SPECTRUM SYNTHESIS OF ASYMPTOTIC GIANT BRANCH STARS IN THE SMALL MAGELLANIC CLOUD - A SEARCH FOR ENVELOPE BURNING

We have investigated the atmospheric abundances of luminous upper AGB stars of the SMC, searching in particular for evidence of the hypothetical envelope burning process. To this end we have computed synthetic spectra with varying C, N and O abundances selected by considering the effect of the processes of the 3rd dredge up and a simple-minded recipe for envelope burning. The synthetic spectra were compared to observations of a small sample of SMC upper AGB stars thought to be prime candidates for the occurrence of envelope burning. We find all of the stars have C/O ratios appropriate for thermally pulsing AGB stars dredging up C. Our simple model envelope burning abundance stars are too N-rich to match the observations, but in some stars the CN bands are stronger than predicted by our C dredge up spectra. This suggests a smaller N enhancement which could be provided by less effective operation of the envelope burning process. We suggest that the relative ZrO bandstrengths of these stars may also indicate envelope burning is occurring. Supporting evidence for this interpretation is found in the spectrum of one star observed at maximum pulsational phase in which strong 13CN bands are seen. Our results are in agreement with the earlier work of Smith & Lambert who find some of these stars to be Li-rich from which they suggest the stars possess a 'hot bottom convective envelope' - the required environment for envelope burning.