2011MNRAS.416..710F

The recent paper by Heyer et al. indicates that observations of size, linewidth and column density of interstellar clouds do not agree with simple virial equilibrium (SVE) 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 SVE as suggested by Dobbs et al., by observational underestimation of the masses as suggested by Heyer et al. or by an external pressure acting as an additional confining force as suggested earlier by Heyer et al. The data of Heyer et al. 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 P_e/k= 10⁴–10⁷/cm3 K, then most of the clouds could be in pressure-bounded virial equilibrium. In this paper we discuss two consequences of the external pressure. First, we show that the observational data are consistent with the hypothesis 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. Secondly, we show that the external pressure modifies the well-known size–linewidth relationship first proposed by Larson so that the proportionality is no longer constant but depends on the external pressure.