Supernova Driving. IV. The Star-formation Rate of Molecular Clouds

We compute the star-formation rate (SFR) in molecular clouds (MCs) that originate ab initio in a new, higherresolution simulation of supernova-driven turbulence. Because of the large number of well-resolved clouds with self-consistent boundary and initial conditions, we obtain a large range of cloud physical parameters with realistic statistical distributions, which is. an unprecedented sample of star-forming regions to test SFR models and to interpret observational surveys. We confirm the dependence of the SFR per free-fall time, SFRff, on the virial parameter, alpha(vir), found in previous simulations, and compare a revised version of our turbulent fragmentation model with the numerical results. The dependences on Mach number, M , gas to magnetic pressure ratio, beta, and compressive to solenoidal power ratio, chi at fixed alpha(vir) are not well constrained, because of random scatter due to time and cloud-to-cloud variations in SFRff. We find that SFRff in MCs can take any value in the range of. 0 <= SFRff less than or similar to 0.2, and its probability distribution peaks at a value of. SFRff approximate to 0.025, consistent with observations. The values of SFRff and the scatter in the SFRff-alpha(vir) relation are consistent with recent measurements in nearby MCs and in clouds near the Galactic center. Although not explicitly modeled by the theory, the scatter is consistent with the physical assumptions of our revised model and may also result in part from a lack of statistical equilibrium of the turbulence, due to the transient nature of MCs.