ON THE FORMATION OF MASSIVE C-O WHITE DWARFS: THE LIFTING EFFECT OF ROTATION

摘要

The effect of stellar rotation on the late evolution of intermediate-mass stars has been explored, employing very simple numerical methods. At the epoch of central He exhaustion and C-O core formation, even an initially small rotation may induce, for the first time, a nonnegligible effect on the evolutionary outcome, as a consequence of the huge contraction of the core radius that occurs at this stage. The role of various hydrodynamic instabilities is discussed, verifying the consistency of our approach. The most important characteristic of the rotating models is the slow increase in temperature in the region where He burning occurs. As a consequence, the elapsed time before the onset of the second dredge-up is greater than that found for nonrotating models, and the C-O core mass is markedly increased during the early asymptotic giant branch (AGB) phase. In such a way, massive C-O white dwarfs, near the Chandrasekhar limiting mass for nonrotating stars, would be formed after envelope ejection during the thermal pulse phase. The properties of early AGB and thermally pulsing models obtained by including rotation are compared to those of nonrotating models.