AI 中文总结
研究多相星际介质中\(σ_s^2 - \mathcal{M}\)关系的适用性，通过云工厂模拟分子云复合体密度PDF，测试其在各相中的情况，发现该关系在不同相适用性不同，受测量方式影响，对部分相不适用，对H₂分布对数正态部分暂成立，还存在CO密度PDF宽度被高估问题。
AI 中文摘要
分子云的密度概率分布函数（PDF）是恒星形成分析理论的关键组成部分。在等温湍流的理想化模拟中，密度PDF的宽度\(σ_s^2\)取决于介质的声马赫数\(\mathcal{M}\)。\(σ_s^2 - \mathcal{M}\)关系被广泛用于将云尺度湍流与密度PDF联系起来，并进一步与恒星形成活动相关联，但其在多相星际介质（ISM）各相中是否适用仍未得到验证。在本研究中，我们评估\(σ_s^2 - \mathcal{M}\)关系是否适用于ISM的各个相。我们研究了云工厂模拟中分子云复合体的密度PDF，这是一组详细的放大模拟，能自洽地生成湍流多相ISM。我们测试了\(σ_s^2 - \mathcal{M}\)关系在热电离介质（HIM）、暖电离介质（WIM）、暖中性介质（WNM）、冷中性介质（CNM）、分子相以及由CO追踪的高屏蔽分子相中是否成立。我们发现经典的\(σ_s^2 - \mathcal{M}\)关系在不同相之间的适用性不同，并且强烈依赖于\(σ_s^2\)和\(\mathcal{M}\)的测量方式。该关系无法捕捉WNM和CNM密度分布的宽度，可能的影响因素包括非等温性和大规模相干运动。相比之下，我们发现\(σ_s^2 - \mathcal{M}\)关系对于H₂分布的对数正态部分暂时成立。经典关系系统地高估了CO密度PDF的宽度，这是由于CO作为分子气体示踪剂的选择性所致。
英文摘要
The density probability distribution (PDF) of molecular clouds is a crucial component of analytical theories of star formation. In idealised simulations of isothermal turbulence, the width of the density PDF, $σ_s^2$, is dependent on the sonic Mach number of the medium, $\mathcal{M}$. The $σ_s^2-\mathcal{M}$ relation is widely used to connect cloud-scale turbulence to the density PDF, and further to star formation activity, yet its validity within individual phases of the multi-phase interstellar medium (ISM) remains untested. In this study, we evaluate whether the $σ_s^2-\mathcal{M}$ relation is applicable to individual phases of the ISM. We study the density PDFs of molecular cloud complexes in the Cloud Factory simulations; a suite of detailed zoom-in simulations that self-consistently generate a turbulent, multi-phase ISM. We test whether the $σ_s^2-\mathcal{M}$ relation holds in the hot ionised medium (HIM), warm ionised medium (WIM), warm neutral medium (WNM), cold neutral medium (CNM), the molecular phase, and the highly-shielded molecular phase traced by CO. We find the applicability of the classical $σ_s^2-\mathcal{M}$ relation to vary between phases and depend strongly on how $σ_s^2$ and $\mathcal{M}$ are measured. The relation fails to capture the widths of the WNM and CNM density distributions, with possible contributing factors including non-isothermality and large-scale coherent motions. In contrast, we find the $σ_s^2-\mathcal{M}$ relation to tentatively hold for the log-normal portion of the H$_2$ distribution. The width of the CO density PDF is systematically overpredicted by the classical relation, resulting from the selective nature of CO as a molecular gas tracer.
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