Nucléosynthèse dans les étoiles de la branche asymptotique: du coeur dégénéré à l'envellope circumstellaire

Abstract : Observations of Asymptotic Giant Branch stars (AGB stars) reveal heavy elements with life times shorter than the AGB phase duration. These elements, called ``s'' elements, have been processed in these stars, and mixed up to the surface where they are observed. However, recent models don't success in predicting the right amount of ``s'' elements compared to the observations. In addition, AGB stars with abundance ratio $C/O>1$ mainly have low luminosities. It's the ``dredge-up'' phenomenon which is responsible for the carbon star formation: the convective enveloppe goes down in the Hydrogen burning shell and bring up some Carbon which has been processed lower in the Helium burning region and put there by the convective instabilities called ``thermal pulses''. However, the depths of the modelled dredge-up are too low to explain the high ratio of carbon stars. To have an overview of the whole problem, the stellar evolution code of Grenoble has been improved to model better the physics of these stars (equation of state, mixing processus in particular). Tests have been realized to determine the changes due to these improvements. Secondly, circumstellar observations have been performed to allow us to estimate what kind of quantities can be obtained and their utilities to constrain the nucleosynthesis. Finaly, the fact that the circumstellar abundances are egal to the abundances processed by the star or not have been studied with a radiative transfer code. Consequently, we have:\\ - determined, in the same stellar evolution code, the influence of different physical or numerical factors on the depth of the dredge-up. In particular, it has been showed that only one physical process (overshooting) is responsible for both a deeper dredge-up, so stars with a higher C/O ratio, and the production of a \nuc(C)(13) pocket, first stage for the nucleosynthesis of ``s'' elements. \\ - enlarged the capabilities of modelling of the code: computation of central Helium flash for low mass stars, computation of reliable PMS (Pre Main Sequence) tracks for stars down to 0.1\Mo...\\ - determined precisely a non solar \iso(Cl)(35)(37) isotopic ratio in the circumstellar enveloppe of the carbon star CW Leo, ratio very sensitive to the occurence of ``s'' process and which allow to constrain the mass of this star.\\ - determined the mass loss rate, $\dot(M)$ and the \iso(C)(12)(13) ratio in \nuc(C)(13) enriched stars, this ratio being very sensitive to all the stages of nucleosynthesis or mixing. These ewo values, $\dot(M)$ and \iso(C)(12)(13), are not compatible and tend to prouve that these stars could not belong to the asymptotic giant branch or that we should change our approach of the modelling of the evolved stellar phases.
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Emmanuel Dufour. Nucléosynthèse dans les étoiles de la branche asymptotique: du coeur dégénéré à l'envellope circumstellaire. Astrophysique [astro-ph]. Université Joseph-Fourier - Grenoble I, 2000. Français. ⟨tel-00009195⟩

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