Dust grain growth and the formation of the extremely primitive star SDSS J102915+172927
Dust grain growth and the formation of the extremely primitive star SDSS J102915+172927
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尘埃颗粒的生长和极其原始恒星的形成 SDSS J102915 172927
DOI：
10.1093/mnras/stu178
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发表时间：
2014
影响因子：
4.8
通讯作者：
Alessandro Chieffi
中科院分区：
文献类型：
--
作者：
Gen Chiaki;Raffaella Schneider;Takaya Nozawa;Kazuyuki Omukai;Marco Limongi;Naoki;Yoshida;Alessandro Chieffi
低金属丰度恒星形成区的尘埃颗粒可能是第一批低质量恒星形成的原因。激活粉尘引起的破碎的最低条件要求气体预先浓缩到超过临界粉尘与气体的质量比。最近发现的银晕恒星SDSSJ102915+172927的恒星质量为0.8 M⊙，金属丰度为Z∼4.5×10−5Z⊙，是尘埃诱导的低质量恒星形成的最佳候选者。事实上，只要至少0.4 M⊙的尘埃在POP III超新星喷出物中凝聚，允许反向激波进行适度破坏，就可以克服临界尘埃与气体质量比。在这里，我们表明，即使第一颗超新星中的尘埃形成效率较低，或者确实发生了强烈的尘埃破坏，母气云坍塌期间的颗粒生长足够快，足以激活尘埃冷却和碎裂成低质量恒星。我们发现，碳颗粒没有经历颗粒生长，因为在低于H∼106 cm−3的密度下，碳原子被锁定在CO分子中。硅酸盐和磁铁矿颗粒在1012 cm−3的密度范围内共生气相物种，直到它们的气相丰度降至零，达到凝结效率≈1。相应地，粉尘与气体质量比的增加允许尘埃诱导的冷却和碎裂在1012 cm−3处被激活，然后坍塌的云变得光学厚度为连续辐射。
Dust grains in low-metallicity star-forming regions may be responsible for the formation of the first low-mass stars. The minimal conditions to activate dust-induced fragmentation require the gas to be pre-enriched above a critical dust-to-gas mass ratio. The recently discovered Galactic halo star SDSS J102915+172927 has a stellar mass of 0.8 M⊙and a metallicity ofZ∼ 4.5 × 10−5Z⊙and represents an optimal candidate for the dust-induced low-mass star formation. Indeed, the critical dust-to-gas mass ratio can be overcome provided that at least 0.4 M⊙of dust condenses in Pop III supernova ejecta, allowing for moderate destruction by the reverse shock. Here, we show that grain growth during the collapse of the parent gas cloud is sufficiently rapid to activate dust cooling and fragmentation into low-mass stars, even if dust formation in the first supernovae is less efficient or strong dust destruction does occur. We find that carbon grains do not experience grain growth because at densities belownH∼ 106cm−3carbon atoms are locked into CO molecules. Silicates and magnetite grains accrete gas-phase species in the density range 109<nH< 1012cm−3, until their gas-phase abundance drops to zero, reaching condensation efficiencies ≈1. The corresponding increase in the dust-to-gas mass ratio allows dust-induced cooling and fragmentation to be activated at 1012<nH< 1014cm−3, before the collapsing cloud becomes optically thick to continuum radiation.