Shock-triggered formation of magnetically-dominated clouds

TL;DR

This study uses magnetohydrodynamic simulations to explore how shocks in hot media can trigger the formation of magnetically dominated, dense molecular clouds from atomic clouds, shedding light on star formation regions.

Contribution

It demonstrates the process by which shocks induce thermal instability and magnetic dominance, leading to molecular cloud formation in a magnetized environment.

Findings

Weak and moderate shocks can produce giant molecular clouds.

Magnetic fields become dominant during cloud formation.

Dense shells form and fragment, facilitating star formation.

Abstract

To understand the formation of a magnetically dominated molecular cloud out of an atomic cloud, we follow the dynamical evolution of the cloud with a time-dependent axisymmetric magnetohydrodynamic code. A thermally stable warm atomic cloud is initially in static equilibrium with the surrounding hot ionised gas. A shock propagating through the hot medium interacts with the cloud. As a fast-mode shock propagates through the cloud, the gas behind it becomes thermally unstable. The $\beta$ value of the gas also becomes much smaller than the initial value of order unity. These conditions are ideal for magnetohydrodynamic waves to produce high-density clumps embedded in a rarefied warm medium. A slow-mode shock follows the fast-mode shock. Behind this shock a dense shell forms, which subsequently fragments. This is a primary region for the formation of massive stars. Our simulations show that only weak and moderate-strength shocks can form cold clouds which have properties typical of giant molecular clouds.