The Lifetimes and Evolution of Molecular Cloud Cores

We discuss the lifetimes and evolution of clumps and cores formed asturbulent density fluctuations in nearly isothermal molecular clouds. In thenon-magnetic case, clumps are unlikely to reach a hydrostatic state, andinstead are expected to either proceed directly to collapse, or else``rebound'' towards the mean pressure and density of the parent cloud.Rebounding clumps are delayed in their re-expansion by their self-gravity. Froma simple virial calculation, we find re-expansion times of a few free-falltimes. In the magnetic case, we present a series of driven-turbulence,ideal-MHD isothermal numerical simulations in which we follow the evolution ofclumps and cores in relation to the magnetic criticality of their ``parentclouds'' (the numerical boxes). In subcritical boxes, magnetostatic clumps donot form. A few moderately-gravitationally bound clumps form which however aredispersed by the turbulence in < 1.3 Myr. An estimate of the ambipolardiffusion (AD) time scale t_AD in these cores gives t_AD > 1.3 Myr, onlyslightly longer than the dynamical times. In supercritical boxes, some coresbecome locally supercritical and collapse in typical times ~ 1 Myr. We alsoobserve longer-lived supercritical cores that however do not collapse becausethey are smaller than the local Jeans length. Fewer clumps and cores form inthese simulations than in their non-magnetic counterpart. Our results suggestthat a) A fraction of the cores may not form stars, and may correspond to someof the observed starless cores. b) Cores may be out-of-equilibrium structures,rather than quasi-magnetostatic ones. c) The magnetic field may help reduce thestar formation efficiency by reducing the probability of core formation, ratherthan by significantly delaying the collapse of individual cores.