Asymptotic Giant Branch stars at low metallicity: the challenging interplay between mass loss and molecular opacities

TL;DR

This study examines how mass loss and molecular opacities influence the chemical yields of low-metallicity Asymptotic Giant Branch stars, highlighting uncertainties especially in less massive stars and the importance of opacity treatment.

Contribution

It provides a detailed analysis of the uncertainties in AGB star yields due to mass loss and molecular opacities, especially at low metallicity, and identifies key mass ranges where these effects are critical.

Findings

Yields of massive AGB stars are mainly influenced by convection efficiency.

Uncertainties in yields are largest for stars with 2.0-3.0Msun, especially for nitrogen and sodium.

Mass loss and convective boundary treatments critically affect third dredge-up in low-mass stars.

Abstract

We investigate the main physical properties of low-metallicity Asymptotic Giant Branch stars, with the aim of quantifying the uncertainties that presently affect the predicted chemical yields of these stars, associated to mass loss and description of molecular opacities. We find that above a threshold mass, M ~ 3.5Msun for Z=0.001, the results are little dependent on the opacity treatment, as long as hot-bottom burning prevents the surface C/O ratio from exceeding unity; the yields of these massive AGB stars are expected to be mostly determined by the efficiency of convection, with a relatively mild dependence on the mass-loss description. A much higher degree of uncertainty is associated to the yields of less massive models, which critically depend on the adopted molecular opacities. An interval of masses exists, say 2.0-3.0Msun, (the exact range depends on mass loss), in which HBB may be even extinguished following the cooling produced by the opacity of C-bearing molecules. The yields of these stars are the most uncertain, the variation range being the largest (up to ~ 2dex) for the nitrogen and sodium yields. For very low-mass models, not experiencing hot-bottom burning (M< 1.5Msun),the description of mass loss and the treatment of the convective boundaries are crucial for the occurrence of the third dredge-up, with sizable consequences on the CNO yields.