The evolution of low-metallicity asymptotic giant branch stars and the formation of carbon-enhanced metal-poor stars

TL;DR

This study explores the evolution of low-metallicity AGB stars, identifying conditions for flash-driven mixing and carbon ingestion, and examines their role in forming carbon-enhanced metal-poor stars, with implications for understanding stellar nucleosynthesis.

Contribution

It provides new insights into the metallicity-dependent behavior of AGB stars, including the occurrence of flash-driven mixing and carbon ingestion, and assesses their impact on CEMP star formation.

Findings

Flash-driven mixing does not occur above Z=10-5 for any mass.

Carbon ingestion occurs in 2-4 M stars and may be a weaker form of flash-driven mixing.

Models struggle to fully explain the observed frequency of CEMP stars with high carbon and nitrogen enhancements.

Abstract

We investigate the behaviour of asymptotic giant branch (AGB) stars between metallicities Z = 10-4 and Z = 10-8 . We determine which stars undergo an episode of flash-driven mixing, where protons are ingested into the intershell convection zone, as they enter the thermally pulsing AGB phase and which undergo third dredge-up. We find that flash-driven mixing does not occur above a metallicity of Z = 10-5 for any mass of star and that stars above 2 M do not experience this phenomenon at any metallicity. We find carbon ingestion (CI), the mixing of carbon into the tail of hydrogen burning region, occurs in the mass range 2 M to around 4 M . We suggest that CI may be a weak version of the flash-driven mechanism. We also investigate the effects of convective overshooting on the behaviour of these objects. Our models struggle to explain the frequency of CEMP stars that have both significant carbon and nitrogen enhancement. Carbon can be enhanced through flash-driven mixing, CI or just third dredge up. Nitrogen can be enhanced through hot bottom burning and the occurrence of hot dredge-up also converts carbon into nitrogen. The C/N ratio may be a good indicator of the mass of the primary AGB stars.