On the Rotational Evolution of Solar‐ and Late‐Type Stars, Its Magnetic Origins, and the Possibility of Stellar Gyrochronology

We propose a simple interpretation of the rotation period data for solar- andlate-type stars. The open cluster and Mt. Wilson star observations suggest thatrotating stars lie primarily on two sequences, initially called I and C. Somestars lie in the intervening gap. These sequences, and the fractional numbersof stars on each sequence evolve systematically with cluster age, enabling usto construct crude rotational isochrones allowing `stellar gyrochronology', aprocedure, upon improvement, likely to yield ages for individual field stars.The age and color dependences of the sequences allow the identification of theunderlying mechanism, which appears to be primarily magnetic. The majority ofsolar- and late-type stars possess a dominant Sun-like, or Interface magneticfield, which connects the convective envelope both to the radiative interior ofthe star and to the exterior where winds can drain off angular momentum. Thesestars spin down Skumanich-style. An age-decreasing fraction of young G, K, andM stars, which are rapid rotators, possess only a Convective field which is notonly inefficient in depleting angular momentum, but also incapable of couplingthe surface convection zone to the inner radiative zone, so that only the outerzone is spun down, and on an exponential timescale. These stars do not yetpossess large-scale dynamos. The large-scale magnetic field associated with thedynamo, apparently created by the shear between the decoupled radiative andconvective zones, (re)couples the convective and radiative zones and drives astar from the Convective to the Interface sequence through the gap on atimescale that increases as stellar mass decreases. (Abstract is truncatedhere.)Comment: 32 pages, 6 figures (4 in color), accepted by Ap