The formation and evolution of carbon stars

In the Magellanic Clouds, carbon stars form at the top of the asymptotic giant branch in the star clusters younger than about 8 billion yr. The mechanism, devised 10 yr ago by Iben (1975) is termed 'third dredge up', and raises carbon produced in helium shell flashes to the stellar surface. Clusters younger than about 0.8 billion yr, however, have giant branches populated by M stars. A number of factors could be responsible for this result. The higher metallicity of younger clusters could play a part. Envelope burning could convert the newly created carbon to nitrogen. However, the evidence seems to favor the hypothesis that thermal pulses are not able to establish themselves in stars of 5 solar masses or more before the stellar envelope is completely eroded away by rapid mass loss. The Magellanic Clouds have proved to be an invaluable laboratory for the study of stellar evolution on the asymptotic giant branch, but recent improvements in instrumentation have given access to all the galaxies of the Local Group. Spectroscopy and photometry are presented of carbon stars in NGC 6822, IC 1613, M31, WLM, M33, Andromeda II, and NGC 205. This will permit comparison of evolutionary processes on the asymptotic giant branch in a number of different environments.