Journal Article

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

Herbert H. B. Lau, Richard J. Stancliffe and Christopher A. Tout

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 396, issue 2, pages 1046-1057
Published in print June 2009 | ISSN: 0035-8711
Published online June 2009 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2009.14772.x
The evolution of low-metallicity asymptotic giant branch stars and the formation of carbon-enhanced metal-poor stars

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We investigate the behaviour of asymptotic giant branch (AGB) stars between metallicities Z= 10−4 and 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 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.

Keywords: stars: AGB and post-AGB; stars: carbon; stars: evolution

Journal Article.  9870 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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