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arXiv 1909.07591astro-ph.SR
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A型亚矮星的形成

The formation of subdwarf A-type stars

Jinlong Yu, Zhenwei Li, Chunhua Zhu, Zhaojun Wang, Helei Liu, Sufen Guo, Zhanwen Han, Xuefei Chen, Guoliang Lü

摘要:

亚矮星A型星(英语:Subdwarf A-type stars)是一种光谱富含氢的天体,其表面重力与热亚矮星相似,但有效温度低于零年水平分支(ZAHB)。它们被认为是贫金属主序星(MS)或极低质量的白矮星(ELM WD)。本文利用恒星演化程序--恒星天体物理学实验模块(MESA),采用单演化模型,研究了双简并系统(DDs)中(前)ELM WDs和贫金属主序星演化形成的sdAs。我们发现ELM WDs和贫金属MS的演化轨迹都可以解释sdAs的观测特性。然而,这两个群体之间的比例是不确定的。本文采用盘中ELM WDs和晕中贫金属MS的二元布居合成方法,得到了不同恒星星族年龄下ELM WDs和贫金属MS的布居,并计算了它们的比例。我们发现,对于10 Gyr的恒星星族,贫金属MS与sdA种群的比例为$\sim$ 98.5%,这与大多数sdA($>$ 95%)是贫金属MS的结论一致。当恒星星族年龄从5 Gyr增加到13.7 Gyr时,ELM WD(贫金属MS)占sdA星族的比例从0.1%(99.9%)增加(减少)到20%(80%).

英文摘要:

Subdwarf A-type stars (sdAs) are objects that have hydrogen-rich spectra with surface gravity similar to that of hot subdwarf stars but effective temperature below the zero-age horizontal branch (ZAHB). They are considered to be metal-poor main sequence (MS) stars or extremely low-mass white dwarfs (ELM WDs). In this work, using the stellar evolution code, Modules for Experiments in Stellar Astrophysics (MESA), we investigate the sdAs formed both by the evolution of (pre-)ELM WDs in double degenerate systems (DDs) and metal-poor main sequence with the single evolution models. We find that both of the evolutionary tracks of ELM WDs and metal-poor MSs can explain the observation properties of sdAs. However, the proportion between these two populations are uncertain. In this work, we adopt the method of binary population synthesis of both ELM WDs in the disk and metal-poor MS in the halo to obtain the populations of ELM WDs and metal-poor MSs at different stellar population ages and calculate their proportions. We find that the proportion of metal-poor MSs to sdA population for a stellar population of 10 Gyr is $\sim$98.5 percent, which is consistent with the conclusion that most sdAs ($>$95 percent) are metal-poor MSs. And the proportion of ELM WDs (metal-poor MSs) to sdA population increases (decreases) from 0.1\% (99.9\%) to 20\% (80\%) with the stellar population ages from 5 to 13.7 Gyr.

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