Does External Pressure Explain Recent Results for Molecular Clouds?

TL;DR

This paper investigates how external pressure influences molecular cloud stability and observed properties, suggesting that varying external pressures can explain deviations from virial equilibrium and modify the size-linewidth relation.

Contribution

It demonstrates that external pressure variations can account for observed cloud dynamics and alters the classical size-linewidth relationship, providing a new perspective on cloud stability.

Findings

External pressure explains deviations from virial equilibrium.

Most clouds are at a critical mass for stability influenced by pressure.

External pressure modifies the size-linewidth relationship.

Abstract

The recent paper by Heyer et al (2009) indicates that observations of size, linewidth and column density of interstellar clouds do not agree with simple virial equilibrium (VE) as a balance between gravitational and kinetic energies in the sense that the clouds either have too much kinetic energy or too little mass to be bound. This may be explained by violation of VE as suggested by Dobbs et al 2011, by observational underestimation of the masses as suggested by Heyer et al 2009, or by an external pressure acting as an additional confining force as suggested earlier by Heyer et al 2004. The data of Heyer et al. 2009 cannot be explained with a single value for the external pressure, but if different clouds in the sample have different external pressures in the range of Pe/k = E4 to E7 cm-3 K, then most of the clouds could be in pressure virial equilibrium (PVE). In this paper we discuss two consequences of the external pressure. First, we show that the observational data are consistent with the hypothesis (Chie\'ze 1987) that most clouds are at a critical mass for dynamical stability determined solely by the pressure. Above this mass a cloud is unstable to gravitational collapse or fragmentation. Second, we show that the external pressure modifies the well-known size-linewidth relationship first proposed by Larson (1981) so that the proportionality is no longer constant but depends on the external pressure.