The Rotation of the Sun (Review Paper)

Abstract

The author’s 1964 article in Nature on the sun’s rotation is rediscussed in the light of new data. This article suggested that the sun might be oblate because of a gravitational quadrupole moment induced by a core rotating with a period < 2 days. Angular momentum lost from the core by molecular diffusion was assumed to be transferred to the solar wind which kept the sun’s surface rotating slowly. The estimated solar wind torque was found to be in good agreement with the torque calculated from a solution of the diffusion equation.

Subsequent to the 1964 paper the oblateness was observed. Also the solar wind torque was ‘observed’ to be in good agreement with the early estimate. New observations discussed here seem to be important if the sun can be safely assumed to be a typical star and not an exception. It has been found by Kraft (1967) that very young solar type stars (Pleiades) are rotating with roughly the same angular velocity postulated for the solar core. As determined from observations on the Hyades in comparison with the Pleiades, the rotation of young solar-type stars is slowing at a rate consistent with a stellar wind torque equal to that of the solar wind acting on the outer 20–30% of the star by radius. The slowing of the rotation in young stars is accompanied by a depletion of lithium, but not berylium. This implies that only the outer 45% of the star by radius, or 5% by mass, is slowed by the solar wind. The rapid rotation of the inner 95% of the mass is sufficient to generate the observed oblateness. The rate of depletion of lithium, determined from observations on solar-type stars of various ages, is consistent with the rate of angular momentum loss assuming a reasonable model for the transport of angular momentum to the convective zone.

This research was supported in part by the National Science Foundation and the Office of Naval Research of the U.S. Navy.