Astronomical Institute, Tohoku University, Sendai, Japan.
Nature. 2018 Jul;559(7715):585-588. doi: 10.1038/s41586-018-0329-2. Epub 2018 Jul 25.
The chemical compositions of stars encode those of the gas from which they formed, providing important clues regarding the formation histories of galaxies. A powerful diagnostic is the abundance of α elements (O, Mg, Si, S, Ca and Ti) relative to iron, [α/Fe]. The α elements are synthesized and injected into the interstellar medium by type II supernovae, which occur about ten million years after their originating stars form; by contrast, iron is returned to the interstellar medium by type Ia supernovae, which occur after a much longer timescale of roughly one billion years. Periods of rapid star formation therefore tend to produce high-[α/Fe] stellar populations (because only type II supernovae have time to contribute to interstellar-medium enrichment as the stellar population forms), whereas low-[α/Fe] stars require periods of star formation that last more than a few billion years (over which timescales type Ia supernovae begin to affect the elemental composition of the interstellar medium more strongly than type II supernovae). The existence of two distinct groups of stars in the solar neighbourhood, one with high [α/Fe] and the other with low [α/Fe], therefore suggests two different origins, but the mechanism by which this bimodal distribution arose remains unknown. Here we use a model of disk-galaxy evolution to show that the two episodes of star formation predicted by the 'cold flow' theory of galactic gas accretion also explain the observed chemical bimodality. In this scenario, the high-[α/Fe] stars form early, during an initial phase of accretion that involves infalling streams of cold primordial gas. There is then a hiatus of around two billion years until the shock-heated gas in the galactic dark-matter halo has cooled as a result of radiation and can itself commence accretion. The low-[α/Fe] stars form during this second phase. The peaks in these two star-formation episodes are separated by around five billion years. In addition, the large-scale variation in the abundance patterns of these two stellar populations that has been observed for the Milky Way is partially explained by the spatial variation in this gas-accretion history.
恒星的化学成分编码了它们形成时的气体化学成分，为星系的形成历史提供了重要线索。一个有力的诊断方法是α元素（O、Mg、Si、S、Ca 和 Ti）相对于铁的丰度[α/Fe]。α元素是由 II 型超新星合成并注入星际介质的，它们发生在其起源恒星形成后约一千万年；相比之下，铁是由 Ia 型超新星返回星际介质的，它们发生在更长的大约十亿年的时间尺度上。因此，快速的恒星形成期往往会产生高[α/Fe]恒星群体（因为只有 II 型超新星有时间在恒星群体形成时对星际介质的富化做出贡献），而低[α/Fe]恒星则需要持续数十亿年以上的恒星形成期（在这段时间内，Ia 型超新星开始比 II 型超新星更强烈地影响星际介质的元素组成）。太阳附近存在两组截然不同的恒星，一组具有高[α/Fe]，另一组具有低[α/Fe]，因此表明有两种不同的起源，但这种双峰分布的形成机制仍不清楚。在这里，我们使用盘星系演化模型表明，星系气体吸积的“冷流”理论所预测的两个恒星形成期也解释了观测到的化学双峰。在这种情况下，高[α/Fe]恒星形成得更早，在涉及冷原始气体流入流的初始吸积阶段形成。然后，大约有 20 亿年的停顿，直到由于辐射而冷却下来的星系暗物质晕中的热激气体本身可以开始吸积。低[α/Fe]恒星在第二阶段形成。这两个恒星形成期的峰值相隔约 50 亿年。此外，对银河系观测到的这两种恒星群体丰度模式的大规模变化部分可以通过这种气体吸积历史的空间变化来解释。
