Dark Cloud Cores and Gravitational Decoupling from Turbulent Flows

We test the hypothesis that the starless cores may be gravitationally boundclouds supported largely by thermal pressure by comparing observed molecularline spectra to theoretical spectra produced by a simulation that includeshydrodynamics, radiative cooling, variable molecular abundance, and radiativetransfer in a simple one-dimensional model. The results suggest that thestarless cores can be divided into two categories: stable starless cores thatare in approximate equilibrium and will not evolve to form protostars, andunstable pre-stellar cores that are proceeding toward gravitational collapseand the formation of protostars. The starless cores might be formed from theinterstellar medium as objects at the lower end of the inertial cascade ofinterstellar turbulence. Additionally, we identify a thermal instability in thestarless cores. Under par ticular conditions of density and mass, a core may beunstable to expansion if the density is just above the critical density for thecollisional coupling of the gas and dust so that as the core expands thegas-dust coupling that cools the gas is reduced and the gas warms, furtherdriving the expansion.Comment: Submitted to Ap