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 ($\rm [N/Fe]\approx 0.5$), lithium, a low \iso{12}{C}/\iso{13}{C} 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 elements carbon, nitrogen, oxygen and lithium correspond well with the observations. Furthermore, we find that the J stars may represent a short and luminous stage in the evolution of an early-R star.