THE SPIN-UP OF CONTRACTING RED SUPERGIANTS

We report on a mechanism which may lead to a spin-up of the surface of a rotating single star leaving the Hayashi line, which is much strong er than the spin-up expected from the mere contraction of the star. By analyzing rigidly rotating, convective stellar envelopes, we qualitat ively work out the mechanism through which these envelopes may be spun up or down by mass loss through their lower or upper boundary, respec tively. We find that the first case describes the situation in retreat ing convective envelopes, which tend to retain most of the angular mom entum while becoming less massive, thereby increasing the specific ang ular momentum in the convection zone and thus in the layers close to t he stellar surface. We explore the spin-up mechanism quantitatively in a stellar evolution calculation of a rotating 12 M. star, which is fo und to be spun up to critical rotation after leaving the red supergian t branch. We discuss implications of this spin-up for the circumstella r matter around several types of stars, i.e., post-AGE stars, B[e] sta rs, pre-main sequence stars, and, in particular, the progenitor of Sup ernova 1987A.