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Astron. Astrophys. 346, L37-L40 (1999)

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1. Introduction

It is known since long that thermally pulsing Asymptotic Giant Branch (TP-AGB) stars provide a site for the so called s-process, i.e., the slow neutron capture process which forms neutron-rich isotopes heavier than iron Clayton 1968). Heavy elements primarily produced by the s-process are overabundant at the surface of AGB stars (Smith & Lambert 1990), including technetium (Little et al. 1987) which has no stable isotope and which is produced as 99Tc ([FORMULA]yr) in the s-process. In particular the roughly solar magnesium isotopic pattern found in s-process enriched AGB stars has demonstrated that the 13C([FORMULA],n) rather than the 22Ne([FORMULA],n) neutron source is likely to operate the s-process in AGB stars (Guélin et al. 1995, Lambert et al. 1995).

Evidence for in situ s-processing is found exclusively in carbon stars (Smith & Lambert 1990), which correspond to a late evolutionary stage on the TP-AGB where the stars have large 12C enrichments in their envelopes (Iben & Renzini 1983, Wallerstein & Knapp 1998). The 12C enrichment implies that these stars contain, at certain times, a region at the bottom of their hydrogen-rich envelope where 12C is abundant. This region where protons and 12C coexist may then perhaps form 13C through 12C(p,[FORMULA])13N([FORMULA])13C. Although this scenario is unrivaled, the formation of a layer which is rich in protons and 12C in TP-AGB models has proven to be difficult, and its existence had hitherto to be assumed ad hoc in all s-process calculations (Gallino et al. 1998). Iben & Renzini (1982) found a 13C layer in low metalicity ABG models. Recently, Herwig et al. (1997) have obtained a 13C-rich layer in TP-AGB models of solar metallicity, by invoking a diffusive overshoot layer at convective boundaries. Here, we investigate for the first time effects of rotationally induced mixing processes on the TP-AGB.

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© European Southern Observatory (ESO) 1999

Online publication: June 17, 1999