STATISTICS OF CORE LIFETIMES IN NUMERICAL SIMULATIONS OF TURBULENT, MAGNETICALLY SUPERCRITICAL MOLECULAR CLOUDS

摘要

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 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 the threshold density used to define them, n_(thr). The number ratios are measured indirectly in the simulations, due to resolution limitations. Our approach contains one free parameter, the lifetime of a protostellar object, T_(yso) (Class 0 + Class Ⅰ stages), that is outside the realm of the simulations. Assuming a value of T_(yso) = 0.46 Myr, we obtain number ratios of starless to stellar cores ranging from 4-5 at n_(thr) = 1.5 × 10~4 cm~(-3) to ～1 at n_(thr) = 1.2 × 10~5 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 n_(thr) = 1.5 × 10~4 cm~(-3), but becomes negligible at n_(thr) = 1.2 × 10~5 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.