Rotational velocity evolution on and prior to the main sequence

Abstract

Nearly all of the rotational velocity data for stars less massive than the sun and younger than the Hyades has been obtained in the past six years. The new data show that low mass T Tauri stars typically have rotational velocities of order 15 km-sec⁻¹. These stars, however, apparently arrive on the main sequence as quite rapid rotators, with rotational velocities up to 200 km-sec⁻¹. Once on the main sequence, winds rapidly decrease the rotational velocities to of order 10 kmsec^−l in only a few times 10⁷ years for G and K dwarfs, with the spindown timescale increasing with decreasing stellar mass. A model which predicts rotational velocity evolution for young stars that roughly matches the observations has been proposed by Endal and Sofia (1981. That model, however, predicts little spindown of the radiative core of solar mass stars, in apparent contradiction to current helioseismology results. New models for the rotational spindown of low mass stars fit the solar interior rotation well, but do not, as yet, provide a satisfactory fit to the surface rotational velocities of young stars (Sofia 1987.