论恒星形成动力学模型中阈值的出现
On the Appearance of Thresholds in the Dynamical Model of Star Formation
摘要：
恒星形成率（SFR）的表面密度和气体表面密度之间的肯尼库特-施密特（KS）关系有三个不同的幂律，这可能是由一个模型导致的，在这个模型中，气体以动力学速率的固定分数坍缩。当观测到的气体具有可探测的特征密度时，其幂律斜率为1;当气体的厚度近似恒定时，总气体的幂律斜率为1.5，就像在星系的主盘中那样;当气体的厚度由自引力调节并且速度色散近似恒定时，总气体的幂律斜率为2，就像在螺旋星系、矮不规则星系和巨分子云的外部那样。当一种示踪剂（HCN）在线性部分上而另一种（CO）在1.5部分上时，所观察到的星形成效率（每单位CO的SFR）与致密气体分数（HCN/CO）的标度来自KS关系。阈值密度或柱密度与一个恒定的SFR每单位气体质量以上的阈值被认为是选择效应的观察，因为是恒星形成的观察，只有在密集的部分云。该模型允许推导出所有三个KS关系使用密度的概率分布函数，没有恒星形成的阈值。失败的星系和系统与子KS SFR的预测有气体是由一个平衡的温暖阶段，热牛仔裤长度超过Toomre长度为主。预测分子气体为主的年轻星系的平方关系。
英文摘要：
The Kennicutt-Schmidt (KS) relationship between the surface density of the star formation rate (SFR) and the gas surface density has three distinct power laws that may result from one model in which gas collapses at a fixed fraction of the dynamical rate. The power law slope is 1 when the observed gas has a characteristic density for detection, 1.5 for total gas when the thickness is about constant as in the main disks of galaxies, and 2 for total gas when the thickness is regulated by self-gravity and the velocity dispersion is about constant, as in the outer parts of spirals, dwarf irregulars, and giant molecular clouds. The observed scaling of the star formation efficiency (SFR per unit CO) with the dense gas fraction (HCN/CO) is derived from the KS relationship when one tracer (HCN) is on the linear part and the other (CO) is on the 1.5 part. Observations of a threshold density or column density with a constant SFR per unit gas mass above the threshold are proposed to be selection effects, as are observations of star formation in only the dense parts of clouds. The model allows a derivation of all three KS relations using the probability distribution function of density with no thresholds for star formation. Failed galaxies and systems with sub-KS SFRs are predicted to have gas that is dominated by an equilibrium warm phase where the thermal Jeans length exceeds the Toomre length. A squared relation is predicted for molecular gas-dominated young galaxies.
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