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Corinne Charbonnel

Publications and source records attributed to Corinne Charbonnel.

78 records · Page 5Linked to original sources

Nucleosynthesis in intermediate mass AGB stars

We present a summary of the main sites for nucleosynthesis in intermediate mass Asymptotic Giant Branch (AGB) stars. We then discuss some detailed evolutionary models and how these have been used to create a synthetic evolution code which calculates the nucleosynthesis very rapidly, enabling us to investigate changes in some uncertain parameters in AGB evolution, such as mass-loss and dredge-up. We then present results for C, C/O, Mg and Al. We also discuss the changes due to the recent NACRE compilation of reaction rates.

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The hot side of the lithium dip - LiBeB abundances beyond the main sequence

We extend to the case of A and early-F type stars our study of the transport of matter and angular momentum by wind-driven meridional circulation and shear turbulence. We show that our fully consistent treatment of the same hydrodynamical processes which can account for C and N anomalies in B type stars (Talon et al. 1997) and for the shape of the hot side of the Li dip in the open clusters (Talon & Charbonnel 1998) also explains LiBeB observations in stars with Teff higher than 7000 K on the main sequence as well as in their evolved counterparts.

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Element Segregation in Low Metallicity Stars and the Primordial Lithium Abundance

Observational constraints on the primordial lithium abundance are important for the evaluation of the baryonic density of the universe. Its precise determination however suffers from uncertainties due to the possible depletion of this element inside the stars. Here we present and discuss new results on the lithium abundances in pop II stars obtained with most recent stellar models including the best available physics. We show that it is possible to account for the general behavior of lithium observed in pop II stars without any free parameters. Macroscopic motions inside the stars are needed but it is not necessary to specify their exact nature to interpret the observational data. This constatation allows to derive a parameter-free value for the lithium primordial abundance : log(Lip) = 2.35 +/- .10 in the log(H) = 12 scale.

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Mixing processes during the evolution of red giants with moderate metal deficiencies : the role of molecular-weight barriers

We have assembled accurate abundance data for Li, C, and N as well as the 12C/13C ratio for five field giants with [Fe/H]=-0.6 including Arcturus and two stars in the globular cluster 47 Tuc. Using their very precise Mbol values obtained from HIPPARCOSparallaxes, we can place them into an evolutionary sequence. The sequence shows that the 12C/13C ratios ratios drops from 20 to near 7 between Mbol=+1 and +0.5, while Li disappears. At the same time the 12C/14N ratio diminishes by 0.2 to 0.4 dex. The two stars in 47 Tuc with Mbol near -2.0 show even lower 12C14N ratios by 0.4 dex indicating further mixing as they evolved to the top of the red giant branch. These observations confirm the existence of an extra-mixing process that becomes efficient on the red giant branch only when the low-mass stars reach the so-called luminosity function bump. We use the values of the carbon isotopic ratio observed in our sample to get constraints on the mu-barriers that may shield the central regions of a star from extra-mixing. We show that the same value of the critical gradient of molecular weight leads to 12C/13C ratios observed at different metallicities. This ``observational critical mu-gradient'' is in very good agreement with the one which is expected to stabilize meridional circulation. This result provides strong clues on the nature of the extra-mixing which occurs on the RGB, and indicates that it is related to rotation.

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Nucleosynthesis of light elements inside thermally pulsing AGB stars I: the case of intermediate-mass stars

The structural and nucleosynthetic evolution of 3, 4, 5, 6 and \mass{7} stars with two metallicities ($Z = 0.005$ and 0.02) has been computed in detail, from the early pre-main sequence phase up to the thermally pulsing (TP) AGB phase or the onset of off-center carbon burning. Typically 10 to 20 thermal pulses have been followed for each TP-AGB object. This homogeneous and quite large set of models allows us to present an overview of the thermal pulse properties as well as of the nucleosynthesis accompanying the TP-AGB phase of intermediate-mass stars. More specifically, after a brief description of the previous evolutionary stages, predictions are given for the isotopic ratios involving C, N, O, Ne, Mg, Al and Si. Also the surface abundances of 7Li, 19F and 23Na are reported. As the asymptotic phase of the thermal pulses has been reached for each star, we also indicate how these abundances will probably evolve until the stars completely loose their envelope, by including the evolution of the nucleosynthesis itself. This article, in its paper form, has been shortened at a level of roughly 60 \% as required by directives coming from the A\&A editors. The complete article (50 pages containing 37 figures) is only available in electronic form at http://gag.observ-gr.fr/liens/starevol/evol.html

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A consistent explanation for $^{12}$C/$^{13}$C, $^7$Li, and $^3$He anomalies in red giant stars

The observations of carbon isotopic ratios in evolved stars suggest that non standard mixing is acting in low mass stars as they are ascending the red giant branch. We propose a simple consistent mechanism, based on the most recent developments in the description of rotation-induced mixing by Zahn (1992), which simultaneously accounts for the low $^{12}$C/$^{13}$C ratios in globular cluster and field Pop II giants and for the lithium abundances in metal-poor giant stars. It also leads to the destruction of $^3$He produced on the main sequence in low mass stars. This should both naturally account for the recent measurements of $^3$He/H in galactic HII regions and allow for high values of $^3$He observed in some planetary nebulae.

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