Evolution of Oxygen-rich and Carbon Stars on the Asymptotic Giant Branch

For many years, it has been commonly believed that oxygen-rich (M) stars evolve first to S stars and then to carbon (C) stars. However, the details of the transition are not understood. It is now accepted that the overabundance of carbon ([C/O] > 1) in some asymptotic giant branch (AGB) stars is due to the dredge up of products of a capture and s-process elements after a number of thermal pulses (Iben 1975). Effects of convective overshooting and semiconvection in the dredge up process have also been considered (Castellani et al. 1985, Lattanzio 1986). The dredge up of carbon into the photosphere leads to the formation of carbon-based molecules, which absorption bands become the basis of spectral classification. During the past decade, it is recognized that the carbon-richness of a star not only manifests itself in the photospheric spectrum, but also in the circumstellar environment as well. Probes of the circumstellar envelopes in the infrared and radio regions provide new means to characterize the chemical properties of the star. These methods are particularly useful in cases where the photosphere is heavily obscured by circumstellar absorption. Table 1 summarizes the photospheric and circumstellar spectral characteristics of M and C stars.