Slow Star Formation in Dense Gas: Evidence and Implications

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  • 12 June 2006
Abstract
It has been known for more than 30 years that star formation in giant molecular clouds is slow, with only ~1% of the gas forming stars every free-fall time. Here we present evidence that it is equally slow in the much denser gas from which star clusters form. This has important implications for models of star formation, since competing models make differing predictions for the characteristic density at which star formation should transition from slow, with only a few percent of the mass forming stars per free-fall time, to rapid, with order unity of the mass going into stars in a free-fall time. We find that the data are strongly inconsistent with models that explain low star formation rates by appealing to unbound molecular clouds or regulation of star formation solely by galactic-scale gravitational instability, and broadly consistent with models of star formation regulated by either turbulence or magnetic fields in virialized objects. The turbulence-regulated star formation model of Krumholz & McKee quantitatively reproduces the infrared-HCN luminosity correlation recently reported by Gao & Solomon. Slow star formation also implies that the process of star cluster formation cannot be one of global collapse, but must instead proceed over many free-fall times. This suggests that turbulence in star-forming clumps does not decay away in a single crossing time, and that the competitive accretion mechanism does not operate in typical cluster-forming molecular clumps.

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