The role of semiconvection in bringing carbon to the surface of asymptotic giant branch stars of small core mass

Abstract

In a low-mass asymptotic giant branch star which has a small carbon-oxygen core, a layer of carbon-rich material is produced just below the hydrogen-helium discontiniuty during the peak of a thermal pulse. Following pulse peak, this layer expands outward and cools until the opacity at the edge of the layer becomes large enough to force convective motions to arise. As a consequence of the fact that the opacity is a strong function of the carbon abundance, a semiconvection region is established which carries carbon-rich material outward into hydrogen-rich material. As the base of the convective envelope moves inward in mass, fresh carbon is brought to the surface for core masses smaller than found previously, thus potentially removing a long-standing discrepancy between theoretically predicted and observed properties of carbon star distributions.