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 Ps of the surrounding medium there is a critical value Pcr above which the core cannot be in magnetohydrostatic equilibrium and collapses; Pcr 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 aeff for the gravity diluted by magnetic force. The cloud core begins dynamical contraction when Pcr has decreased below Ps by some mechanism. Dissipation of turbulence is the most important process in reducing Pcr. 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 Φcr or with small aeff ≈ 1 - (Φ/Φcr)2, Pcr will not decrease below Ps even when turbulence has completely dissipated; this will happen only in very low mass cores because of the sharp mass dependence of Pcr. Such cores begin dynamical contraction after aeff 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 Pcr on Φ at Φ ≈ Φcr. 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 ≈ Φcr must be low. It is shown to be implausible that cloud cores form from magnetically subcritical condensations via ambipolar diffusion.