Early-type R stars and J stars are a special type of carbon star , having enhanced nitrogen ( [ N / Fe ] \approx 0.5 ) , lithium , a low ^ { 12 } { C } / ^ { 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 .