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

We examine the effects of different assumptions about the initial cond itions, angular momentum loss law, and angular momentum transport on t he angular momentum evolution of 0.5-1.2 solar mass stars. We first pe rform a parameter variation study to test the sensitivity of the surfa ce rotation rate as a function of mass and age to changes in the initi al conditions and input physics. We then check to see if the distribut ion of initial conditions for a given physical scenario is consistent for open clusters of different ages. The behavior of the rapid rotator s is highly sensitive to the saturation threshold for angular momentum loss (omega(crit)), above which angular momentum loss scales linearly with the rotation rate. Very high values for omega(crit) suppress rap id rotation prior to the main sequence, and very low values permit rap id rotation to survive for too long. For solid-body (SB) and different ial rotation (DR) models, higher mass models rotate more rapidly than lower mass models for the same initial conditions and omega(crit). DR models differ from SB models in both the direct effect of core-envelop e decoupling and a change in the calibration of the angular momentum l oss 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 a nd longer disk lifetimes for the SE models (10 or more Myr). In additi on, the slowly rotating DR models spin down during the early main sequ ence more than the slowly rotating SB models do. When compared with th e cluster data, the observed mass dependence of the rapid rotator phen omenon can be reproduced only with a mass-dependent omega(crit) for bo th the SB and DR models. A scaling of omega(crit) inversely proportion al to the convective overturn timescale can reproduce the observed mas s-dependent spindown. The observed spindown of the slow rotators in th e young open clusters is in better agreement with the DR than the SB m odels. We also discuss observational tests to distinguish different cl asses of models using low-mass stars and rotation periods in open clus ters.