Universal Properties of Dense Clumps in Magnetized Molecular Clouds Formed through Shock Compression of Two-phase Atomic Gases

TL;DR

This study uses 3D magnetohydrodynamical simulations to explore how dense clumps form in magnetized molecular clouds, revealing universal properties of dense clumps regardless of initial flow conditions, which may explain the universality of the stellar initial mass function.

Contribution

It demonstrates that dense clumps in molecular clouds develop universal statistical properties independent of initial collision parameters, supported by an analytic virial parameter formula.

Findings

Dense clumps exhibit universal statistical properties at high densities.

Internal velocity dispersions tend to subsonic levels.

The virial parameter formula aligns well with simulation results.

Abstract

We investigate the formation of molecular clouds from atomic gas by using three-dimensional magnetohydrodynamical simulations,including non-equilibrium chemical reactions, heating/cooling processes, and self-gravity by changing the collision speed $V_0$ and the angle $\theta$ between the magnetic field and colliding flow. We found that the efficiency of the dense gas formation depends on $\theta$. For small $\theta$, anisotropic super-Alfv\'enic turbulence delays the formation of gravitationally unstable clumps. An increase in $\theta$ develops shock-amplified magnetic fields along which the gas is accumulated, making prominent filamentary structures. We further investigate the statistical properties of dense clumps identified with different density thresholds. The statistical properties of the dense clumps with lower densities depend on $V_0$ and $\theta$ because their properties are inherited from the global turbulence structure of the molecular clouds. By contrast, denser clumps appear to have asymptotic universal statistical properties, which do not depend on the properties of the colliding flow significantly. The internal velocity dispersions approach subsonic and plasma $\beta$ becomes order of unity. We develop an analytic formula of the virial parameter which reproduces the simulation results reasonably well. This property may be one of the reasons for the universality of the initial mass function of stars.