Does Deuterium Enable the Formation of Primordial Brown Dwarfs?
Does Deuterium Enable the Formation of Primordial Brown Dwarfs?
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氘是否能够形成原始棕矮星？
DOI：
10.1086/312529
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发表时间：
1999
期刊：
影响因子：
--
通讯作者：
S. Inutsuka
中科院分区：
文献类型：
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作者：
H. Uehara;S. Inutsuka
我们研究了原始气体云的热力学和动力学演化与更新的氘化学。我们考虑一个碎片的冲击波后冷却片，预计形成的大规模云（108 M的崩溃）和冲击波由于超新星爆炸。首先，我们研究了原始冲击中的分子形成。我们表明，几乎所有的氘可以转换为HD宇宙的年龄内的情况下，具有维里温度为106 K和崩溃在z > 1的红移。当激波后片由于引力不稳定性而碎裂时，H2和HD的丰度分别为~10-2和~10-5，比膨胀宇宙中分子形成后再复合的结果高103-104倍。为了研究碎片的后续演变，我们对球形/圆柱形云进行了一维模拟，其中初始条件（例如，化学成分的丰度分数、温度）从冲击的结果导出。研究发现，在圆柱形坍缩的情况下，HD分子的冷却使云的温度保持在100 K以下，并且云几乎等温地演化。当云的光学厚度达到HD谱线发射（~1010 cm-3）时，圆柱形云的引力破碎变得有效，金斯质量变得相当于0.1 M。这一系列的过程使得原始气体云中的原始低质量恒星，可能还有棕矮星的形成成为可能。
We investigate thermal and dynamical evolution of a primordial gas cloud with an updated deuterium chemistry. We consider a fragment of a postshock-cooled sheet that is expected to form by collapse of a massive cloud (≳108 M☉) and by blast waves due to supernova explosions. At first we investigate molecule formation in a primordial shock. We show that almost all deuterium can be converted to HD within the age of the universe at the collapsed redshift in the case of a cloud that has a virial temperature of ~106 K and collapses at z > 1. When the postshock sheet fragments owing to gravitational instability, the fractional H2 and HD abundances become ~10-2 and ~10-5, respectively, which are 103-104 times higher than the result of molecule formation in the expanding universe after recombination. To study the subsequent evolution of a fragment, we performed one-dimensional simulations of a spherical/cylindrical cloud, of which initial conditions (e.g., fractional abundances of chemical composition, temperature) are derived from the result of the shock. It is found that, in case of a cylindrical collapse, the cooling by HD molecules keeps the temperature of the cloud less than 100 K and the cloud evolves almost isothermally. When the cloud becomes optically thick to the HD line emission (~1010 cm-3) and the gravitational fragmentation of the cylindrical cloud becomes effective, the Jeans mass becomes comparable to 0.1 M☉. This series of processes enables the formation of primordial low-mass stars, and possibly brown dwarfs, in primordial gas clouds.