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

We have determined Li, C, N, O, Na, and Fe abundances, and C-12/C-13 isotop ic ratios for a sample of 62 field metal-poor stars in the metallicity rang e -2 less than or equal to[Fe/H]less than or equal to -1. Stars were select ed in order to have accurate luminosity estimates from the literature, so t hat evolutionary phases could be clearly determined for each star. We furth er enlarged this dataset by adding 43 more stars having accurate abundances for some of these elements and similarly well defined luminosities from th e literature. This large sample was used to show that (small mass) lower-RG B stars (i.e. stars brighter than the first dredge-up luminosity and fainte r 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 caus es the surface Li abundance to decrease by a factor of similar to 20. A sec ond, 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 th e outward expansion of the H-burning shell, in agreement with recent theore tical predictions. In field stars, this second mixing episode only reaches regions of incomplete CNO burning: it causes a depletion of the surface C-1 2 abundance by about a factor of 2.5, and a corresponding increase in the N abundance by about a factor of 4. The C-12/C-13 is lowered to about 6 to 1 0 (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 su ch 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 (inc luding 8 RHB stars) shows any sign of an O depletion or Na enhancement. Thi s means that the second mixing episode is not deep enough to reach regions were ON-burning occurs in field stars.