Gravitational collapse in turbulent molecular clouds. I. Gasdynamical turbulence

Observed molecular clouds often appear to have very low star formation effi ciencies and lifetimes an order of magnitude longer than their free-fall ti mes. Their support is attributed to the random supersonic motions observed in them. We study the support of molecular clouds against gravitational col lapse by supersonic, gasdynamical turbulence using direct numerical simulat ion. Computations with two different algorithms are compared: a particle-ba sed, Lagrangian method (smoothed particle hydrodynamics [SPH]) and a grid-b ased, Eulerian, second-order method (ZEUS). The effects of both algorithm a nd resolution can be studied with this method. We find that, under typical molecular cloud conditions, global collapse can indeed be prevented, but de nsity enhancements caused by strong shocks nevertheless become gravitationa lly unstable and collapse into dense cores and, presumably, stars. The occu rrence and efficiency of local collapse decreases as the driving wavelength decreases and the driving strength increases. It appears that local collap se can be prevented entirely only with unrealistically short wavelength dri ving, but observed core formation rates can be reproduced with more realist ic driving. At high collapse rates, cores are formed on short timescales in coherent structures with high efficiency, while at low collapse rates they are scattered randomly throughout the region and exhibit considerable age spread. We suggest that this naturally explains the observed distinction be tween isolated and clustered star formation.