LITHIUM ABUNDANCES AND OTHER CLUES TO ENVELOPE BURNING IN SMALL MAGELLANIC CLOUD ASYMPTOTIC GIANT BRANCH STARS

We present a chemical analysis of seven luminous asymptotic giant branch stars and one M supergiant of the Small Magellanic Cloud. The abundances are derived from high-resolution spectra by spectrum synthesis using new, spherically symmetric, opacity sampling model atmospheres. The average metallicity is [Fe/H] = -0.5, in accordance with other determinations of the SMC's metallicity. The AGB stars show signs of envelope burning, being Li rich, C poor, and with a low C-12/C-13 ratio. The s-process elements (Rb, Zr, Nd) abundance pattern is different from that in the solar system, and is characteristic of a high exposure at low neutron density. This is not peculiar to the SMC, however, but seems to be an effect of metallicity: Galactic Ba, CH, and S-type stars at the same metallicity display similar s-process element abundance patterns. Apparently, the C-13 neutron source is operating in these intermediate-mass thermally pulsing AGB stars.