标题： HCN/CO 比率是否可以追踪气体的恒星形成比例？ 二． CO 和 HCN 发射率的变化 显示英文标题

标题： Does the HCN/CO ratio trace the star-forming fraction of gas? II. Variations in CO and HCN Emissivity

摘要： 我们利用正常恒星形成星系以及更极端系统的云团观测性质，来建模 HCN 和 CO 的$J=1-0$转移发射率。这些模型与 HCN 和 CO 的$J=1-0$转移的观测结果进行了比较。我们将这些模型发射率与恒星形成重力湍流模型的预测相结合，探讨激发和光学深度对 CO 和 HCN 辐射的影响，并评估观测到的 HCN/CO 比值是否追踪了分子云中引力束缚致密气体（$f_\mathrm{grav}$）的比例。我们的建模得出的 HCN/CO 比值和发射率与观测测量结果一致。CO 辐射在不同环境中表现出各种光学深度，从正常星系中的光学厚到极端系统中的适度光学薄。HCN 只是适度光学厚，且在普通和极端星系中都存在显著次热激发。我们发现 HCN/CO 与$f_\mathrm{grav}$呈反相关，这与恒星形成的重力湍流模型预测相符。相反，这个比率追踪的是中等密度气体（$n>10^{3.5}\ \mathrm{cm}^{-3}$），低于标准致密气体阈值$n>10^{4.5}\ \mathrm{cm}^{-3}$。CO 发射率的变化强烈依赖于光学深度，因为云气动力学变化。HCN 发射率更强烈地依赖于激发，因此并不直接追踪 CO 发射率的变化。我们得出结论，如果恒星形成的临界密度在分子云中发生变化，那么单一线谱比值（例如 HCN/CO）不会始终追踪引力束缚的恒星形成气体的比例。这项工作强调了在应用 HCN 转换因子以估算近邻星系中致密（即$n>10^{4.5}\ \mathrm{cm}^{-3}$）气体含量时需要考虑的重要不确定性。 显示英文摘要

摘要： We model emissivities of the HCN and CO $J=1-0$ transitions using measured properties of clouds found in normal star forming galaxies and more extreme systems. These models are compared with observations of HCN and CO $J=1-0$ transitions. We combine these model emissivities with predictions of gravoturbulent models of star formation, explore the impact of excitation and optical depth on CO and HCN emission, and assess if observed HCN/CO ratios track the fraction of gravitationally-bound dense gas, $f_\mathrm{grav}$, in molecular clouds. Our modeled HCN/CO ratios and emissivities are consistent with measurements from observations. CO emission shows a range of optical depths across different environments, from optically thick in normal galaxies to moderately optically thin in extreme systems. HCN is only moderately optically thick, with significant subthermal excitation in both normal and extreme galaxies. We find an anticorrelation between HCN/CO and $f_\mathrm{grav}$ as predicted by gravoturbulent models of star formation. Instead this ratio tracks gas at moderate densities ($n>10^{3.5}\ \mathrm{cm}^{-3}$), which is below the standard dense gas threshold of $n>10^{4.5}\ \mathrm{cm}^{-3}$. Variations in CO emissivity depend strongly on optical depth, due to variations in the dynamics of the cloud gas. HCN emissivity depends more strongly on excitation, and thus does not directly track variations in CO emissivity. We conclude that a single line ratio, such as HCN/CO, will not consistently track the fraction of gravitationally-bound, star-forming gas if the critical density for star formation varies in molecular clouds. This work highlights important uncertainties that need to be considered when observationally applying an HCN conversion factor in order to estimate the dense (i.e. $n>10^{4.5}\ \mathrm{cm}^{-3}$) gas content in nearby galaxies.