White dwarf-red giant mergers, early-type R stars, J stars and lithium

Early-type R stars and J stars are a special type of carbon star, having enhanced nitrogen ([N/Fe] approximate to 0.5), lithium, a low C-12/C-13 ratio (<15) and no s-element enhancements. The merger of a helium white dwarf with a red giant is regarded to be a possible model for the origin of early-type R stars, but the details of nucleosynthesis are not clear. In this paper, we investigate three possible channels for helium white dwarf + red giant mergers, and find that, amongst the three, only a high-mass helium white dwarf subducted into a low-core-mass red giant can make an early-type R star. Nucleosynthesis of the elements carbon, nitrogen, oxygen and lithium corresponds well with the observations. Furthermore, we find that the J stars may represent a short and luminous stage in the evolution of an early-type R star.