Mixing along the red giant branch in metal-poor field stars

We have determined Li, C, N, O, Na, and Fe abundances, and ¹²C/¹³C isotopic ratios for a sample of 62 field metal-poor stars in the metallicity range -2<=[Fe/H]<= -1. Stars were selected in order to have accurate luminosity estimates from the literature, so that evolutionary phases could be clearly determined for each star. We further enlarged this dataset by adding 43 more stars having accurate abundances for some of these elements and similarly well defined luminosities from the literature. This large sample was used to show that (small mass) lower-RGB stars (i.e. stars brighter than the first dredge-up luminosity and fainter than that of the RGB bump) have abundances of light elements in agreement with predictions from classical evolutionary models: only marginal changes occur for CNO elements, while dilution within the convective envelope causes the surface Li abundance to decrease by a factor of ~ 20. A second, distinct mixing episode occurs in most (perhaps all) small mass metal-poor stars just after the RGB bump, when the molecular weight barrier left by the maximum inward penetration of the convective shell is canceled by the outward expansion of the H-burning shell, in agreement with recent theoretical predictions. In field stars, this second mixing episode only reaches regions of incomplete CNO burning: it causes a depletion of the surface ¹²C abundance by about a factor of 2.5, and a corresponding increase in the N abundance by about a factor of 4. The ¹²C/¹³C is lowered to about 6 to 10 (close to but distinctly higher than the equilibrium value of 3.5), while practically all remaining Li is burnt. However an O-Na anti-correlation such as typically observed amongst globular cluster stars, is not present in field stars. None of the 29 field stars more evolved than the RGB bump (including 8 RHB stars) shows any sign of an O depletion or Na enhancement. This means that the second mixing episode is not deep enough to reach regions were ON-burning occurs in field stars. Based in part on observations made at the ESO La Silla Observatory

Tables 1, 2, 3, 5 and 6 are available in electronic form only at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html