Is H+3 cooling ever important in primordial gas?
Is H+3 cooling ever important in primordial gas?
复制标题
H 3 冷却对于原始气体是否重要？
DOI：
10.1111/j.1365-2966.2008.14156.x
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发表时间：
2008
影响因子：
4.8
通讯作者：
D. Savin
中科院分区：
文献类型：
--
作者：
S. Glover;D. Savin
对不含金属的第三族恒星形成的研究通常主要集中在H2冷却所扮演的角色，因为相对于其他可能的分子或离子冷却剂，它的化学丰度很大。然而，虽然H2在低气体密度下通常是最重要的冷却剂，但由于其达到局部热力学平衡（LTE）的低临界密度以及在其发射线中产生的大的不透明度，其在高气体密度下不是有效的冷却剂。因此，其他化学物质的排放可能在冷却高密度原始气体方面发挥重要作用。一个特别有趣的候选者是H +分子离子。这种离子的LTE冷却速率大约是H2的十亿倍，与其他原始分子离子如H +或HeH +不同，它不容易通过与H或H2的碰撞而从气体中去除。至少在一个天体物理学背景下-气体巨星的高层大气中-它已经被认为是一种重要的冷却剂，但它在原始气体冷却中的作用以前很少得到研究。在本文中，我们使用新开发的H +冷却函数和迄今为止发表的最详细的原始化学模型来研究原始气体中H +冷却的潜在重要性。我们表明，虽然H +是，在大多数情况下，第三个最重要的冷却剂在致密的原始气体（后H2和HD），它仍然是不重要的，因为它的贡献不超过百分之几的总冷却。我们还表明，在足够强的宇宙射线或X射线通量照射的气体中，H +可以成为气体中占主导地位的冷却剂，虽然所需的通量的大小使这种情况不太可能发生。
Studies of the formation of metal-free Population III stars usually focus primarily on the role played by H2 cooling, on account of its large chemical abundance relative to other possible molecular or ionic coolants. However, while H2 is generally the most important coolant at low gas densities, it is not an effective coolant at high gas densities, owing to the low critical density at which it reaches local thermodynamic equilibrium (LTE) and to the large opacities that develop in its emission lines. It is therefore possible that emission from other chemical species may play an important role in cooling high-density primordial gas. A particularly interesting candidate is the H + molecular ion. This ion has an LTE cooling rate that is roughly a billion times larger than that of H2, and unlike other primordial molecular ions such as H + or HeH + , it is not easily removed from the gas by collisions with H or H2 .I t is already known to be an important coolant in at least one astrophysical context – the upper atmospheres of gas giants – but its role in the cooling of primordial gas has received little previous study. In this paper, we investigate the potential importance of H + cooling in primordial gas using a newly developed H + cooling function and the most detailed model of primordial chemistry published to date. We show that although H + is, in most circumstances, the third most important coolant in dense primordial gas (after H2 and HD), it is nevertheless unimportant, as it contributes no more than a few per cent of the total cooling. We also show that in gas irradiated by a sufficiently strong flux of cosmic rays or X-rays, H + can become the dominant coolant in the gas, although the size of the flux required renders this scenario unlikely to occur.