3. The data sample
3.1. Giants with measured 12C/13C ratios
We collected in the literature all the available observations of 12C/13C ratios in stars of luminosity classes III and IV-III in the spectral ranges F to K. The whole sample contains 191 stars, with [Fe/H] values between -2.8 and 0.3. Our 85 field stars have been observed by Lambert & Ries (1981), Cottrell & Sneden (1986), Sneden et al. (1986), Shetrone et al. (1993), Shetrone (1996), CBW98. The data for galactic cluster giants are from GB91 for NGC 2682 (M67) and Gilroy (1989) for NGC 752, NGC 2360, IC 4756. For globular clusters, we use the data from Brown & Wallerstein (1989) for NGC 104, NGC 5139, NGC 6121 (M4), NGC 6656 (M22), from Smith & Suntzeff (1989) for NGC 6121 and NGC 6656, from Suntzeff & Smith (1991) for NGC 6121 and NGC 6752, from Shetrone (1996) for NGC 6205 (M13), from Briley et al. (1997) for NGC 6838, and from CBW98 for 47 Tuc. When different values of the 12C/13C ratio are given by different authors for the same star, we take the most recent determination. We use the [Fe/H] values quoted in the respective papers.
3.2. Evolutionary status of our sample stars
The determination of the 12C/13C ratio in our sample is not homogeneous. This is however not a problem for our statistical study : We consider that a star has undergone extra-mixing when its 12C/13C is smaller than 15, i.e., well below the minimum standard post dredge-up prediction.
It is crucial however to have secure constraints on the evolutionary state of our sample stars. Indeed, as shown by GB91 and CBW98, the extra-mixing becomes efficient on the RGB only after the luminosity function bump. Our statistics must thus be done only for stars that have already passed this evolutionary state. The luminosity of the bump slightly depends on the initial metallicity. In the [Fe/H] range we consider, is higher than -0.2 (Fusi Pecci et al. 1990).
For the field stars, we use the and the parallaxes given in the HIPPARCOS catalogue to get . For the globular cluster stars, we take the values given in the respective papers quoted above. Precise absolute magnitudes depend on the cluster distance modulus, which is probably still uncertain. However, it matters only here to be sure that the cluster stars that we take into account in our final statistics are more luminous than the bump.
In our sample, we may have clump or AGB stars. Among the 106 cluster giants, 10 objects have probably already reached this phase. However, their 12C/13C ratios does not differ (except in one case discussed in Sect. 4.3) from the ones of the stars that are ascending the RGB for the first time and have passed the bump. Including them in the sample does not modify the final statistics.
© European Southern Observatory (ESO) 1998
Online publication: July 27, 1998