The chemical evolution of r-process elements from neutron star mergers: the role of a 2-phase interstellar medium
The chemical evolution of r-process elements from neutron star mergers: the role of a 2-phase interstellar medium
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DOI：
10.1093/mnras/stz1126
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发表时间：
2019-01
影响因子：
4.8
通讯作者：
R. Schönrich;D. Weinberg
中科院分区：
文献类型：
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作者：
R. Schönrich;D. Weinberg
中子星合并（NM）是重r过程元素（如铕）的合理来源，但之前的化学演化模型要么未能重现观测到的银河系厚盘星（含[Fe/H] ~ -1）的铕趋势，要么只是采用了不切实际的短合并时间尺度。通过分析论证和数值模拟，我们证明了采用单相星际介质（ISM）和金属丰度无关产率的模型不能再现显示[Eu/ α] >或[Eu/Fe] > [α /Fe]的α元素，如Mg和Si。然而，如果我们允许两相ISM，即热相冷却时间\tau_{cool}为1 Gyr阶，并且相对于核心坍缩超新星（ccSNe）的α元素产率，将更大比例的NM产率直接注入冷恒星形成阶段，那么这个问题就很容易解决。我们发现冷相注入比f_{c，NM}/f_{c，ccSN}为2阶，特征合并时间标度\tau_NM=150 Myr的模型与观测结果吻合良好。我们发现，在中等金属丰度下观测到的超级太阳[Eu/alpha]表明，除了ccSNe外，还有相当一部分Eu来自NM或其他来源，这些非ccsn产额在早期的ISM恒星形成阶段优先沉积。
Neutron star mergers (NM) are a plausible source of heavy r-process elements such as Europium, but previous chemical evolution models have either failed to reproduce the observed Europium trends for Milky Way thick disc stars (with [Fe/H] ~ -1) or have done so only by adopting unrealistically short merger timescales. Using analytic arguments and numerical simulations, we demonstrate that models with a single-phase interstellar medium (ISM) and metallicity-independent yields cannot reproduce observations showing [Eu/alpha] > 0 or [Eu/Fe] > [alpha/Fe] for alpha-elements such as Mg and Si. However, this problem is easily resolved if we allow for a 2-phase ISM, with hot-phase cooling times \tau_{cool} of order 1 Gyr and a larger fraction of NM yields injected directly into the cold star-forming phase relative to alpha-element yields from core collapse supernovae (ccSNe). We find good agreement with observations in models with a cold phase injection ratio f_{c,NM}/f_{c,ccSN} of order 2, and a characteristic merger timescale \tau_NM=150 Myr. We show that the observed super-solar [Eu/alpha] at intermediate metallicities implies that a significant fraction of Eu originates from NM or another source besides ccSNe, and that these non-ccSN yields are preferentially deposited in the star-forming phase of the ISM at early times.