3D NLTE analysis of the most iron-deficient star, SMSS0313-6708
3D NLTE analysis of the most iron-deficient star, SMSS0313-6708
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最缺铁恒星的 3D NLTE 分析 SMSS0313-6708
DOI：
10.1051/0004-6361/201629202
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发表时间：
2016
期刊：
影响因子：
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通讯作者：
J. Leenaarts
中科院分区：
文献类型：
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作者：
T. Nordlander;A. Amarsi;K. Lind;M. Asplund;P. Barklem;A. Casey;R. Collet;J. Leenaarts
早期宇宙中星星形成的模型取决于吸积、碎裂和辐射反馈的细节。不同的模拟预测不同的初始质量函数的第一颗恒星，从主要的低质量（0.1-10 Msol），大质量（10-100 Msol），甚至超大质量（100-1000 Msol）。第一颗恒星的质量分布应该会在仍然存在的低质量第二代和更晚一代贫金属恒星上留下独特的化学印记。SMSS 0313 -6708的化学成分是已知星星中Ca和Fe丰度最低的，表明它是由一颗大质量超新星富集的。然而，在贫金属恒星中，即使是微弱的光谱线也可能受到强烈的3D和NLTE效应的影响。如果这些影响被忽略或处理不当，推断丰度的错误可能会显着偏离污染超新星的推断属性。我们使用3D NLTE辐射转移重新测定SMSS 0313 -6708的化学成分，以获得Li、Na、Mg、Al、Ca和Fe的准确丰度。模型原子采用现实的碰撞率，没有校准的自由参数。我们发现3D NLTE中Fe的丰度比1D LTE高0.8 dex，Mg，Al和Ca的丰度高0.5 dex，而Li和Na的丰度不受影响，在0.03 dex以内。特别是，我们的[Fe/H]上限现在大了十倍，在[Fe/H] NLTE（即使用平均3D模型）。该较高的估计值是由于保守的上限估计、更新的NLTE数据和3D-NLTE差异，所有这些都导致较高的丰度测定。我们发现，SMSS 0313 -6708的化学成分修正值与20-60 Msol的低能量（1-2 x 10^51 erg）爆炸的大质量祖星以及10 Msol的极低爆炸能量（< 10^51 erg）的祖星的超新星产额相匹配。
Models of star formation in the early universe depend on the details of accretion, fragmentation and radiative feedback. Different simulations predict different initial mass functions of the first stars, ranging from predominantly low mass (0.1-10 Msol), to massive (10-100 Msol), or even supermassive (100-1000 Msol). The mass distribution of the first stars should lead to unique chemical imprints on the low-mass second and later generation metal-poor stars still in existence. The chemical composition of SMSS0313-6708, which has the lowest abundances of Ca and Fe of any star known, indicates it was enriched by a single massive supernova. However, even weak spectral lines may be affected by strong 3D and NLTE effects in metal-poor stars. If these effects are ignored or treated incorrectly, errors in the inferred abundances may significantly bias the inferred properties of the polluting supernovae. We redetermine the chemical composition of SMSS0313-6708 using 3D NLTE radiative transfer to obtain accurate abundances for Li, Na, Mg, Al, Ca and Fe. The model atoms employ realistic collisional rates, with no calibrated free parameters. We find significantly higher abundances in 3D NLTE than 1D LTE by 0.8 dex for Fe, and 0.5 dex for Mg, Al and Ca, while Li and Na are unaffected to within 0.03 dex. In particular, our upper limit for [Fe/H] is now a factor ten larger, at [Fe/H] NLTE (i.e. using averaged 3D models). This higher estimate is due to a conservative upper limit estimation, updated NLTE data, and 3D- NLTE differences, all of which lead to a higher abundance determination. We find that the revised chemical composition of SMSS0313-6708 matches supernova yields for massive progenitors of 20-60 Msol exploding with low energies (1-2 x 10^51 erg), as well as progenitors of 10 Msol with very low explosion energies (< 10^51 erg).