Variation of the core lifetime and fragmentation scale in molecular clouds as an indication of ambipolar diffusion

Abstract

Ambipolar diffusion likely plays a pivotal role in the formation and evolution of dense cores in weakly ionized molecular clouds. Linear analyses show that the evolutionary times and fragmentation scales are significantly greater than the hydrodynamic (Jeans) values even for clouds with mildly supercritical mass-to-flux ratios. We use values of fragmentation scales and growth times that correspond to typical ionization fractions within a molecular cloud, and apply these in the context of the observed estimated lifetime of prestellar cores and the observed number of such embedded cores forming in a parent clump. By varying a single parameter – the mass-to-flux ratio – over the range of observationally measured densities, we fit the range of estimated prestellar core lifetimes (∼0.1 to a few Myr) identified with Herschel as well as the number of embedded cores formed in a parent clump measured in Perseus with the Submillimeter Array. Our model suggests that the prestellar cores are formed with a transcritical mass-to-flux ratio and higher densities correspond to somewhat higher mass-to-flux ratios, but the normalized mass-to-flux ratio μ remains in the range 1 ≲ μ ≲ 2. Our best-fit model exhibits B ∝ n0.43 for prestellar cores because of the partial flux-freezing caused by ambipolar diffusion.