The Chemical Evolution of the Galaxy: The Two-Infall Model
The Chemical Evolution of the Galaxy: The Two-Infall Model
复制标题
DOI：
10.1086/303726
复制
发表时间：
1996-09
期刊：
影响因子：
--
通讯作者：
C. Chiappini;F. Matteucci;R. Gratton
中科院分区：
文献类型：
--
作者：
C. Chiappini;F. Matteucci;R. Gratton
我们提出了一个新的银河系化学演化模型，该模型假设了两个主要的落入幕，分别为晕厚盘和薄盘的形成。我们没有试图显式地考虑光晕的演化，因为我们的模型更适合于计算圆盘(厚和薄)的演化，但我们隐含地假设，光晕形成的时标与厚盘形成的时标具有相同的量级。形成薄盘的时间比形成厚盘的时间长得多，说明形成薄盘的气体不仅来自厚盘，而且主要来自星际介质。薄盘形成的时间刻度被认为是银心距离的函数，这导致了银河系建筑的一幅由内而外的照片。该模型考虑了最新的核合成处方，并在恒星形成过程中采用了一个阈值，正如最近的观察所表明的那样，这自然会在厚盘阶段结束时产生恒星形成速率的中断。模型的结果与太阳近邻和整个圆盘的一组扩展的观测约束进行了比较。在这些限制中，最严格的是G矮星的金属丰度分布，现在有了新的数据。我们的模型很好地符合这些新数据。该模型还预测了气体质量、恒星形成率、超新星速率以及16种化学元素丰度随时间和银心距离的变化。我们发现，为了重现这些约束中的大多数，对于(晕环)厚盘来说，时间标度为≤1 Gyr，对于在太阳附近形成的薄盘，时间标度为8 Gyr。我们预测盘内区(R<；1R☉)的径向丰度梯度比外区更陡峭，最近的丰度测定证实了这一结果，并且在银河系寿命期间，内区的丰度梯度变陡。讨论了在恒星形成过程开始时假定一个阈值气体密度的重要性和优点。
We present a new chemical evolution model for the Galaxy that assumes two main infall episodes, for the formation of the halo-thick disk and thin disk, respectively. We do not try to take into account explicitly the evolution of the halo since our model is better suited for computing the evolution of the disk (thick plus thin), but we implicitly assume that the timescale for the formation of the halo was of the same order as the timescale for the formation of the thick disk. The formation of the thin disk is much longer than that of the thick disk, implying that the infalling gas forming the thin disk comes not only from the thick disk but mainly from the intergalactic medium. The timescale for the formation of the thin disk is assumed to be a function of Galactocentric distance, leading to an inside-out picture for the Galaxy's building. The model takes into account the most up-to-date nucleosynthesis prescriptions and adopts a threshold in the star formation process, which naturally produces a hiatus in the star formation rate at the end of the thick-disk phase, as suggested by recent observations. The model results are compared with an extended set of observational constraints both for the solar neighborhood and for the whole disk. Among these constraints, the tightest is the metallicity distribution of the G-dwarf stars, for which new data are now available. Our model fits these new data very well. The model also predicts the evolution of the gas mass, the star formation rate, the supernova rates, and the abundances of 16 chemical elements as functions of time and Galactocentric distance. We show that, in order to reproduce most of these constraints, a timescale of ≤1 Gyr for the (halo) thick disk and of 8 Gyr for the thin disk's formation in the solar vicinity are required. We predict that the radial abundance gradients in the inner regions of the disk (R < 1 R☉) are steeper than in the outer regions, a result confirmed by recent abundance determinations, and that the inner gradients steepen during the Galactic lifetime. The importance and the advantages of assuming a threshold gas density for the onset of the star formation process are discussed.