Which Molecular Cloud Structures Are Bound?

TL;DR

This study examines molecular cloud structures across various tracers and finds that the fraction of bound structures varies with observational methods, impacting star formation rate estimates.

Contribution

It provides a comparative analysis of molecular cloud structures using different tracers, revealing how observational methods influence the perceived bound fraction and star formation predictions.

Findings

Bound fraction increases with tracer density and galaxy radius.

Estimated star formation rate from bound clouds is significantly lower.

Virial parameter decreases with structure mass and surface density.

Abstract

We analyze surveys of molecular cloud structures defined by tracers ranging from CO $J = 1-0$ through $^{13}$CO $J = 1-0$ to dust emission together with NH$_3$ data. The mean value of the virial parameter and the fraction of mass in bound structures depends on the method used to identify structures. Generally, the virial parameter decreases and the fraction of mass in bound structures increases with the effective density of the tracer, the surface density and mass of the structures, and the distance from the center of a galaxy. For the most complete surveys of structures in the Galaxy defined by CO $J = 1-0$, the fraction of mass that is in bound structures is 0.19. For catalogs of other galaxies based on CO $J = 2-1$, the fraction is 0.35. These results offer substantial alleviation of the fundamental problem of slow star formation. If only clouds found to be bound are counted and they are assumed to collapse in a free-fall time at their mean cloud density, the sum over all clouds in a complete survey of the Galaxy yields a predicted star formation rate of 46 solar masses per year, a factor of 6.5 less than if all clouds are bound.