Rotational Evolution of Solar-Type Stars. I. Main-Sequence Evolution

A simple, parameterized model for the redistribution of angular momentum within the interiors of solar-type stars is presented. By incorporating it with a description of angular momentum loss through the action of a magnetically coupled wind, tracing the rotational histories of low-mass dwarf stars for a variety of initial conditions and parameter specifications is accomplished. The results of calculations are discussed for the rotational evolution of a 1 solar mass star, from the time of its arrival on the zero-age main sequence to the age of the present-day sun. Best agreement is with observational constraints for (1) a surface magnetic field strength which is largely independent of surface angular velocity omega for rapid rotation, and approximately linearly dependent on omega for approximately solar values, and (2), a time scale for angular momentum transfer from the core to the convection zone which remains essentially constant throughout the evolution, with magnitude of a about 10 million yr. This value is approximately equal to the initial wind braking time. These results are discussed in the context of a qualitative description of angluar momentum redistribution by magnetic fields in the radiative interiors of solar-type stars.