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- W2044194531 abstract "We revisit the problem of the formation of dense protostellar cores due to ambipolar diffusion within magnetically supported molecular clouds, and derive an analytical expression for the core formation timescale. The resulting expression is similar to the canonical expression ≃ � 2 ff /�ni ∼ 10�ff (whereff is the free-fall time andni is the neutral-ion collision time), except that it is multiplied by a numerical factor C(µc0), where µc0 is the initial central mass-to-flux ratio normalized to the critical value for gravitational collapse. C(µc0) is typically ∼ 1 in highly subcritical clouds (µc0 ≪ 1), although certain conditions allow C(µc0) ≫ 1. For clouds that are not highly subcritical, C(µc0) can be much less than unity, with C(µc0) → 0 for µc0 → 1, significantly reducing the time required to form a supercritical core. This, along with recent observations of clouds with mass-to-flux ratios close to the critical value, may reconcile the results of ambipolar diffusion models with statistical analyses of cores and YSO's which suggest an evolutionary timescale ∼ 1 Myr for objects of mean density ∼ 10 4 cm −3 . We compare our analytical relation to the results of numerical simulations, and also discuss the effects of dust grains on the core formation timescale. Subject headings: diffusion — dust, extinction — ISM: abundances — ISM: clouds — ISM: magnetic fields — MHD — plasmas — stars: formation" @default.
- W2044194531 created "2016-06-24" @default.
- W2044194531 creator A5058056781 @default.
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- W2044194531 date "2001-01-20" @default.
- W2044194531 modified "2023-09-27" @default.
- W2044194531 title "On the Timescale for the Formation of Protostellar Cores in Magnetic Interstellar Clouds" @default.
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- W2044194531 doi "https://doi.org/10.1086/318348" @default.
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