THE DEPENDENCE OF STAR FORMATION EFFICIENCY ON GAS SURFACE DENSITY

Studies by Lada et al. and Heiderman et al. have suggested that star formation mostly occurs above a threshold in gas surface density Sigma of Sigma(c) similar to 120 M-circle dot pc(-2) (A(K) similar to 0.8). Heiderman et al. infer a threshold by combining low-mass star-forming regions, which show a steep increase in the star formation rate per unit area Sigma(SFR) with increasing Sigma, and massive cores forming luminous stars which show a linear relation. We argue that these observations do not require a particular density threshold. The steep dependence of Sigma(SFR), approaching unity at protostellar core densities, is a natural result of the increasing importance of self-gravity at high densities along with the corresponding decrease in evolutionary timescales. The linear behavior of Sigma(SFR) versus Sigma in massive cores is consistent with probing dense gas in gravitational collapse, forming stars at a characteristic free-fall timescale given by the use of a particular molecular tracer. The low-mass and high-mass regions show different correlations between gas surface density and the area A spanned at that density, with A similar to Sigma(-3) for low-mass regions and A similar to Sigma(-1) for the massive cores; this difference, along with the use of differing techniques to measure gas surface density and star formation, suggests that connecting the low-mass regions with massive cores is problematic. We show that the approximately linear relationship between dense gas mass and stellar mass used by Lada et al. similarly does not demand a particular threshold for star formation and requires continuing formation of dense gas. Our results are consistent with molecular clouds forming by galactic hydrodynamic flows with subsequent gravitational collapse.