The Early Stage of Molecular Cloud Formation by Compression of Two-phase Atomic Gases

TL;DR

This study uses 3D magnetohydrodynamic simulations to explore how the angle between atomic gas flows and magnetic fields influences molecular cloud formation, revealing a critical angle that determines the efficiency of cloud development.

Contribution

It introduces an analytic model for the critical angle of cloud formation based on magnetic field strength, density, and collision speed, advancing understanding of magnetic effects in cloud formation.

Findings

Critical angle for cloud formation is less than ~15° for magnetic fields >1 μG.

Turbulence is maintained when flow is aligned with magnetic fields, promoting cloud growth.

Strong magnetic fields suppress dense cloud formation at large angles.

Abstract

We investigate the formation of molecular clouds from atomic gas by using three-dimensional magnetohydrodynamic simulations, including non-equilibrium chemical reactions and heating/cooling processes. We consider super-Alfv\'enic head-on colliding flows of atomic gas possessing the two-phase structure that consists of HI clouds and surrounding warm diffuse gas. We examine how the formation of molecular clouds depends on the angle $\theta$ between the upstream flow and the mean magnetic field. We find that there is a critical angle $\theta_\mathrm{cr}$ above which the shock-amplified magnetic field controls the post-shock gas dynamics. If the atomic gas is compressed almost along the mean magnetic field ($\theta\ll\theta_\mathrm{cr}$), super-Alfv\'enic anisotropic turbulence is maintained by the accretion of the highly inhomogeneous upstream atomic gas. As a result, a greatly extended turbulence-dominated post-shock layer is generated. Around $\theta\sim \theta_\mathrm{cr}$, the shock-amplified magnetic field weakens the post-shock turbulence, leading to a dense post-shock layer. For $\theta\gg \theta_\mathrm{cr}$, the strong magnetic pressure suppresses the formation of cold dense clouds. Efficient molecular cloud formation is expected if $\theta$ is less than a few times $\theta_\mathrm{cr}$. Developing an analytic model and performing a parameter survey, we obtain an analytic formula for the critical angle as a function of the mean density, collision speed, and field strength of the upstream atomic gas. The critical angle is found to be less than $\sim 15^\circ$ as long as the field strength is larger than $1~\mu$G, indicating that the probability of occurrence of compression with $\theta<\theta_\mathrm{cr}$ is limited if shock waves come from various directions.