Star Formation in Magnetic Clouds

Abstract

This paper reexamines the widely accepted assumption that low-mass stars form mainly in magnetically subcritical cloud cores and high-mass stars form in magnetically supercritical ones. Cloud cores, as well as molecular clouds, are shown to be magnetically supercritical because although the cores are generally observed as portions of a molecular cloud having considerably higher column densities than their surroundings, magnetically subcritical condensations embedded in a cloud are not very likely to have higher column densities than their surroundings, and because it is difficult to maintain the nonthermal velocity dispersions widely observed in the cores for a significant fraction of their lifetimes if the cores are magnetically subcritical. In a magnetically supercritical condensation, which we call a core, for the pressure P s of the surrounding medium there is a critical value P cr above which the core cannot be in magnetohydrostatic equilibrium and collapses; P cr depends sharply on the core mass, on the effective sound velocity in the core, which includes the effect of turbulence, and on the effective coefficient a eff for the gravity diluted by magnetic force. The cloud core begins dynamical contraction when P cr has decreased below P s by some mechanism. Dissipation of turbulence is the most important process in reducing P cr . Therefore, in most cases, the timescale of star formation in each core is the dissipation time of turbulence, which is several times the free-fall time of the core. For the cores of magnetic flux Φ very close to the critical flux Φ cr or with small a eff ≈ 1 - (Φ/Φ cr ) 2 , P cr will not decrease below P s even when turbulence has completely dissipated; this will happen only in very low mass cores because of the sharp mass dependence of P cr . Such cores begin dynamical contraction after a eff has increased somewhat because of magnetic flux loss from their central parts by ambipolar diffusion; for this to happen, only a slight loss of magnetic flux is needed because of sharp dependence of P cr on Φ at Φ ≈ Φ cr . The timescale of star formation in this case is not much different from the dissipation time of the turbulence, though the probability that the cores have Φ ≈ Φ cr must be low. It is shown to be implausible that cloud cores form from magnetically subcritical condensations via ambipolar diffusion.