Evidence from Disrupted Halo Dwarfs that r-process Enrichment via Neutron Star Mergers is Delayed by ≳500 Myr
Evidence from Disrupted Halo Dwarfs that r-process Enrichment via Neutron Star Mergers is Delayed by ≳500 Myr
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DOI：
10.3847/2041-8213/ac5589
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发表时间：
2021-10
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通讯作者：
R. Naidu;A. Ji;C. Conroy;A. Bonaca;Y. Ting 丁;D. Zaritsky;L. V. van Son;F. Broekgaarden;
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作者：
R. Naidu;A. Ji;C. Conroy;A. Bonaca;Y. Ting 丁;D. Zaritsky;L. V. van Son;F. Broekgaarden;
r-过程元素的天体物理起源仍然难以捉摸。中子星星合并（NSM）和特殊类型的核心坍缩超新星（rCCSNe）是主要的候选者。由于这些通道的独特的特征时间尺度（rCCSNe：提示，NSMs：延迟），测量r-过程丰富的星系相似的质量，但不同的星星形成的持续时间可能会证明信息。最近在银河系的恒星晕中发现了两个被破坏的矮星，Kraken和Gaia-Sausage Enceladus（GSE），正好提供了这个机会：两者都有M = 108 M，但不同的星星形成持续时间为102 Gyr和103.6 Gyr。在这里，我们提出了从这些系统中的31颗恒星的R 50，000麦哲伦/MIKE光谱，检测所有恒星中的r-过程元素Eu。两个星系的恒星都有相似的[Mg/H]<$−1，但Kraken的[Eu/Mg]<$−0.1，而GSE的[Eu/Mg]<$0.2。通过简单的模型，我们认为NSM富集必须延迟500-1000万年才能产生这种差异。rCCSNe也必须有贡献，特别是在早期，否则在延迟期间形成的恒星将是不含Eu的。在这张照片中，rCCSNe占Kraken中Eu的1.50%，GSE中的1.25%，以及像人马座这样的星星形成时间延长的矮星中的1.15%。NSM浓缩的延迟时间比恒星群体合成的合并延迟时间长10×-100×-这并不一定令人惊讶，因为浓缩延迟包括NSM喷出物并入后续恒星的时间。例如，这可能是由于纳塔尔撞击，导致富含r的物质远离恒星形成气体沉积，然后在这些星系中冷却108-109年。
The astrophysical origins of r-process elements remain elusive. Neutron star mergers (NSMs) and special classes of core-collapse supernovae (rCCSNe) are leading candidates. Due to these channels’ distinct characteristic timescales (rCCSNe: prompt, NSMs: delayed), measuring r-process enrichment in galaxies of similar mass but differing star formation durations might prove informative. Two recently discovered disrupted dwarfs in the Milky Way’s stellar halo, Kraken and Gaia-Sausage Enceladus (GSE), afford precisely this opportunity: Both have M ⋆ ≈ 108 M ⊙ but differing star formation durations of ≈2 Gyr and ≈3.6 Gyr. Here we present R ≈ 50,000 Magellan/MIKE spectroscopy for 31 stars from these systems, detecting the r-process element Eu in all stars. Stars from both systems have similar [Mg/H] ≈ −1, but Kraken has a median [Eu/Mg] ≈ −0.1 while GSE has an elevated [Eu/Mg] ≈ 0.2. With simple models, we argue NSM enrichment must be delayed by 500–1000 Myr to produce this difference. rCCSNe must also contribute, especially at early epochs, otherwise stars formed during the delay period would be Eu free. In this picture, rCCSNe account for ≈50% of the Eu in Kraken, ≈25% in GSE, and ≈15% in dwarfs with extended star formation durations like Sagittarius. The inferred delay time for NSM enrichment is 10×–100× longer than merger delay times from stellar population synthesis—this is not necessarily surprising because the enrichment delay includes time taken for NSM ejecta to be incorporated into subsequent generations of stars. For example, this may be due to natal kicks that result in r-enriched material deposited far from star-forming gas, which then takes ≈108–109 yr to cool in these galaxies.