On the Origin of the Ultrafast Rotators in Young Star Clusters

Abstract

In this paper we present a series of evolutionary models of low-mass stars, including rotation, that begin on the deuterium main sequence as suggested by Palla & Stahler. We find that angular momentum loss must saturate on the pre-main sequence if we are to account for the presence of the ultrafast rotators (UFRs) in young star clusters. This is in agreement with the saturation in chromospheric emission observed by Stauffer and others. The saturation threshold required to produce the lower envelope of the observations is different for G, K, and M stars, with lower mass stars saturating at lower angular velocities. The result seems to indicate that convection is an important contributor to the magnetic fields of low-mass stars.