Molecular Opacities for Low‐Mass Metal‐poor AGB Stars Undergoing the Third Dredge‐up

Abstract

The concomitant overabundances of C, N, and s-process elements arecommonly ascribed to the complex interplay of nucleosynthesis, mixing,and mass loss taking place in asymptotic giant branch (AGB) stars. Atlow metallicity, the enhancement of C and/or N can be up to 1000 timeslarger than the original iron content and significantly affects thestellar structure and its evolution. For this reason, the interpretationof the already available and still growing amount of data concerningC-rich metal-poor stars belonging to our Galaxy as well as to dwarfspheroidal galaxies would require reliable AGB stellar models for lowand very low metallicities. In this paper we address the question ofcalculation and use of appropriate opacity coefficients, which take intoaccount the C enhancement caused by the third dredge-up. A possible Nenhancement, caused by the cool bottom process or by the engulfment ofprotons into the convective zone generated by a thermal pulse and thesubsequent huge third dredge-up, is also considered. Based on up-to-datestellar models, we illustrate the changes induced by the use of theseopacity coefficients on the physical and chemical properties expectedfor these stars.