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Astron. Astrophys. 320, 115-124 (1997)

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Could intermediate-mass AGB stars produce star-to-star abundance variations in globular-cluster red giants?

P.A. Denissenkov 1, 2, A. Weiss 1 and J. Wagenhuber 1

1 Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, D-85740 Garching, Germany
2 Astronomical Institute of the St. Petersburg University, Bibliotechnaja Pl. 2, Petrodvorets, 198904 St. Petersburg, Russia

Received 25 March 1996 / Accepted 7 August 1996

Abstract

We performed detailed nucleosynthesis calculations in hydrogen and helium burning shells as well as for hot bottom burning at the base of convective envelope in a low-metallicity ([FORMULA]) intermediate-mass star during thermal-pulsing AGB evolution. Based on complete stellar models, up-to-date simple analytical expressions were used to describe the model star and its evolution. Our study concentrated on surface abundances of light elements, such as C, N, O, Na and Al, and their isotopes in order to test a hypothesis of Cottrell and Da Costa (1981) frequently invoked to explain star-to-star abundance variations in globular-cluster red giants. It is shown that this hypothesis of primordial contamination of intracluster matter by nuclear products from early generation AGB stars fails to reproduce the observed O depletion and Al enhancement. We propose an alternative mechanism which combines some primordial composition anomalies with atmospheric abundance effects produced by deep mixing in globular-cluster red giants. We also suggest observational tests for the verification of our model.

Key words: stars: abundances – stars: AGB, post-AGB – stars: giant – stars: interiors – globular clusters: general

Send offprint requests to: A. Weiss (weiss@mpa-garching.mpg.de)

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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