High Mass Star Formation in Galactic and Extragalactic Molecular Clouds

Abstract

We utilize CO and IR surveys to determine high mass star formation rates in molecular clouds. For galactic molecular cloud - H II region complexes, the average far IR luminosity per unit cloud mass, (L IR /M VT ) = 4 (LΘ/MΘ), is independent of the cloud mass for clouds from 104 to 5 × 106 MΘ. Since the source of the far IR luminosity is primarily young massive stars, the rate of star formation per unit mass is independent of cloud mass. This argues against nonlinear processes such as supernova-induced star formation, or star formation-induced star formation which would result in higher star formation efficiencies in the most massive clouds. The moderate star formation rate observed, which corresponds to a gas depletion time within the clouds of ~ 2 × 109 years for formation of high mass stars (M > 2 MΘ), suggests that the star formation process is self limiting. A comparison of galactic clouds with L IR and L CO from external galaxies (Sage 1987, Solomon and Sage 1987) shows that isolated or weakly interacting galaxies have a far IR to CO luminosity ratio L IR /L CO which can be interpreted as arising from star formation rates in giant molecular clouds similar to those in the Milky Way. Galaxies with tidal tails or bridges and merging systems have a much higher L IR /L CO and thus a much higher star formation rate per unit H2 mass than any galactic molecular cloud. Thus the star formation process must be fundamentally different than in galactic molecular clouds. The regulation which keeps the star formation rate modest in the galactic disk clouds is clearly not operating in these closely interacting systems.