1212.4543 (Andreas Burkert et al.)

Density thresholds in and efficiencies of star formation    [PDF]

Andreas Burkert, Lee Hartmann

Recent studies by Lada et al. (2010) and Heiderman et al. (2010) have suggested that efficient star formation occurs above an approximate threshold in gas surface density Sigma of Sigma_c = 120 Msun/pc^3 (A_K=0.8). We find no precise threshold for star formation; the impression of such results from a continuous and steep power-law increase of the ratio of protostellar mass to molecular gas mass with Sigma, approaching unity at protostellar core densities, corresponding to Sigma=1000 Msun/pc^3. We argue that this increase in star formation efficiency results from the increasing importance of self-gravity with increasing density, along with the consequent decrease in evolutionary timescales. The observations are consistent with models in which regions of more diffuse molecular gas with column densities corresponding to A_V=1-2 are initially formed by converging galactic hydrodynamic flows which subsequently collapse gravitationally, producing a power-law relation between surface density and the area A spanned at that density of A=Sigma^(-3.2). We show that the finding of a strong correlation between the amount of gas above Sigma_c and the young stellar population by Lada et al. (2010) with ratios q of dense gas mass to stellar mass of 3View original: http://arxiv.org/abs/1212.4543