THE NEW MODEL OF CHEMICAL EVOLUTION OF r-PROCESS ELEMENTS BASED ON THE HIERARCHICAL GALAXY FORMATION. I. Ba AND Eu
THE NEW MODEL OF CHEMICAL EVOLUTION OF r-PROCESS ELEMENTS BASED ON THE HIERARCHICAL GALAXY FORMATION. I. Ba AND Eu
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DOI：
10.1088/0004-637x/783/2/132
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发表时间：
2014-01
期刊：
影响因子：
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通讯作者：
Y. Komiya;S. Yamada;T. Suda;M. Fujimoto
中科院分区：
文献类型：
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作者：
Y. Komiya;S. Yamada;T. Suda;M. Fujimoto
我们在层级式星系形成框架下，利用半解析合并树，研究了银河系晕早期演化阶段中r -过程元素的化学增丰情况。在本文中，我们聚焦于极贫金属（EMP）恒星中的重r -过程元素钡（Ba）和铕（Eu），并对其天文起源场所给出限制。我们的模型考虑了因星际介质（ISM）吸积导致的EMP恒星表面丰度变化。我们还考虑了星系风对星系际介质的金属增丰以及由此产生的原星系预增丰。我们的层级模型能很好地再现观测到的r -过程丰度的趋势和离散情况，该模型中约10%处于低质量端（约10个太阳质量）的核心坍缩超新星作为主要的r -过程源，且恒星形成效率约为10⁻¹⁰ 年⁻¹ 。对于作为r -过程源的中子星合并，其并合时标必须约为10⁷ 年，且事件发生率要比目前在银河系中观测到的高约100倍。我们发现，对于[Ba/H] < - 3.5的恒星，星际介质的吸积是r -过程元素的主要来源。在这个模型中，[Fe/H] < - 3的大多数恒星在形成时不含r -过程元素，但其表面会被星际介质的吸积所污染。预增丰影响约4%的原星系，然而，在EMP恒星表面，其影响小于星际介质的吸积。
We investigate the chemical enrichment of r-process elements in the early evolutionary stages of the Milky Way halo within the framework of hierarchical galaxy formation using a semi-analytic merger tree. In this paper, we focus on heavy r-process elements, Ba and Eu, of extremely metal-poor (EMP) stars and give constraints on their astronomical sites. Our models take into account changes of the surface abundances of EMP stars by the accretion of interstellar medium (ISM). We also consider metal-enrichment of intergalactic medium by galactic winds and the resultant pre-enrichment of proto-galaxies. The trend and scatter of the observed r-process abundances are well reproduced by our hierarchical model with ∼10% of core-collapse supernovae in low-mass end (∼10 M☉) as a dominant r-process source and the star formation efficiency of ∼10−10 yr−1. For neutron star mergers as an r-process source, their coalescence timescale has to be ∼107 yr, and the event rates ∼100 times larger than currently observed in the Galaxy. We find that the accretion of ISM is a dominant source of r-process elements for stars with [Ba/H] < −3.5. In this model, a majority of stars at [Fe/H] < −3 are formed without r-process elements, but their surfaces are polluted by the ISM accretion. The pre-enrichment affects ∼4% of proto-galaxies, and yet, is surpassed by the ISM accretion in the surface of EMP stars.