Is HE 0107–5240 A Primordial Star? The Characteristics of Extremely Metal-Poor Carbon-Rich Stars
Is HE 0107–5240 A Primordial Star? The Characteristics of Extremely Metal-Poor Carbon-Rich Stars
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DOI：
10.1086/422135
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发表时间：
2004-02
期刊：
影响因子：
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通讯作者：
T. Suda;M. Aikawa;M. Machida;M. Fujimoto;Icko Iben, Jr.,
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文献类型：
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作者：
T. Suda;M. Aikawa;M. Machida;M. Fujimoto;Icko Iben, Jr.,
我们讨论了HE0107-5240的起源，它的金属丰度为[Fe/H]=-5.3，是迄今观测到的最贫铁的恒星。它的发现对我们宇宙中第一代恒星的可观性问题有着重要的影响。与其他金属丰度很小的恒星(-4≲[Fe/H]≲-2.5)一样，HE0107-5240表现出一种特殊的丰度模式，包括C、N和O的较大增强，以及较温和的Na增强。观测到的丰度模式可以用第一代双星的核合成和质量转移来解释，第一代双星在出生后，从混合了超新星喷发的原始云中吸积物质。我们根据我们目前对极端贫金属、低质量模型恒星的演化和核合成的理解来阐述双星情景，并讨论了将这一情景与其他情景区分开来的可能性。在我们的图片中，铁峰元素出现在组成恒星的表层，是由于受到污染的原始星云的气体积累而产生的，污染发生在双星诞生后。为了解释观测到的碳、氮、氧和钠的增强，以及12C/13C的比率，我们假设目前观测到的恒星，曾经是双星中的第二颗恒星，是从化学演化的伴星中积累而来的，现在是一颗白矮星。为了估计以双星形式传输的物质中的丰度，我们依赖于用最新输入物理构建的模型恒星的计算结果。跟踪了氦闪光驱动的对流区中的核合成，使我们能够从根本上解释观测到的O和Na丰度的来源，并讨论S过程元素的丰度。根据观测的丰度，我们得出结论：HE0107-5240是从一个较宽的双星(初始分离约20AU)演化而来的，初始质量在1.2M-3M☉范围内。在假定该系统由一颗白矮星和一颗红巨星组成的情况下，估计了目前的双星分离和周期分别为≃34AU和≃150年。重S过程元素的丰度分布可能是理解HE0107-5240成因的关键。如果HE0107-5240是由已经被铁族元素污染的气体形成的第二代恒星，则应该观察到[Pb/Fe]≃1-2的增强。如果没有探测到主线S过程元素的增强，HE0107-5240可能是初级质量小于2.5M☉的双星系统中的第一代次要物质，来自原始成分的气体，产生于大爆炸中，随后受到来自母云的气体的吸积而受到表面污染-通过与超新星的喷出物混合而富集。
We discuss the origin of HE 0107-5240, which, with a metallicity of [Fe/H] = -5.3, is the most iron-poor star yet observed. Its discovery has an important bearing on the question of the observability of first-generation stars in our universe. In common with other stars of very small metallicity (-4 ≲ [Fe/H] ≲ -2.5), HE 0107-5240 shows a peculiar abundance pattern, including large enhancements of C, N, and O, and a more modest enhancement of Na. The observed abundance pattern can be explained by nucleosynthesis and mass transfer in a first-generation binary star, which, after birth, accretes matter from a primordial cloud mixed with the ejectum of a supernova. We elaborate the binary scenario on the basis of our current understanding of the evolution and nucleosynthesis of extremely metal-poor, low-mass model stars and discuss the possibility of discriminating this scenario from others. In our picture, iron-peak elements arise in surface layers of the component stars by accretion of gas from the polluted primordial cloud, pollution occurring after the birth of the binary. To explain the observed C, N, O, and Na enhancements, as well as the 12C/ 13C ratio, we suppose that the currently observed star, once the secondary in a binary, accreted matter from a chemically evolved companion, which is now a white dwarf. To estimate the abundances in the matter transferred in the binary, we rely on the results of computations of model stars constructed with up-to-date input physics. Nucleosynthesis in a helium-flash-driven convective zone into which hydrogen has been injected is followed, allowing us to explain the origin in the primary of the observed O and Na enrichments and to discuss the abundances of s-process elements. From the observed abundances, we conclude that HE 0107-5240 has evolved from a wide binary (of initial separation ~20 AU) with a primary of initial mass in the range 1.2-3 M☉. On the assumption that the system now consists of a white dwarf and a red giant, the present binary separation and period are estimated at ≃34 AU and a period of ≃150 yr, respectively. We also conclude that the abundance distribution of heavy s-process elements may hold the key to a satisfactory understanding of the origin of HE 0107-5240. An enhancement of [Pb/Fe] ≃ 1-2 should be observed if HE 0107-5240 is a second-generation star, formed from gas already polluted with iron-group elements. If the enhancement of main-line s-process elements is not detected, HE 0107-5240 may be a first-generation secondary in a binary system with a primary of mass less than 2.5 M☉, born from gas of primordial composition, produced in the big bang, and subsequently subjected to surface pollution by accretion of gas from the parent cloud metal-enriched by mixing with the ejectum of a supernova.