恒星形成速率-致密气体质量关系的新参数化：包含气体密度梯度
A new parameterization of the star formation rate-dense gas mass relation: embracing gas density gradients
摘要：
众所周知，分子云和团块内部的气体密度梯度与均匀密度的气体储层相比会提高它们的恒星形成率。在分子云和团块的稠密气体质量$M_{dg}$和恒星形成率$SFR$之间的关系中，可以观察到这种效应，气体密度梯度越陡，$SFR/M_{dg}$的$SFR/M_{dg}$比率越高。本文的内容是双重的。首先，我们基于Parmentier（2019）引入的放大因子的概念来重新定义稠密气体关系（即$M_{dg}$和$SFR$之间的关系）。$SFR/M_{dg}$比不仅取决于气体的平均自由落体时间和每自由落体时间的（固有）恒星形成效率，还取决于气体密度分布的对数斜率$-p$和团簇中心恒定密度区域的相对范围。其次，我们表明，附近的分子云遵循新定义的稠密气体关系，提供其稠密气体质量的体积密度标准的基础上定义的。我们还发现了银河系中央分子区（CMZ）的致密分子云的相同趋势，尽管与附近的云相比，这一分子云的比例缩小了$10$。讨论了邻近云和CMZ云在$(p, SFR/M_{dg})$参数空间中各自的位置。
英文摘要：
It is well-established that a gas density gradient inside molecular clouds and clumps raises their star formation rate compared to what they would experience from a gas reservoir of uniform density. This effect should be observed in the relation between dense-gas mass $M_{dg}$ and star formation rate $SFR$ of molecular clouds and clumps, with steeper gas density gradients yielding higher $SFR/M_{dg}$ ratios. The content of this paper is two-fold. Firstly, we build on the notion of magnification factor introduced by Parmentier (2019) to redefine the dense-gas relation (i.e. the relation between $M_{dg}$ and $SFR$). Not only does the $SFR/M_{dg}$ ratio depend on the mean free-fall time of the gas and on its (intrinsic) star formation efficiency per free-fall time, it also depends on the logarithmic slope $-p$ of the gas density profile and on the relative extent of the constant-density region at the clump center. Secondly, we show that nearby molecular clouds follow the newly-defined dense-gas relation, provided that their dense-gas mass is defined based on a volume density criterion. We also find the same trend for the dense molecular clouds of the Central Molecular Zone (CMZ) of the Galaxy, although this one is scaled down by a factor of $10$ compared to nearby clouds. The respective locii of both nearby and CMZ clouds in the $(p, SFR/M_{dg})$ parameter space is discussed.
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