Molecular opacities for low-mass metal-poor AGB stars undergoing the Third Dredge Up

TL;DR

This paper investigates the calculation of molecular opacities in low-metallicity AGB stars, emphasizing the impact of C and N enhancements due to the third dredge-up on stellar models and their observable properties.

Contribution

It introduces updated opacity coefficients that incorporate C and N enhancements, improving the accuracy of models for low-metallicity AGB stars undergoing the third dredge-up.

Findings

Enhanced opacities significantly alter stellar structure predictions.

C and N overabundances impact the interpretation of observational data.

Models with new opacities better match observed properties of metal-poor AGB stars.

Abstract

The concomitant overabundances of C, N and s-process elements are commonly ascribed to the complex interplay of nucleosynthesis, mixing and mass loss taking place in Asymptotic Giant Branch stars. At low metallicity, the enhancement of C and/or N may be up to 1000 times larger than the original iron content and significantly affects the stellar structure and its evolution. For this reason, the interpretation of the already available and still growing amount of data concerning C-rich metal-poor stars belonging to our Galaxy as well as to dwarf spheroidal galaxies would require reliable AGB stellar models for low and very low metallicities. In this paper we address the question of calculation and use of appropriate opacity coefficients, which take into account the C enhancement caused by the third dredge up. A possible N enhancement, caused by the cool bottom process or by the engulfment of protons into the convective zone generated by a thermal pulse and the subsequent huge third dredge up, is also considered. Basing on up-to-date stellar models, we illustrate the changes induced by the use of these opacity on the physical and chemical properties expected for these stars.