恒星形成在稠密的气体中，但“稠密”是什么意思？
Star Formation Occurs in Dense Gas, but What Does "Dense" Mean?
摘要：
我们报告了一个项目的结果，映射HCN和HCO+ J = 1-0的排放量对样本的分子云在银河系内部，所有包含密集的团块，积极参与恒星的形成。我们比较了这两个分子线示踪剂毫米连续发射和消光，从13 CO推断，作为分子云中致密气体的示踪剂。来自致密气体的每种示踪剂的线光度的分数，如AV > 8等所测量的，在云与云之间变化很大。在所有情况下，总光度的相当一部分（在大多数情况下，大部分）出现在AV > 8等阈值以下的气体中，并且在强毫米连续辐射区域之外。对于其他星系的HCN光度的测量可能主要是由这种气体在较低的表面密度。对总谱线光度的重要贡献，甚至占主导地位的贡献，可以出现在具有典型云整体密度的气体中（密度约为每立方厘米100）。从HCN或HCO+排放本身定义致密团块，类似于以前的研究，导致广泛的团块性质，其中一些比以前的研究大得多，密度小。HCN和HCO+具有类似的追踪该样品中云的致密气体的能力。对于低维里参数的两个云团，13 CO对稠密气体的示踪效果较差，而对于其它四个云团，13 CO对稠密气体的示踪效果也同样好（或差）。
英文摘要：
We report results of a project to map HCN and HCO+ J = 1-0 emission toward a sample of molecular clouds in the inner Galaxy, all containing dense clumps that are actively engaged in star formation. We compare these two molecular line tracers with millimeter continuum emission and extinction, as inferred from 13CO, as tracers of dense gas in molecular clouds. The fraction of the line luminosity from each tracer that comes from the dense gas, as measured by AV > 8 mag, varies substantially from cloud to cloud. In all cases, a substantial fraction (in most cases, the majority) of the total luminosity arises in gas below the AV > 8 mag threshold and outside the region of strong mm continuum emission. Measurements of the luminosity of HCN toward other galaxies will likely be dominated by such gas at lower surface density. Substantial, even dominant, contributions to the total line luminosity can arise in gas with densities typical of the cloud as a whole (densities about 100 per cubic cm). Defining the dense clump from the HCN or HCO+ emission itself, similarly to previous studies, leads to a wide range of clump properties, with some being considerably larger and less dense than in previous studies. HCN and HCO+ have similar ability to trace dense gas for the clouds in this sample. For the two clouds with low virial parameters, the 13CO is definitely a worse tracer of the dense gas, but for the other four, it is equally good (or bad) at tracing dense gas.
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