Abundances in a Large Sample of Stars in M3 and M13
  We have carried out a detailed abundance analysis for 21 elements in a sample of 25 stars with a wide range in luminosity from luminous giants to stars near the main sequence turnoff in the globular cluster M13 and in a sample of 13 stars distributed from the tip to the base of the RGB in the GC M3. Most elements, including Fe, show no trend with Teff, and low scatter around the mean between the top of the RGB and near the main sequence turnoff. We have detected an anti-correlation between O and Na abundances, observed previously among the most luminous RGB stars in both of these clusters, in both M3 and in M13 over the full range of luminosity of our samples, i.e. in the case of M13 to near the main sequence turnoff. M13 shows a larger range in both O and Na abundance than does M3 at all luminosities, in particular having a few stars at its RGB tip with unusually strongly depleted O. Although CN burning must be occurring in both M3 and in M13, and ON burning is required for M13, we combine our new O abundances with published C and N abundances to confirm with quite high precision that the sum of C+N+O is constant near the tip of the giant branch, and we extend this down to the bump in the luminosity function. Star I-5 in M13 has large excesses of Y and of Ba, with no strong enhancement of Eu, suggesting an s-process event contributed. The mean abundance ratios for M3 and for M13 are identical to within the errors. The abundance ratios for 13 Galactic GCs with recent detailed abundance analyses are compared to those of published large surveys of metal-poor halo field stars. For most elements, the agreement is very good, suggesting a common chemical history for the halo field and cluster stars. 
Judith G. Cohen and Jorge Melendez