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

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1. Introduction

For some time now it has been clear that evolved low mass stars exhibit chemical anomalies which are not predicted by standard stellar evolution theory. By "standard" we refer to the hypotheses that stellar convective regions are instantly mixed and that no transport of chemicals occurs in the radiative regions. In addition, standard models are those without rotation at any depth. With these assumptions, changes in the surface abundances prior to the asymptotic giant branch (AGB) stage are only expected to be due to convective dilution during the first dredge-up phase. In low mass red giant branch (RGB) stars, the convective envelope reaches only regions where 12 C was processed to 13 C and 14 N. Consequently, the carbon isotopic ratio declines (from 90 to about 20-30), the carbon abundance drops (by about 30%) and nitrogen increases (by about 80%), but oxygen and all heavier element abundances remain unchanged. According to the standard scenario, the surface abundances then stay unaltered as the convective envelope slowly withdraws outward in mass during the end of the RGB evolution.

However, observational data reveal a different reality. The first discrepancy came among the first stellar [FORMULA] ratios published (Day et al. 1973), when Arcturus was found to have [FORMULA]. The discrepancy was aggravated by the analysis by Lambert & Ries (1981) of C, N, and O abundances in a sample of red giants, including Arcturus. In fact, in most of the metal-deficient field and globular cluster evolved stars, the observed conversion of 12 C to 13 C and 14 N greatly exceeds the levels expected from standard stellar models; the [FORMULA] ratio even reaches the near-equilibrium value in many Pop II RGB stars (Sneden, Pilachowski & Vandenberg 1986; Smith & Suntzeff 1989; Brown & Wallerstein 1989, 1992; Gilroy & Brown 1991, henceforth GB91; Brown, et al. 1990, henceforth BWO; Bell et al. 1990; Suntzeff & Smith 1991; Shetrone et al. 1993, henceforth SSP93; Briley et al. 1994, 1997). This problem actually also occurs, but to a somewhat lower extent, in evolved stars belonging to open clusters with turnoff masses lower than [FORMULA] (Gilroy 1989; GB91).

In addition, Population II evolved stars present other chemical anomalies. In halo giants, the lithium abundance continues to decrease after the completion of the first dredge-up (Pilachowski et al. 1993). A continuous decline in carbon abundance with increasing stellar luminosity along the RGB is observed in globular clusters such as M92 (Carbon 1982; Langer et al. 1986), M3 and M13 (Suntzeff 1981), M15 (Trefzger et al. 1983), NGC 6397 (Bell et al. 1979, Briley et al. 1990), NGC 6752 and M4 (Suntzeff & Smith 1991). In some globular clusters (M92, Pilachowski 1988; M15, Sneden et al. 1991; M13, Brown et al. 1991, Kraft et al. 1992; [FORMULA] Cen, Paltoglou & Norris 1989), giants exhibit evidence in their atmospheres for O [FORMULA] N processed material. In addition to the O versus N anticorrelation, the existence of Na and Al versus N correlations and Na and Al versus O anticorrelations in a large number of globular cluster red giants has been clearly confirmed (Drake et al. 1992, Kraft et al. 1992, 1993, Norris & Da Costa 1995, Kraft 1994, Kraft et al. 1997, Shetrone 1996, Zucker et al. 1996).

These observations suggest that low mass stars undergo a non-standard mixing process which adds to the first dredge-up and modifies the surface abundances. The first and only evidence on the evolutionary state at which this non-standard mixing actually becomes effective came from observations of the C/N and [FORMULA] ratios (respectively by Brown 1987 and by GB91) in evolved stars of M67. In this old open cluster, which has a turnoff mass of 1.2 [FORMULA] (VandenBerg 1985, Meynet et al. 1993) and a value of [Fe/H]=0.0 [FORMULA] 0.1, the observational and theoretical first dredge-up appear to be in complete agreement (Charbonnel 1994, henceforth C94). Indeed, the [FORMULA] ratio changes at the base of the giant branch from a value [FORMULA] 40 to 21-25 between [FORMULA] and [FORMULA] ; C/N changes by about a factor of 4 in this same interval. The onset of the dredge-up occurs at a luminosity in accord with standard theory, and the observed post-dilution ratios are in very good agreement with the standard predictions at this point. However, in M67 an additional mixing event begins at about [FORMULA] where [FORMULA] drops from 21-25 to 11-15. This luminosity actually corresponds to the so-called "bump" in the RGB luminosity function (Fusi-Pecci et al. 1990), i.e., to the evolutionary point where the hydrogen burning shell crosses the chemical discontinuity created by the convective envelope at its maximum extent. As pointed out by C94, this observational fact strongly suggests that prior to this evolutionary point the mean molecular weight gradient created during the first dredge-up acts as a barrier to any mixing below the convective envelope. After this point, however, the gradient of molecular weight above the hydrogen-burning shell is much lower, and extra-mixing is free to act.

Since the observations in M67, there have been no new sets of abundance data in any cluster, either globular or galactic, to establish the evolutionary point at which the extra-mixing begins to occur. With 4-meter class telescopes it is almost impossible to observe red giants in globular clusters at the luminosities at which [FORMULA] is expected to drop from near 20 to less than 10 because the molecular bands become weaker with increasing temperature and gravity. Furthermore, the attainable signal-to-noise of the spectra diminishes with decreasing stellar luminosity. It is crucial, however, for our understanding of the nature of the process to compare conditions in stars of different populations and metallicities.

To this end, we address here the problem of the onset of the extra-mixing, and of its possible metallicity dependance, by placing in an evolutionary sequence five field giants with moderate metal deficiencies and two giants of the globular cluster 47 Tuc for which we have assembled accurate abundance data for Li, C and N as well as for the [FORMULA] ratio. Our observational data are presented in Sect. 2. We enlarge our sample by including in our discussion the old disk giants with [FORMULA] ratios derived by SSP93. Very precise absolute magnitudes were recomputed from HIPPARCOS parallaxes for all the stars we consider. As discussed in Sect. 3, the sequence allows us to confirm that the discrepancy between observations of mixing-sensitive species and standard theory really appears at the luminosity-function bump (LFB) on the RGB, but not at lower luminosities. Since our sample is moderately metal-deficient, we can study the metallicity-dependence of the extra-mixing by comparing our results with the observations in M67. In Sect. 4, we investigate the nature of the factor that stabilizes the star against extra-mixing before it reaches the luminosity-function bump, and discuss the role of molecular-weight (µ) barriers. From simple considerations, we derive from our data the "observational" critical µ-gradient, ([FORMULA], that shields the central regions of a star from extra-mixing. If we assume that extra-mixing is free to act down to the layers defined by the same value of ([FORMULA] whatever the stellar mass and metallicity, then [FORMULA] ratios can be explained both for Pop I and II giants. Finally, we discuss the implications for the nature of the extra-mixing mechanism itself on the RGB, and bring clues that it may be related to rotation-induced processes.

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© European Southern Observatory (ESO) 1998

Online publication: March 10, 1998