhancements and depletions are consistent with the conditions needed to explain some aspects of the chemistry of chondritic meteorites and formation of giant planets. The levels of concentration and depletion, as well as their locations, depend strongly on the combined effects of the gaseous disk evolution, the formation of rapidly migrating rubble, and the growth of immobile planetesimals. Understanding how these processes operate simultaneously is critical to developing our models for meteorite parent body formation in the Solar System and giant planet formation throughout the galaxy. We present examples of evolution under a range of plausible assumptions and demonstrate how the chemical evolution of the inner region of a protoplanetary disk is intimately connected to the physical processes which occur in the outer regions.     

The formation of a disk galaxy within a slowly growing dark halo

The formation of a disk galaxy within a slowly growing dark halo is simulated with a new chemo-dynamical model. The model describes the evolution of the stellar populations, the multi-phase ISM and all important interaction. I find, that the galaxy forms radially from inside-out and vertically from top-to-bottom. The derived stellar age distributions show that the inner halo is the oldest component, followed by the outer halo, the triaxial bulge, the halo-disk transition region and the disk. Despite the still idealized model, the final galaxy resembles present-day disk galaxies in many aspects. In particular, the stellar metallicity distribution in the halo of the model resembles the one of M31. The bulge in the model shows, at least two stellar subpopulations, an early collapse population and a population that formed later out of accreted disk mass. In the stellar metallicity distribution of the disk, I find a pronounced ‘G-dwarf problem’ which is the result of a pre-enrichment of the disk ISM with metal-rich gas from the bulge. This revised version was published online in September 2006 with corrections to the Cover Date.     

Chemical evolution of two-component galaxies

In order to confirm and refine the results obtained in a previous paper (hereafter referred to as Paper II), the chemical evolution of two-component (spheroid+disk) galaxies is derived rejecting the instantaneous recycling approximation, by means of numerical computations, accouting for (i) the collapse phase of the gas, assumed to be uniform in density and composition, and (ii) a birth-rate stellar function where μ can be related to the usual density term whilex is a volume term first introduced in Paper II; computations are performed relatively to the solar neighbourhood and to model galaxies which closely resemble the real morphological sequence: in both cases, numerical results are compared with analytical ones. As regards the solar neighbourhood, we find the parametersn andq changed to about the value 10–20% with respect to the analytical one, andZ(t) (i.e., metallicity curve) values lowered to about the value?25% with respect to the analytical one; moreover, spheroid star birth is intermediate between an initial burst and an uniform generation, while more than 90% with respect to the present-day (gas+star) disk is primary. As regards other galaxies, we find that when the disk component is dominant (i.e., in late-type spirals or irregulars) the numerical present-day values of the gas mass-fraction μ(T) and of the metallicityZ(T) differ by a few per cent from the corresponding analytical ones, while the main part of present-day disks is primary; on the contrary, when the spheroid component is dominant, μ(T) andZ(T) are ≈50% different from the corresponding analytical ones, while a substantial fraction of present-day disks is secondary: moreover, super-metallic effect takes place, as a direct consequence of rejecting instantaneous recycling. However, the qualitative conclusions of Paper II continue to hold, so we can state that analytical models constitute a zero-th order approximation in describing the chemical evolution of both solar neighbourhood and other galaxies; numerical models of this paper constitute a first-order approximation, while higher order approximations could be made by rejecting the hypothesis of uniform density and composition, and making use of detailed dynamical models.     

Gap formation in the dust layer of 3D protoplanetary disks

We numerically model the evolution of dust in a protoplanetary disk using a two-phase (gas+dust) Smoothed Particle Hydrodynamics (SPH) code, which is non-self-gravitating and locally isothermal. The code follows the three dimensional distribution of dust in a protoplanetary disk as it interacts with the gas via aerodynamic drag. In this work, we present the evolution of a disk comprising 1% dust by mass in the presence of an embedded planet for two different disk configurations: a small, minimum mass solar nebular (MMSN) disk and a larger, more massive Classical T Tauri star (CTTS) disk. We then vary the grain size and planetary mass to see how they effect the resulting disk structure. We find that gap formation is much more rapid and striking in the dust layer than in the gaseous disk and that a system with a given stellar, disk and planetary mass will have a different appearance depending on the grain size and that such differences will be detectable in the millimetre domain with ALMA. For low mass planets in our MMSN models, a gap can open in the dust disk while not in the gas disk. We also note that dust accumulates at the external edge of the planetary gap and speculate that the presence of a planet in the disk may facilitate the growth of planetesimals in this high density region.