Li Abundances in Luminous Magellanic Cloud AGB stars

Astronomy and Astrophysics – Astrophysics

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Scientific paper

Improved model atmospheres including spherical geometry and better opacities treated in Opacity Sampling (Plez, Brett, Nordlund, 1992, Astron. Astrophys., in press) have been used to extract Li abundances in Magellanic Cloud AGB and Supergiant stars. The effect of sphericity on the derivation of stellar parameters and chemical composition has been investigated. The C/O ratio has been found to be a critical parameter that has to be carefully estimated in S type stars, before any reliable abundance may be determined. This is due to the rapid variation of TiO partial pressure with the C/O ratio when it nears one. This results in changes in the atmospheric structure and in the veiling of the spectrum by the numerous TiO lines. Preliminary results will be discussed that suggest Li enhancement in the most luminous AGB stars. This confirms the discovery by Smith and Lambert (1989, Ap.J. letters, 345, L75 and 1990, Ap.J. Letters, 361, L69) of Li enrichment in luminous S stars of the Magellanic Clouds and supports their conclusion that the excess Li was produced as a result of envelope burning. This research is supported in part by the NSF (AST 91-15090) and the Robert A. Welch Foundation of Houston, Texas.

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