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Astron. Astrophys. 357, 669-676 (2000)

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6. Conclusions

New observations of four transitions, belonging to two chlorine bearing molecules, have allowed to derive an accurate and reliable measurement of the 35Cl/37Cl ratio towards IRC+10216, the circumstellar envelope of the carbon-rich AGB star CW Leo. It represents the first observational evidence for a significant deviation from solar of an isotopic ratio of elements heavier than the CNO in this circumstellar envelope. The high accuracy of the new data presented here for Cl, combined with the carbon and magnesium isotopic ratios previously measured in IRC+10216, allow nucleosynthesis models to constrain the initial stellar mass better than previously possible (Guélin et al. 1995; Forestini & Charbonnel 1997). The inferred mass is low, M [FORMULA] 2 [FORMULA] , as often assumed in IRC+10216 envelope modelling. Our results are obtained with the Reimers (1975) choice for the mass loss rate and with the TDU efficiency provided by the Schwarzschild criterion inside the FRANEC evolutionary code. We have argued, from the dependence of Cl isotope abundances on the pulse number, and from independent constraints provided by carbon, magnesium and oxygen isotopes, that our results for the stellar mass remain valid independently of several details (mass loss prescriptions, TDU) of the specific stellar code adopted. A dedicated check from s-process calculations in different evolutionary codes would be clearly useful.

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000