The formation of cool stars from cloud cores

Abstract

We consider models of the processes by which a molecular cloud acquires dense cores, a (magnetized) rotating core collapses to give a protostar plus nebular disk, and a powerful stellar wind sets in to reverse the accretion flow and reveal the central object as a pre-main-sequence star. At each stage, we rely on a combination of theory and observation to fix the basic parameters of the model. We show that core formation in a molecular cloud is an inevitable byproduct of ambipolar diffusion in a magnetized self-gravitating medium of low fractional ionization. We find that the gravitational collapse of a uniformly-rotating isothermal core, which possesses a 1/r² density profile in its inner parts, has simple analytic properties. And we propose that strong stellar winds in T Tauri stars represent a phase of readjustment in the angular momentum distribution after deuterium burning drives convection throughout a strongly differentially-rotating protostar. We conclude that the major missing link in this picture is the evolutionary behavior of massive nebular disks that may accumulate around protostars. Otherwise, there seems to be a satisfying connection between the cloud cores observed by molecular-line radio astronomers and the active stellar atmospheres of young stars studied by optical and x-ray astronomers.