Rapid Molecular Cloud and Star Formation: Mechanisms and Movies

TL;DR

This paper uses 3D simulations to show how flow-driven atomic gas accumulation and gravity lead to rapid molecular cloud formation and star formation, with CO formation and core development occurring quickly after cloud assembly.

Contribution

It demonstrates that global gravity and flow dynamics can explain rapid molecular cloud and star formation, supported by detailed simulations and CO formation analysis.

Findings

Rapid CO formation occurs after cloud assembly due to gravity-induced density increase.

Global gravity drives high-pressure dense gas necessary for star core formation.

Turbulence is secondary, mainly a consequence of gravitational forces.

Abstract

We demonstrate that the observationally inferred rapid onset of star formation after parental molecular clouds have assembled can be achieved by flow-driven cloud formation of atomic gas, using our previous three-dimensional numerical simulations. We post-process these simulations to approximate CO formation, which allows us to investigate the times at which CO becomes abundant relative to the onset of cloud collapse. We find that global gravity in a finite cloud has two crucial effects on cloud evolution. (a) Lateral collapse (perpendicular to the flows sweeping up the cloud) leads to rapidly increasing column densities above the accumulation from the one-dimensional flow. This in turn allows fast formation of CO, allowing the molecular cloud to ``appear'' rapidly. (b) Global gravity is required to drive the dense gas to the high pressures necessary to form solar-mass cores, in support of recent analytical models of cloud fragmentation. While the clouds still appear ``supersonically turbulent'', this turbulence is relegated to playing a secondary role, in that it is to some extent a consequence of gravitational forces.