Gravitational Binding and Star Formation in Molecular Clouds of the Milky Way

Abstract

The gravitational binding and star-forming properties of molecular clouds (MCs) in the Milky Way (MW) are estimated from CO cloud observations and from a model of pressure-bounded virial equilibrium (PVE). Two CO surveys are analyzed with the standard CO conversion factor. The main results are: (1) For each survey the cloud virial parameter $\alpha_{vir}$ increases by a factor ~2 from galactocentric radius $R_{gal}$ = 4 kpc to 15 kpc. (2) PVE models match these trends only if the surface densities of survey clouds and nearby stars are comparable. This evidence of environmental influence resembles that seen in other disk galaxies. (3) Many survey clouds form stars even though their virial parameter exceeds the critical value $\alpha_{vir}\approx2$. In PVE such clouds with constant velocity dispersion have stable equilibrium and cannot form stars by simple global collapse. (4) However, simulations show that $\alpha_{vir}\approx2$ clouds with dissipating turbulence may form filaments, cores and protostars with little global contraction. Such clouds can match the MW star formation rate if their protostellar cores have mass fraction ~10$^{-3}$. A simple model predicts that the star-forming age of a cloud is proportional to the ratio of its YSOs to its mass. (5) Clouds within ~500 pc of the Sun are predicted to have star-forming ages 1-10 Myr and average YSO age ~2 Myr, matching evolutionary models. The Orion A cloud is predicted to have ~60 Class 0 protostars, ~2900 YSOs and efficiency $SFE\approx0.02$, in good agreement with observed estimates.