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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.

The Lifetimes and Evolution of Molecular Cloud Cores