Why are most molecular clouds not gravitationally bound?

TL;DR

This paper explains why most molecular clouds are not gravitationally bound by proposing that cloud collisions and stellar feedback maintain high internal velocities, allowing only dense regions to form stars.

Contribution

It introduces a model where cloud-cloud collisions and feedback regulate cloud dynamics, explaining the unbound nature of molecular clouds and star formation in dense regions.

Findings

Virial parameter distributions match observations under the model.

Star formation efficiency in bound regions is about 5-10%.

Gas composition in clouds changes over a few million years.

Abstract

The most recent observational evidence seems to indicate that giant molecular clouds are predominantly gravitationally unbound objects. In this paper we show that this is a natural consequence of a scenario in which cloud-cloud collisions and stellar feedback regulate the internal velocity dispersion of the gas, and so prevent global gravitational forces from becoming dominant. Thus, while the molecular gas is for the most part gravitationally unbound, local regions within the denser parts of the gas (within the clouds) do become bound and are able to form stars. We find that the observations, in terms of distributions of virial parameters and cloud structures, can be well modelled provided that the star formation efficiency in these bound regions is of order 5 - 10 percent. We also find that in this picture the constituent gas of individual molecular clouds changes over relatively short time scales, typically a few Myr.