Clues for non-standard mixing on the red giant branch from C-12/C-13 and C-12/N-14 ratios in evolved stars

Details Clues for non-standard mixing on the red giant branch from C-12/C-13 and C-12/N-14 ratios in evolved stars Clues for non-standard mixing on the red giant branch from C-12/C-13 and C-12/N-14 ratios in evolved stars

Feb 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...282..811c&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 282, no. 3, p. 811-820

Astronomy and Astrophysics

Astrophysics

198

Carbon 12, Carbon 13, Mass Ratios, Nitrogen, Red Giant Stars, Stellar Composition, Stellar Evolution, Stellar Interiors, Stellar Models, Abundance, Mathematical Models, Metallicity, Stellar Mass

Scientific paper

Based on new grids of standard stellar models computed by the Geneva group (Schaller et al. 1992; Schaerer et al. 1993a, 1993b; Charbonnel et al. 1993a) with the recent OPAL opacity tables (Rogers & Iglesias 1992; Iglesias et al. 1992), we map the theoretical evolution of the C-12/C-13 and C-12/C-14 ratios along the red giant branch of low and intermediate mass stars(1 less than or = M*/solar mass less than or = 7) for the different metallicities (Z = 0.001, 0.004, 0.008, 0.020, 0.040). The predicted post-dredge up values lie between 18 and 26 for C-12/C-13 and between 0.8 and 2.5 for C-12/N-14. The influence of the stellar mass and initial chemical composition is discussed. Comparisons of the evolutionary predictions with observations of the surface ratios in evolved stars of different populations reveal that (1) the observational and theoretical abundances at the first dredge-up are in agreement, but (2) an extra-mixing process occurs on the giant branch which additionally lowers the post-dredge up surface ratios in stars with masses smaller than about 2 solar mass. We present clues that the extra-mixing needed to account for the observed giant branch CNO anomalies is only efficient of the red giant branch when the hydrogen-burning shell reaches the chemical discontinuity created by the convective envelope at its maximum extent. We discuss the nature of the extra-mixing process and suggest future improvements to the stellar modelisation.

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