Astronomy & Astrophysics

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Issue		A&A Volume 415, Number 3, March I 2004
Page(s)		1089 - 1097
Section		Stellar structure and evolution
DOI		10.1051/0004-6361:20034281

A&A 415, 1089-1097 (2004)
DOI: 10.1051/0004-6361:20034281

Nucleosynthesis of s-elements in rotating AGB stars

L. Siess¹, S. Goriely¹ and N. Langer²

¹ Institut d'Astronomie et d'Astrophysique, Université Libre de Bruxelles, CP 226, 1050 Brussels, Belgium
² Astronomical Institute, Utrecht University, The Netherlands

(Received 5 September 2003 / Accepted 6 November 2003 )

Abstract
We analyze the s-process nucleosynthesis in models of rotating AGB stars, using a complete nuclear network covering nuclei up to Polonium. During the stage of thermal pulses, the extreme shear field that develops at the base of the convective envelope leads to the injection of protons into the adjacent $\rm \kern-0.8ptC$ -rich core. Subsequent proton captures lead to overlapping $\rm \kern-0.8ptN$ -rich and $\rm \kern-0.8ptC$ -rich layers. While the $\rm \kern-0.8ptC$ nuclei release neutrons due to $\alpha$ -captures during the interpulse phase, the persistence of mixing due to differential rotation produces a contamination of the whole $\rm \kern-0.8ptC$ -rich layer with $\rm \kern-0.8ptN$ . The result is a quenching of the s-process efficiency. Our study emphasizes the sensitivity of the s-process nucleosynthesis to the strength and duration of the shear mixing phase. Uncertainties in the rate of $\rm \kern-0.8ptC$ ( $\alpha$ ,n) turn out to have small effects on the resultant distribution of s-elements. Finally, we show that in this framework, a deeper third dredge-up tends to further inhibit the production of s-elements.

Key words: nucleosynthesis -- stars: AGB and post-AGB -- nuclear reactions -- abundances

Offprint request: L. Siess, siess@astro.ulb.ac.be

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