Classification of extremely metal-poor stars: absent region in A(C)-[Fe/H] plane and the role of dust cooling
Classification of extremely metal-poor stars: absent region in A(C)-[Fe/H] plane and the role of dust cooling
复制标题
DOI：
10.1093/mnrasl/slx163
复制
发表时间：
2017-10
影响因子：
4.8
通讯作者：
G. Chiaki;N. Tominaga;T. Nozawa
中科院分区：
文献类型：
--
作者：
G. Chiaki;N. Tominaga;T. Nozawa
极贫金属（EMP）恒星是记录了星系形成早期化学富集历史的活化石。通过最近的大规模观测活动，已经获得了电磁脉冲恒星的统计样本。这促使我们重新考虑它们的分类和形成条件。从观测到的C-增强EMP （CE-EMP）和C-正常EMP （CN-EMP）的Acr(C) ~ 6和[Fe/H]cr ~ -5的碳和铁丰度下限，我们证实了尘埃热发射的气体冷却对于它们的母云的破碎是必不可少的，形成了这样的低质量，即长寿命的恒星，并且主要的颗粒分别是碳和硅酸盐。我们将物质i的颗粒半径r_i^{cool}和关键元素j的凝聚效率f_{ij}分别限定为r_C^{cool}/f_{C， C} = 10 um和r_Sil^{cool}/f_{Sil， Mg} = 0.1 um，从而得到了Acr(C)和[Fe/H]cr，给出了每颗EMP恒星形成的普遍条件10^{[C/H]-2.30} + 10^[Fe/H] > 10^{-5.07}。与传统的CE-和CN-EMP恒星之间的边界[C/Fe] = 0.7不同，这个条件表明了一个物理上有意义的边界[C/Fe]_{b} = 2.30，在这个边界之上和之下，碳和硅酸盐颗粒分别是主要的冷却剂。
Extremely metal-poor (EMP) stars are the living fossils with records of chemical enrichment history at the early epoch of galaxy formation. By the recent large observation campaigns, statistical samples of EMP stars have been obtained. This motivates us to reconsider their classification and formation conditions. From the observed lower-limits of carbon and iron abundances of Acr(C) ~ 6 and [Fe/H]cr ~ -5 for C-enhanced EMP (CE-EMP) and C-normal EMP (CN-EMP) stars, we confirm that gas cooling by dust thermal emission is indispensable for the fragmentation of their parent clouds to form such low-mass, i.e., long-lived stars, and that the dominant grain species are carbon and silicate, respectively. We constrain the grain radius r_i^{cool} of a species i and condensation efficiency f_{ij} of a key element j as r_C^{cool}/f_{C, C} = 10 um and r_Sil^{cool}/f_{Sil, Mg} = 0.1 um to reproduce Acr(C) and [Fe/H]cr, which give a universal condition 10^{[C/H]-2.30} + 10^[Fe/H] > 10^{-5.07} for the formation of every EMP star. Instead of the conventional boundary [C/Fe] = 0.7 between CE- and CN-EMP stars, this condition suggests a physically meaningful boundary [C/Fe]_{b} = 2.30 above and below which carbon and silicate grains are dominant coolants, respectively.