M and S stars in the LMC globular clusters

Spectroscopic observations of 16 oxygen-rich AGB stars in LMC clusters reveal 12 M and four S stars, all radial velocity members of the LMC. Twenty S stars are known in the intermediate age clusters of the LMC, with most of the accessible possible examples having been observed. These data, together with other information on the clusters, confirm an earlier finding that the M -> S and S -> C transitions occur at higher luminosity in the younger clusters. The correlation between age and metal abundance of LMC clusters creates ambiguity but there is evidence that metal abundance is important, with higher transition luminosities in more metal rich clusters. The lifetime of the S star stage of evolution is a substantial fraction of the life of a carbon star, contrary to theoretical expectation, although a prediction that S stars will be more common relative to C stars at higher metal content is in accordance with observation. The absence of pure S, SC or CS stars may be in conflict with the current calibration of the S and C spectral types in terms of C/O. Four old open clusters in the Galaxy were searched for S stars without success. (author)