Statistics of Core Lifetimes in Numerical Simulations of Turbulent, Magnetically Supercritical Molecular Clouds
Abstract
We present measurements of the mean dense core lifetimes in numerical simulations of magnetically supercritical, turbulent, isothermal molecular clouds, in order to compare with observational determinations. "Prestellar" lifetimes (given as a function of the mean density within the cores, which in turn is determined by the density threshold nthr used to define them) are consistent with observationally reported values, ranging from a few to several free-fall times. We also present estimates of the fraction of cores in the "prestellar", "stellar'', and "failed" (those cores that redisperse back into the environment) stages as a function of nthr. The number ratios are measured indirectly in the simulations due to their resolution limitations. Our approach contains one free parameter, the lifetime of a protostellar object tyso (Class 0 + Class I stages), which is outside the realm of the simulations. Assuming a value tyso = 0.46 Myr, we obtain number ratios of starless to stellar cores ranging from 4-5 at nthr = 1.5 x 104 cm-3 to 1 at nthr = 1.2 x 105 cm-3, again in good agreement with observational determinations. We also find that the mass in the failed cores is comparable to that in stellar cores at nthr = 1.5 x 104 cm-3, but becomes negligible at nthr = 1.2 x 105 cm-3, in agreement with recent observational suggestions that at the latter densities the cores are in general gravitationally dominated. We conclude by noting that the timescale for core contraction and collapse is virtually the same in the subcritical, ambipolar diffusion-mediated model of star formation, in the model of star formation in turbulent supercritical clouds, and in a model intermediate between the previous two, for currently accepted values of the clouds' magnetic criticality.