THEORETICAL MODELS OF THE ANGULAR MOMENTUM EVOLUTION OF SOLAR-TYPE STARS

摘要

We examine the effects of different assumptions about the initial conditions, angular momentum loss law, and angular momentum transport on the angular momentum evolution of 0.5-1.2 solar mass stars. We first perform a parameter variation study to test the sensitivity of the surface rotation rate as a function of mass and age to changes in the initial conditions and input physics. We then check to see if the distribution of initial conditions for a given physical scenario is consistent for open clusters of different ages. The behavior of the rapid rotators is highly sensitive to the saturation threshold for angular momentum loss (ω_(crit)), above which angular momentum loss scales linearly with the rotation rate. Very high values for ω_(crit) suppress rapid rotation prior to the main sequence, and very low values permit rapid rotation to survive for too long. For solid-body (SB) and differential rotation (DR) models, higher mass models rotate more rapidly than lower mass models for the same initial conditions and ω_(crit). DR models differ from SB models in both the direct effect of core-envelope decoupling and a change in the calibration of the angular momentum loss law needed to reproduce the solar rotation at the age of the Sun; the effects of both are discussed. Slow rotation in young clusters can be achieved with modest disk lifetimes (3-10 Myr) for the DR models and longer disk lifetimes for the SB models (10 or more Myr). In addition, the slowly rotating DR models spin down during the early main sequence more than the slowly rotating SB models do. When compared with the cluster data, the observed mass dependence of the rapid rotator phenomenon can be reproduced only with a mass-dependent ω_(crit) for both the SB and DR models. A scaling of ω_(crit) inversely proportional to the convective overturn timescale can reproduce the observed mass-dependent spindown. The observed spindown of the slow rotators in the young open clusters is in better agreement with the DR than the SB models. We also discuss observational tests to distinguish different classes of models using low-mass stars and rotation periods in open clusters.