Fragmentation of shocked flows: Gravity, turbulence, and cooling

The observed rapid onset of star formation in molecular clouds requires rapid formation of dense fragments that can collapse individually before being overtaken by global gravitationally driven flows. Many previous investigations have suggested that supersonic turbulence produces the necessary fragmentation, without addressing however the source of this turbulence. Motivated by our previous (numerical) work on the flow-driven formation of molecular clouds, we investigate the expected timescales of the dynamical and thermal instabilities leading to the rapid fragmentation of gas swept up by large-scale flows and compare them with global gravitational collapse timescales. We identify parameter regimes in gas density, temperature, and spatial scale within which a given instability will dominate. We find that dynamical instabilities disrupt large-scale coherent flows through generation of turbulence, while strong thermal fragmentation amplifies the resulting low-amplitude density perturbations, thus leading to small-scale, high-density fragments as seeds for local gravity to act upon. Global gravity dominates only on the largest scales; large-scale, gravitationally driven flows promote the formation of groups and clusters of stars formed by turbulence, thermal fragmentation, and rapid cooling.