We investigate the behaviour of asymptotic giant branch (AGB) stars between metallicities Z= 10a??4 and 10a??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= 10a??5 for any mass of star and that stars above 2a??MaS? 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 2a??MaS? to around 4a??MaS? . 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 Carbon-Enhanced Metal-Poor (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.
|Pages (from-to)||1046 - 1057|
|Number of pages||12|
|Journal||Monthly Notices of the Royal Astronomical Society|
|Publication status||Published - 2009|