Accretion-powered Stellar Winds as a Solution to the Stellar Angular Momentum Problem

Abstract

We compare the angular momentum extracted by a wind from a pre-main-sequence star to the torques arising from the interaction between the star and its Keplerian accretion disk. We find that the wind alone can counteract the spin-up torque from mass accretion, solving the mystery of why accreting pre-main-sequence stars are observed to spin at less than 10% of breakup speed, provided that the mass outflow rate in the stellar winds is ~10% of the accretion rate. We suggest that such massive winds will be driven by some fraction ɛ of the accretion power. For observationally constrained typical parameters of classical T Tauri stars, ɛ needs to be between a few and a few tens of percent. In this scenario, efficient braking of the star will terminate simultaneously with accretion, as is usually assumed to explain the rotation velocities of stars in young clusters.