Models for Carbon-Rich Stars

Models for carbon-rich stars (with an initial homogeneous composition of 50 per cent C'2 and 50 per cent 016 by mass) below the Chandrasekhar limiting mass are presented. Two series of models are pre- sented; one with the newest rates for neutrino-pair energy losses and one without any neutrinos. The minimum mass Mmjn for carbon burning is 0.78 Mo without and 0.97 Mo with the neutrino energy losses, if previously published carbon-burning rates are used. Decreasing this rate by a factor 40, in- creases Mmin from 0.97 Mo to 1.06 Mo. In the presence of neutrinos, carbon burning starts in a shell for the smaller masses, and the star experiences a fairly large carbon flash, similar to the helium flash. Some inhomogeneous models, containing a helium envelope, are also presented, and the maximum mass fraction q~ for a non-burning helium envelope is found. The increase in radius due to a ~ia1l hydrogen outer envelope is then calculated. The relevance of our models to the central stars of planetary nebulae and to very hot white dwarfs is discussed