Population Synthesis of Helium White Dwarf–Red Giant Star Mergers and the Formation of Lithium-rich Giants and Carbon Stars

Abstract

The formation histories of lithium-rich and carbon-rich red giants are not yet understood. It has been proposed that the merger of a helium-core white dwarf with a red giant branch (RGB) star might provide a solution. We have computed an extended grid of post-merger evolution models and combined these with predictions of binary-star population synthesis. The results strongly support the proposal that the merger of a helium white dwarf with an RGB star can provide the progenitors of both lithium-rich red clump stars and early-R carbon stars. The distribution of post-merger models in T eff , log g , log L , the surface abundances of lithium and carbon, and the predicted space densities agree well with the observed distributions of these parameters for Li-rich and early-R stars in the Galaxy.