CO-to-H2转换因子依赖于恒星形成率吗？
Does the CO-to-H2 conversion factor depend on the star formation rate?
摘要：
我们提出了一系列的数值模拟，探讨如何的“X-因子”，$X_{CO}$ -所观察到的综合CO排放和分子氢的柱密度之间的转换因子-随环境条件，其中的分子云被放置。我们的研究主要围绕两个环境条件：宇宙射线电离率（CRIR）和星际辐射场（ISRF）的强度。由于星际介质的这两种性质都起源于大质量恒星，因此本文假设ISRF和CRIR的强度都与本地恒星形成率（SFR）呈线性关系。本研究中的云建模首先涉及运行数值模拟，以捕获云动力学，以及随时间变化的化学和ISM加热和冷却。这些模拟，然后后处理线辐射转移代码创建合成12 CO（1-0）的排放地图，从$X_{CO}$可以计算。我们发现，对于1 E4太阳质量维里化云的平均密度100厘米$^{-3}$，$X_{CO}$是只有弱依赖于当地的SFR，变化了几个超过两个数量级的SFR的一个因素。相比之下，我们发现，对于类似的云，但质量为1 e5太阳质量，X因子将在SFR的相同范围内变化一个数量级，这意味着河外恒星形成规律应谨慎看待。然而，对于密度更大（$10^4$ cm $^{-3}$）的超维里云，如在银河系中心发现的那些云，X因子再次独立于当地的SFR。
英文摘要：
We present a series of numerical simulations that explore how the `X-factor', $X_{CO}$ -- the conversion factor between the observed integrated CO emission and the column density of molecular hydrogen -- varies with the environmental conditions in which a molecular cloud is placed. Our investigation is centred around two environmental conditions in particular: the cosmic ray ionisation rate (CRIR) and the strength of the interstellar radiation field (ISRF). Since both these properties of the interstellar medium have their origins in massive stars, we make the assumption in this paper that both the strength of the ISRF and the CRIR scale linearly with the local star formation rate (SFR). The cloud modelling in this study first involves running numerical simulations that capture the cloud dynamics, as well as the time-dependent chemistry, and ISM heating and cooling. These simulations are then post-processed with a line radiative transfer code to create synthetic 12CO (1-0) emission maps from which $X_{CO}$ can be calculated. We find that for 1e4 solar mass virialised clouds with mean density 100 cm$^{-3}$, $X_{CO}$ is only weakly dependent on the local SFR, varying by a factor of a few over two orders of magnitude in SFR. In contrast, we find that for similar clouds but with masses of 1e5 solar masses, the X-factor will vary by an order of magnitude over the same range in SFR, implying that extra-galactic star formation laws should be viewed with caution. However, for denser ($10^4$ cm$^{-3}$), super-virial clouds such as those found at the centre of the Milky Way, the X-factor is once again independent of the local SFR.
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