Gas and dust from solar metallicity AGB stars

TL;DR

This study models the evolution of solar metallicity AGB stars, analyzing their chemical yields, dust production, and lifetimes, with a focus on the effects of stellar mass and convection treatment, and compares results across different codes.

Contribution

It provides a detailed comparison of AGB star models with different physical assumptions, highlighting the robustness of predictions for low-mass stars and their dust yields.

Findings

Stars ≥ 3.5 M_sun experience hot bottom burning and have short AGB lifetimes.

Low-mass stars become carbon stars with C/O ratios up to 3.

Dust production varies with stellar mass, producing carbon and silicon carbide or silicate dust.

Abstract

We study the asymptotic giant branch (AGB) evolution of stars with masses between $1~M_{\odot} - 8.5~M_{\odot}$. We focus on stars with a solar chemical composition, which allows us to interpret evolved stars in the Galaxy. We present a detailed comparison with models of the same chemistry, calculated with a different evolution code and based on a different set of physical assumptions. We find that stars of mass $\ge 3.5~M_{\odot}$ experience hot bottom burning at the base of the envelope. They have AGB lifetimes shorter than $\sim 3\times 10^5$ yr and eject into their surroundings gas contaminated by proton-capture nucleosynthesis, at an extent sensitive to the treatment of convection. Low mass stars with $1.5~M_{\odot} \le M \le 3~M_{\odot}$ become carbon stars. During the final phases the C/O ratio grows to $\sim 3$. We find a remarkable agreement between the two codes for the low-mass models and conclude that predictions for the physical and chemical properties of these stars, and the AGB lifetime, are not that sensitive to the modelling of the AGB phase. The dust produced is also dependent on the mass: low-mass stars produce mainly solid carbon and silicon carbide dust, whereas higher mass stars produce silicates and alumina dust. Possible future observations potentially able to add more robustness to the present results are also discussed.