# Feeding vs. Falling: The growth and collapse of molecular clouds in a turbulent interstellar medium

## TL;DR

This study uses advanced simulations to explore how molecular clouds form, grow, and collapse in a turbulent interstellar medium, highlighting the roles of accretion, supernova feedback, and gravitational contraction.

## Contribution

The paper provides detailed 3D MHD simulations revealing the combined effects of turbulence, accretion, and supernovae on molecular cloud evolution and their gravitational contraction dynamics.

## Key findings

- Mass accretion involves turbulent and free-fall processes.
- Supernovae both compress and disrupt cloud envelopes.
- Gravitational contraction drives non-thermal motions.

## Abstract

In order to understand the origin of observed molecular cloud properties, it is critical to understand how clouds interact with their environments during their formation, growth, and collapse. It has been suggested that accretion-driven turbulence can maintain clouds in a highly turbulent state, preventing runaway collapse, and explaining the observed non-thermal velocity dispersions. We present 3D, AMR, MHD simulations of a kiloparsec-scale, stratified, supernova-driven, self-gravitating, interstellar medium, including diffuse heating and radiative cooling. These simulations model the formation and evolution of a molecular cloud population in the turbulent interstellar medium. We use zoom-in techniques to focus on the dynamics of the mass accretion and its history for individual molecular clouds. We find that mass accretion onto molecular clouds proceeds as a combination of turbulent and near free-fall accretion of a gravitationally bound envelope. Nearby supernova explosions have a dual role, compressing the envelope, boosting accreted mass, but also disrupting parts of the envelope and eroding mass from the cloud's surface. It appears that the inflow rate of kinetic energy onto clouds from supernova explosions is insufficient to explain the net rate of charge of the cloud kinetic energy. In the absence of self-consistent star formation, conversion of gravitational potential into kinetic energy during contraction seems to be the main driver of non-thermal motions within clouds. We conclude that although clouds interact strongly with their environments, bound clouds are always in a state of gravitational contraction, close to runaway, and their properties are a natural result of this collapse.