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Astron. Astrophys. 332, 204-214 (1998)

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6. Conclusions

We have assembled accurate [FORMULA] ratios, as well as abundance data for C, N, O and Li for five field giants with [Fe/H] [FORMULA] including Arcturus, and for two stars in the globular cluster 47 Tuc, and reviewed previously published data for a number of other field giants. Using HIPPARCOS parallaxes to derive [FORMULA] values, we placed these stars into an evolutionary sequence along the ascent of the RGB. The sequence shows that the [FORMULA] ratios drops from [FORMULA] to near 7 between [FORMULA] = +0.9 and +0.5, while Li disappears. The C/N ratio is also affected, with log ([FORMULA]) diminishing from +0.1 to -0.6 between [FORMULA] +0.9 and -2. Arcturus stands above the curve defined by the other stars by 0.4 dex. These observations confirm the existence of an extra-mixing process that becomes efficient on the RGB only when low-mass stars reach the so-called luminosity-function bump. This result is established for the first time for stars with moderate metal deficiencies.

We used the values of the carbon isotopic ratio observed in our sample to get constraints on the µ-barriers that may shield the central regions of a star from extra-mixing. We showed that the same value of the critical gradient of molecular weight leads to [FORMULA] ratios observed at different metallicities. This observational critical µ-gradient is in very good agreement with the critical µ-gradient which is expected to stabilize meridional circulation. This result brings strong clues on the nature of the extra-mixing which occurs on the RGB, and indicates that it is related to rotation. Observational support for this approach may exist from the variations of oxygen, sodium and aluminum in globular cluster red giants as well as the measurement of rotational velcoities in blue horizontal branch stars in three globular clusters (Kraft, 1994; Kraft et al., 1997; Peterson et al. 1995).

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© European Southern Observatory (ESO) 1998

Online publication: March 10, 1998