Turbulence and star formation in molecular clouds

TLDR

Observations show that molecular cloud velocity dispersions scale with size and mass in a power‑law consistent with Kolmogorov turbulence, indicating a common turbulent hierarchy in gravitationally bound, virialized regions. The data imply that molecular clouds and their substructures arise from supersonic turbulence rather than simple collapse, leading to a turbulent hierarchy that ends at subsonic scales, predicts a minimum protostellar mass of a few tenths of a solar mass, and results in transient clouds that disperse within ~10⁷ yr.

Abstract

Data for many molecular clouds and condensations show that the internal velocity dispersion of each region is well correlated with its size and mass, and these correlations are approximately of power-law form. The dependence of velocity dispersion on region size is similar to the Kolmogoroff law for subsonic turbulence, suggesting that the observed motions are all part of a common hierarchy of interstellar turbulent motions. The regions studied are mostly gravitationally bound and in approximate virial equilibrium. However, they cannot have formed by simple gravitational collapse, and it appears likely that molecular clouds and their substructures have been created at least partly by processes of supersonic hydrodynamics. The hierarchy of subcondensations may terminate with objects so small that their internal motions are no longer supersonic; this predicts a minimum protostellar mass of the order of a few tenths of a solar mass. Massive 'protostellar' clumps always have supersonic internal motions and will therefore develop complex internal structures, probably leading to the formation of many pre-stellar condensation nuclei that grow by accretion to produce the final stellar mass spectrum. Molecular clouds must be transient structures, and are probably dispersed after not much more than 107 yr.