Gravitational Collapse in Turbulent Molecular Clouds. I. Gasdynamical Turbulence

Observed molecular clouds often appear to have very low star formationefficiencies and lifetimes an order of magnitude longer than their free-falltimes. Their support is attributed to the random supersonic motions observed inthem. We study the support of molecular clouds against gravitational collapseby supersonic, gas dynamical turbulence using direct numerical simulation.Computations with two different algorithms are compared: a particle-based,Lagrangian method (SPH), and a grid-based, Eulerian, second-order method(ZEUS). The effects of both algorithm and resolution can be studied with thismethod. We find that, under typical molecular cloud conditions, global collapsecan indeed be prevented, but density enhancements caused by strong shocksnevertheless become gravitationally unstable and collapse into dense cores and,presumably, stars. The occurance and efficiency of local collapse decreases asthe driving wave length decreases and the driving strength increases. Itappears that local collapse can only be prevented entirely with unrealisticallyshort wave length driving, but observed core formation rates can be reproducedwith more realistic driving. At high collapse rates, cores are formed on shorttime scales in coherent structures with high efficiency, while at low collapserates they are scattered randomly throughout the region and exhibitconsiderable age spread. We suggest that this naturally explains the observeddistinction between isolated and clustered star formation.Comment: Minor revisions in response to referee, thirteen figures, accepted to Astrophys.