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arXiv 1105.3073astro-ph.GA
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分子气体是恒星形成的必要条件吗？

Is molecular gas necessary for star formation?

S. C. O. Glover, P. C. Clark

备注：
13 pages, 9 figures. Added new section on density PDF, other minor revisions to match version accepted by MNRAS
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摘要：

在星系尺度上，恒星形成的表面密度似乎与分子气体的表面密度有很好的相关性。这使得许多作者提出，气体的化学状态与其形成恒星的能力之间存在因果关系-换句话说，假设气体在恒星形成之前必须是分子。我们通过使用一系列不同的化学模型，在具有类似于小分子云的性质的致密气体云中模拟恒星形成来测试这一假设，其中一个模型没有遵循分子的形成，气体被假设为始终保持原子状态，我们发现气体中分子的存在对气体形成恒星的能力几乎没有影响：恒星形成在原子气体中和分子气体中一样容易发生。在低密度（< 10^4 cm^-3）下，气体通过C+精细结构发射的冷却效率几乎与通过CO旋转谱线发射的冷却效率一样，而在高密度下，主要的冷却过程涉及能量从气体到尘埃的转移，这意味着分子的存在也不重要。H2的冷却效率特别低，仅占云中总冷却量的1%。我们发现，控制恒星形成速率的最重要因素不是化学成分，而是气体屏蔽星际辐射场的能力。由于这也是气体中分子生存的先决条件，我们的结果支持了这样一种观点，即分子形成和冷气体的形成都与云的柱密度相关-因此其屏蔽自身的能力-而不是彼此直接相关。

英文摘要：

On galactic scales, the surface density of star formation appears to be well correlated with the surface density of molecular gas. This has lead many authors to suggest that there exists a causal relationship between the chemical state of the gas and its ability to form stars -- in other words, the assumption that the gas must be molecular before star formation can occur. We test this hypothesis by modelling star formation within a dense cloud of gas with properties similar to a small molecular cloud using a series of different models of the chemistry, ranging from one in which the formation of molecules is not followed and the gas is assumed to remain atomic throughout, to one that tracks the formation of both H2 and CO. We find that presence of molecules in the gas has little effect on the ability of the gas to form stars: star formation can occur just as easily in atomic gas as in molecular gas. At low densities (< 10^4 cm^-3), the gas is able to cool via C+ fine-structure emission almost as efficiently as via CO rotational line emission, while at higher densities, the main cooling process involves the transfer of energy from gas to dust, meaning that the presence of molecules is again unimportant. Cooling by H2 is particularly inefficient, accounting for as little as 1 percent of the overall cooling in the cloud. Rather than the chemical makeup, we find that the most important factor controlling the rate of star formation is the ability of the gas to shield itself from the interstellar radiation field. As this is also a prerequisite for the survival of molecules within the gas, our results support a picture in which molecule formation and the formation of cold gas are both correlated with the column density of the cloud -- and thus its ability to shield itself -- rather than being directly correlated with each other.

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