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arXiv 1003.1340astro-ph.GA
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分子云中分子氢和一氧化碳丰度的关系

On the Relationship Between Molecular Hydrogen and Carbon Monoxide Abundances in Molecular Clouds

S. C. O. Glover, M. -M. Mac Low

备注：
16 pages, 11 figures. Updated to match version accepted by MNRAS
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摘要：

最常见的分子气体示踪剂是来自CO的线发射。然而，在不同于银河系的环境中，这种示踪剂的可靠性长期受到质疑。我们研究了H2和CO丰度之间的关系，使用一个完全动态模型的磁化湍流耦合到一个化学网络简化到只遵循H2和CO的形成和破坏的主导途径，并包括使用六射线近似的光解。我们发现，H2的丰度主要是由其形成的时间，这是成比例的密度和金属丰度的产品，但不敏感的光解量。光解离只有在视星等的十分之几的振幅下才变得重要，这与观测和先前的理论工作一致。另一方面，CO形式迅速，在一个动态的时间内，但其丰度主要取决于光解，只有一个弱的二级依赖H2丰度。结果表明，在平均视反射率A_V < 3时，CO丰度有一个明显的截止值.在较低的A_V值下，我们发现H_2柱密度与CO发射率之比X_CO与A_V^（-3.5）成正比。这解释了在低金属量系统中观测到的来自CO观测和其他技术（如红外发射）的云量之间的差异。我们的工作预测，低金属丰度系统中的CO明亮云应该比银河系云更大或更密集，或者两者兼而有之。我们的研究结果进一步解释了作为阈值效应的分子云柱密度的窄范围，而不需要维里平衡的假设。

英文摘要：

The most usual tracer of molecular gas is line emission from CO. However, the reliability of that tracer has long been questioned in environments different from the Milky Way. We study the relationship between H2 and CO abundances using a fully dynamical model of magnetized turbulence coupled to a chemical network simplified to follow only the dominant pathways for H2 and CO formation and destruction, and including photodissociation using a six-ray approximation. We find that the abundance of H2 is primarily determined by the amount of time available for its formation, which is proportional to the product of the density and the metallicity, but insensitive to photodissociation. Photodissociation only becomes important at extinctions under a few tenths of a visual magnitude, in agreement with both observational and prior theoretical work. On the other hand, CO forms quickly, within a dynamical time, but its abundance depends primarily on photodissociation, with only a weak secondary dependence on H2 abundance. As a result, there is a sharp cutoff in CO abundance at mean visual extinctions A_V < 3. At lower values of A_V we find that the ratio of H2 column density to CO emissivity X_CO is proportional to A_V^(-3.5). This explains the discrepancy observed in low metallicity systems between cloud masses derived from CO observations and other techniques such as infrared emission. Our work predicts that CO-bright clouds in low metallicity systems should be systematically larger or denser than Milky Way clouds, or both. Our results further explain the narrow range of observed molecular cloud column densities as a threshold effect, without requiring the assumption of virial equilibrium.

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