Abundances in Stars from the Red Giant Branch Tip to Near the Main   Sequence Turn Off in M5
  We present the iron abundance and abundance ratios for 18 elements with respect to Fe in a sample of stars with a wide range in luminosity from luminous giants to stars near the turnoff in the globular cluster M5. The analyzed spectra, obtained with HIRES at the Keck Observatory, are of high dispersion (R=35,000). We find that the neutron capture, the iron peak and the alpha-element abundance ratios show no trend with Teff, and low scatter around the mean between the top of the RGB and near the main sequence turnoff To within the precision of the measurements (~0.1 dex), gravitationally induced heavy element diffusion does not appear to be present among the stars near the main sequence turnoff studied here. Our work and other recent studies suggest that heavy element diffusion is inhibited in the surface layers of metal poor stars. Differences in the Na abundance from star to star which extend to the main sequence turnoff are detected in our sample in M5. The anti-correlation between O and Na abundances, observed in other metal poor globular clusters, is not detected in our sample, but it may be hidden among stars with only upper limits for their O abundances. Overall the abundance ratios of M5 appear very similar to those of M71, with the possible exception of the neutron capture element Ba, where we argue that the apparent difference may be due to difficulties in the analysis. As in M71, the alpha-elements Mg, Ca, Si and Ti are overabundant relative to Fe. The results of our abundance analysis of 25 stars in M5 provide further evidence of abundance variations among specific light elements at unexpectedly low luminosities, which cannot be explained by our current understanding of stellar evolution. 
Solange V. Ramirez and Judith G. Cohen