Onset of Shear Instability in Rotating Red Giants

We incorporate into an evolutionary code a unified treatment of turbulent transport due to convective and shear instabilities in a rotating star. The effects of composition gradients are included in a consistent way. We use the code to follow the evolution of rotating stars from the pre-main sequence to near the tip of the red giant branch. We find that at a certain stage in the red giant evolution of a 1 M... star, when the convectively unstable region occupies an extensive outer envelope, shear instability sets in in a zone just below the H-burning shell. However, at a later time, shear instability also develops in a second (outer) region that lies between the H-burning shell and the convective envelope. The onset of the outer region of shear instability coincides closely in time with a well-defined feature of red giant evolution: the "bump" (so-called because of its effect on the luminosity function of clusters). This evolutionary bump has recently been identified as a significant empirical marker among red giants: cool massive winds have been found to set in abruptly after the star evolves through the bump. The results we report in the present paper suggest that the onset of mass loss in red giants may be correlated with the onset of an outer region of shear instability inside the star. We offer some speculations concerning the possible physics of such a correlation.