第二代恒星的临界金属丰度
Critical Metallicities for Second-Generation Stars
摘要：
第一颗大质量恒星可能会影响第二代恒星的形成，部分原因是它们周围气体的金属富集。我们研究了“临界金属含量”，定义为值，Z_crit，在该值下原始气体通过O I（63.18微米，Si II 34.8微米，Fe II（25.99和35.35微米）和C II（157.74微米）的精细结构线比通过H I或H2线发射更有效地冷却。我们探讨了随时间变化的热力学和破碎的冷却气体在红移z = 10-30，种子的微量重元素从早期超新星驱逐。由于不同的核合成模式（α-过程，Fe-组）产生的丰度比远离太阳值，这些早期的恒星星族很可能受到O，Si和Fe冷却的影响。我们的模型还包括精细结构线和H2与宇宙微波背景（CMB）的辐射耦合，CMB设置了一个温度下限（z = 25-30时为70- 80 K），这可能会增加Jeans质量。H2的形成是由重组时期遗留下来的电子或在维里化过程中产生的电子的催化作用。这些电子形成H^-离子（H + e -> H- + gamma），而H ^-离子又通过缔合分离（H- + H -> H2 + e）形成H2。在z = 10-30的维里化晕中，气体密度（n = 1-100 cm^{-3}）远低于临界密度（n_cr = 10^{5-6} cm^{-3}），在临界密度下（O，Si，Fe）精细结构线达到LTE布居数并产生最有效的冷却。因此，Z_crit在n = 1-100 cm^{-3}时可能最初超过0.01 Z_sun，然后在n = 10^6 cm^{-3}时下降到10^{-3.5} Z_sun，这时，金斯质量可能会印在恒星质量函数上。在0.01 Z_sun和200 K时，10^8 M_sun的原始云将产生红移的精细结构线，在z = 4时，通量在10^{-22}和10^{-21}瓦/米^2之间。
英文摘要：
The first massive stars may influence the formation of second-generation stars, in part by their metal enrichment of the surrounding gas. We investigate the "critical metallicity", defined as the the value, Z_crit, at which primordial gas cools more efficiently by fine-structure lines of O I (63.18 microns, Si II 34.8 microns, Fe II (25.99 and 35.35 microns), and C II (157.74 microns) than by either H I or H2 line emission. We explore the time-dependent thermodynamics and fragmentation of cooling gas at redshifts z = 10-30, seeded by trace heavy elements expelled from early supernovae. Because different modes of nucleosynthesis (alpha-process, Fe-group) produce abundance ratios far from solar values, these early stellar populations are likely to be influenced by O, Si, and Fe cooling. Our models also include radiative coupling of the fine structure lines and H2 to the cosmic microwave background (CMB), which sets a temperature floor (70-80K at z = 25-30) that may increase the Jeans mass. The H2 forms from catalytic effects of electrons left over from the recombination epoch or produced during virialization. These electrons form the H^- ion (H + e -> H- + gamma), which in turn forms H2 through associative detachment (H- + H -> H2 + e). In virialized halos at z = 10-30, the gas densities (n = 1-100 cm^{-3}) are well below the critical densities, n_cr = 10^{5-6} cm^{-3}, at which (O, Si, Fe) fine-structure lines reach LTE populations and produce their most efficient cooling. Thus, Z_crit may initially exceed 0.01 Z_sun at n = 1-100 cm^{-3}, and then drop to 10^{-3.5} Z_sun at n = 10^6 cm^{-3}, where the Jeans mass may be imprinted on the stellar mass function. Primordial clouds of 10^8 M_sun at 0.01 Z_sun and 200K will produce redshifted fine structure lines, with fluxes between 10^{-22} and 10^{-21} Watts/m^2 at z = 4.
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