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R-process enrichment from a single event in an ancient dwarf galaxy

机译：古代矮星系中单个事件的R过程富集

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Elements heavier than zinc are synthesized through the rapid (r) and slow (s) neutron-capture processes(1,2). The main site of production of the r-process elements (such as europium) has been debated for nearly 60 years(2). Initial studies of trends in chemical abundances in old Milky Way halo stars suggested that these elements are produced continually, in sites such as core-collapse supernovae(3,4). But evidence from the local Universe favours the idea that r-process production occurs mainly during rare events, such as neutron star mergers(5,6). The appearance of a plateau of europium abundance in some dwarf spheroidal galaxies has been suggested as evidence for rare r-process enrichment in the early Universe(7), but only under the assumption that no gas accretes into those dwarf galaxies; gas accretion(8) favours continual r-process enrichment in these systems. Furthermore, the universal r-process pattern(1,9) has not been cleanly identified in dwarf spheroidals. The smaller, chemically simpler, and more ancient ultrafaint dwarf galaxies assembled shortly after the first stars formed, and are ideal systems with which to study nucleosynthesis events such as the r-process(10,11). Reticulum II is one such galaxy(12-14). The abundances of non-neutron-capture elements in this galaxy (and others like it) are similar to those in other old stars(15). Here, we report that seven of the nine brightest stars in Reticulum II, observed with high-resolution spectroscopy, show strong enhancements in heavy neutron-capture elements, with abundances that follow the universal r-process pattern beyond barium. The enhancement seen in this 'r-process galaxy' is two to three orders of magnitude higher than that detected in any other ultrafaint dwarf galaxy(11,16,17). This implies that a single, rare event produced the r-process material in Reticulum II. The r-process yield and event rate are incompatible with the source being ordinary core-collapse supernovae(18), but consistent with other possible sources, such as neutron star mergers(19).
机译：通过快速（r）和缓慢（s）中子俘获过程（1,2）合成比锌重的元素。 r-过程元素（例如euro）的生产主要场所已经争论了近60年[2]。对银河系旧光晕恒星化学丰度趋势的初步研究表明，这些元素是不断产生的，例如核心塌陷超新星（3,4）。但是当地宇宙的证据支持r过程产生主要发生在罕见事件中的想法，例如中子星合并（5,6）。有人建议在一些矮球形球星系中出现a含量稳定的平台，这是早期宇宙罕见的r-过程富集的证据（7），但前提是假定这些矮星系中不会积聚任何气体。气体积聚（8）有利于这些系统中的连续r过程富集。此外，在矮球体中还没有清楚地识别出通用的r-过程模式（1,9）。较小，化学上更简单，更古老的超微弱矮星系在第一颗恒星形成后不久就组装起来，是研究核合成事件（如r过程）的理想系统（10,11）。网II就是这样一个星系（12-14）。这个星系（以及其他类似的星系）中非中子俘获元素的丰度与其他老恒星的丰度相似（15）。在这里，我们报告说，用高分辨率光谱法观察到的网状II中9个最亮的恒星中有7个显示出重中子俘获元素的强增强，其丰度遵循了钡以外的通用r过程模式。在这个``r-过程星系''中看到的增强比在其他任何超微弱矮星系中检测到的增强高2至3个数量级（11,16,17）。这暗示着一个罕见事件在网状II中产生了r加工材料。 r过程的收率和事件发生率与普通的核塌陷超新星源不兼容（18），但与其他可能的源如中子星合并（19）相一致。

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  • 来源
    《Nature》 |2016年第7596期|610-613|共4页
  • 作者

    Ji Alexander P.; Frebel Anna; Chiti Anirudh; Simon Joshua D.;

  • 作者单位

    MIT, Dept Phys, Cambridge, MA 02139 USA|MIT, Kavli Inst Astrophys & Space Res, Cambridge, MA 02139 USA|Ctr Evolut Elements, Joint Inst Nucl Astrophys, E Lansing, MI 48824 USA;

    MIT, Dept Phys, Cambridge, MA 02139 USA|MIT, Kavli Inst Astrophys & Space Res, Cambridge, MA 02139 USA|Ctr Evolut Elements, Joint Inst Nucl Astrophys, E Lansing, MI 48824 USA;

    MIT, Dept Phys, Cambridge, MA 02139 USA|MIT, Kavli Inst Astrophys & Space Res, Cambridge, MA 02139 USA;

    Observ Carnegie Inst Washington, Pasadena, CA 91101 USA;

  • 收录信息 美国《科学引文索引》(SCI);美国《工程索引》(EI);美国《生物学医学文摘》(MEDLINE);美国《化学文摘》(CA);
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