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

摘要

We investigate the behaviour of asymptotic giant branch (AGB) stars betweenmetallicities Z = 10-4 and Z = 10-8 . We determine which stars undergo anepisode of flash-driven mixing, where protons are ingested into the intershellconvection zone, as they enter the thermally pulsing AGB phase and whichundergo third dredge-up. We find that flash-driven mixing does not occur abovea metallicity of Z = 10-5 for any mass of star and that stars above 2 M do notexperience this phenomenon at any metallicity. We find carbon ingestion (CI),the mixing of carbon into the tail of hydrogen burning region, occurs in themass range 2 M to around 4 M . We suggest that CI may be a weak version of theflash-driven mechanism. We also investigate the effects of convectiveovershooting on the behaviour of these objects. Our models struggle to explainthe frequency of CEMP stars that have both significant carbon and nitrogenenhancement. Carbon can be enhanced through flash-driven mixing, CI or justthird dredge up. Nitrogen can be enhanced through hot bottom burning and theoccurrence of hot dredge-up also converts carbon into nitrogen. The C/N ratiomay be a good indicator of the mass of the primary AGB stars.