DIFFERENTIAL ROTATION AND THE CONVECTIVE CORE MASS OF UPPER MAIN-SEQUENCE STARS

Accurate two-dimensional models of rotating main-sequence stars have been computed to test the recently proposed hypothesis that stars with a uniform distribution of specific angular momentum have more massive convective cores and, therefore, longer lifetimes than their rigidly rotating and nonrotating counterparts. The main conclusion is that the numerical models do, indeed, confirm this hypothesis at least for stars with masses below 12 M.. Hence, this phenomenon provides yet another possible explanation for intermediate mass blue stragglers and other peculiar stars that may arise from enhanced core-envelope mixing. In more massive configurations, the radiative temperature gradient at the core boundary also increases with an increase in rotation (as in the lower masses) but the adiabatic gradient increases by an even larger amount due to a drop in the radiation pressure. Thus, the convective cores of rotating massive stars always decrease in mass fraction for any distribution of angular velocity.