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arXiv 1503.00537astro-ph.SR
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碳、氮和$α$元素丰度决定了银河系厚盘和薄盘的形成顺序

Carbon, nitrogen and $α$-element abundances determine the formation sequence of the Galactic thick and thin disks

T. Masseron, G. Gilmore

备注：
12 pages, 14 figures, accepted in MNRAS
发表信息：
Monthly Notices of the Royal Astronomical Society 2015 453 (2): 1855-1866
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摘要：

利用DR 12公布的APOGEE数据，我们发现在[ $α$ /Fe]，[Fe/H]和[C/N]定义的空间中，薄盘和厚盘分离得很好。厚盘巨星的[C/N]和[ $α$ /Fe]都比[Fe/H]相近的薄盘星高。我们推断，厚盘是由低质量的恒星比薄盘。考虑到在给定的金属丰度下，恒星质量和年龄之间存在一一对应的关系，我们就能够推断出盘形成的年代学。由[ $α$ /Fe]定义的厚盘和薄盘都在[Fe/H] $\approx$-0.7处收敛，这取决于[C/N]和[C+N/Fe]。我们的结论是：1）大多数厚盘星比薄盘星更早形成; 2）即使[Fe/H]丰度相似，薄盘和厚盘的形成历史也在早期出现分歧; 3）在所有金属丰度下，薄盘中的恒星形成率都低于厚盘。虽然这些一般性的结论仍然是强大的，我们也表明，目前的恒星演化模型不能再现厚盘星的C/N比。出乎意料的是，减少或抑制典型的额外混合是非常常见的场星。虽然受到丰度校准零点的不确定性，这意味着一个强烈的依赖性非典型的额外混合沿红巨星分支的初始组成的恒星，特别是对$α$元素丰度。

英文摘要：

Using the DR12 public release of APOGEE data, we show that thin and thick disk separate very well in the space defined by [$α$/Fe], [Fe/H] and [C/N]. Thick disk giants have both higher [C/N] and higher [$α$/Fe] than do thin disk stars with similar [Fe/H]. We deduce that the thick disk is composed of lower mass stars than the thin disk. Considering the fact that at a given metallicity there is a one-to-one relation between stellar mass and age, we are then able to infer the chronology of disk formation. Both the thick and the thin disks - defined by [$α$/Fe] -- converge in their dependance on [C/N] and [C+N/Fe] at [Fe/H]$\approx$-0.7. We conclude that 1) the majority of thick disk stars formed earlier than did the thin disk stars 2) the formation histories of the thin and thick disks diverged early on, even when the [Fe/H] abundances are similar 3) that the star formation rate in the thin disk has been lower than in the thick disk, at all metallicities. Although these general conclusions remain robust, we also show that current stellar evolution models cannot reproduce the observed C/N ratios for thick disk stars. Unexpectedly, reduced or inhibited canonical extra-mixing is very common in field stars. While subject to abundance calibration zeropoint uncertainties, this implies a strong dependence of non canonical extra-mixing along the red giant branch on the initial composition of the star and in particular on the $α$ elemental abundance.

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