Low-Mass Star Formation Triggered by Supernovae in Primordial Clouds
Low-Mass Star Formation Triggered by Supernovae in Primordial Clouds
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DOI：
10.1086/428030
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发表时间：
2004-12
期刊：
影响因子：
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通讯作者：
M. Machida;K. Tomisaka;F. Nakamura;M. Fujimoto
中科院分区：
文献类型：
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作者：
M. Machida;K. Tomisaka;F. Nakamura;M. Fujimoto
被第一代大质量恒星的超新星遗迹扫过的气体壳层的演化，通过考虑 H2 和 HD 化学并使用动力学的半解析近似来研究。当在银河系前母云中形成的第一代恒星爆炸为超新星时，爆炸能量在 1051-1052 尔格范围内，红移为 z = 10-50，在扫过的气体壳层中形成 H2 和 HD 分子，丰度分别为 ~10-3 和 ~10-5，并有效地将气体壳层冷却到 32-154 K 的温度。如果超新星遗迹能够扫过为了收集质量为 6 × 104 到 8 × 105 M☉ 的周围气体，气体壳将受到其自重力的支配，因此预计会破裂。碎片所需的扫除气体量随着爆炸能量的增加而增加，并随着宿主云的星际气体密度（或红移）而减少，这为第一代超新星触发恒星形成的宿主云的质量提供了下限。而且，碎裂的条件对星际气体的热状态非常敏感。我们的结果表明，对于星际气体的合理温度范围（200-1000 K），第二代恒星的形成可以由总质量（暗和重子）低至几倍 106 M☉ 的原始云中爆炸能量在上述范围内的单个超新星或超新星触发。然而，对于更高温度的星际气体，扫掠气体壳层的碎裂条件需要更大的超新星爆炸能量。我们还跟踪了假设其几何形状（球体和圆柱体）的碎片的后续收缩，并证明当碎片在光学上变得厚至 H2 和 HD 线时，碎片中的 Jeans 质量会降至远低于 1 M☉。然后，这些碎片预计将分裂成质量与金斯质量相当的致密核心，并塌陷形成可以存活至今的低质量恒星。如果气体壳层中的物质与超新星的喷射物充分混合，由此形成的壳层和低质量恒星可能含有平均丰度[Fe/H]≃-3的金属。这种金属丰度与银晕中发现的极度贫金属恒星的金属丰度一致。 [Fe/H] < -5 的低金属丰度恒星，例如最近在银河晕中发现的 HE 0107-5240，很难通过这种机制形成，必须在不同的情况下产生。
The evolution of a gas shell, swept up by the supernova remnant of a massive first-generation star, is studied with H2 and HD chemistry taken into account and with the use of a semianalytical approximation to the dynamics. When a first-generation star, formed in a parent pregalactic cloud, explodes as a supernova with explosion energy in the range of 1051-1052 ergs at redshifts of z = 10-50, H2 and HD molecules are formed in the swept up gas shell at fractional abundances of ~10-3 and ~10-5, respectively, and effectively cool the gas shell to temperatures of 32-154 K. If the supernova remnant can sweep to gather the ambient gas of mass 6 × 104 to 8 × 105 M☉, the gas shell comes to be dominated by its self-gravity and, hence, is expected to fragment. The amount of swept up gas necessary for fragmentation increases with the explosion energy and decreases with the interstellar gas density (or redshift) of the host cloud, which provides a lower boundary to the mass of the host cloud in which star formation is triggered by the first-generation supernova. Also, the condition for fragmentation is very sensitive to the thermal state of interstellar gas. Our result shows that for a reasonable range of temperatures (200-1000 K) of interstellar gas, the formation of second-generation stars can be triggered by a single supernova or hypernova with explosion energy in the above range in a primordial cloud of total (dark and baryonic) mass as low as a few times 106 M☉. For higher temperatures in the interstellar gas, however, the condition for the fragmentation in the swept up gas shell demands a larger supernova explosion energy. We also follow the subsequent contraction of the fragment pieces assuming their geometry (sphere and cylinder) and demonstrate that the Jeans masses in the fragments decrease to well below 1 M☉ by the time the fragments become optically thick to the H2 and HD lines. The fragments are then expected to break up into dense cores whose masses are comparable to the Jeans masses and collapse to form low-mass stars that can survive to the present. If the material in the gas shell is mixed well with the ejecta of the supernova, the shell and low-mass stars thus formed are likely to have metals of abundance [Fe/H] ≃ -3 on average. This metallicity is consistent with those of the extremely metal-poor stars found in the Galactic halo. Stars with low metallicities of [Fe/H] < -5 such as HE 0107-5240, recently discovered in the Galactic halo, are difficult to form by this mechanism and must be produced in different situations.