The formation of (very) slowly rotating stars

Abstract

The slow rotation of some young stars and the extreme rotation periods of some Ap stars have so far defied explanation. The absence of sufficiently efficient braking mechanisms for newly formed stars points to the star formation process itself as the origin. I find that a mode of star formation exists by which a protostar can form without accreting angular momentum. It depends on the survival of a magnetic connection between the accreting matter and the birth cloud. The conditions for this process to operate are analyzed, and illustrated with a generic (scale-free) model. Depending on the initial rotation rate of the accreting matter, either a Keplerian disk forms, or the gas ends up rotating with the rotation period of the cloud, even if it is at a large distance. The boundary in parameter space between these two outcomes is sharp.