Barium and Sodium Abundances in the Globular Clusters M15 and M92

Abstract

Upper giant branch members of the Galactic globular clusters M15 and M92 have been surveyed with the WIYN 3.5 m telescope and Hydra multiobject Ðber-fed spectrometer. Medium-resolution spectra (R4j/djD20000) in the wavelength range 5740È5980 were obtained for 31 stars in M15 and 34 stars A? in M92. The stellar samples range over the top three magnitudes of the cluster color-magnitude dia- grams; the faintest of the stars are at or below the luminosity of the horizontal branch. Ca, Ba, and Na abundances have been derived for all stars from the Ca I j5857.4, the Ba II j5853.7, and the Na I D jj5889.9, 5895.9 lines, and Fe, Si, and Ti abundances have also been derived for the coolest, most lumi- nous stars near the cluster red giant branch tips. The Fe metallicities and abundance ratios of the tip giants are in general agreement with literature values for these two clusters. The derived S[Ca/H]T abundance in M15 is about 0.05 dex larger than in M92, consistent with earlier suggestions of a very small overall metallicity di†erence between these two clusters. Using the Ca abundance as a surrogate for cluster metallicity, the overall relative abundances of Ba indicated by the S[Ba/Ca]T ratios are signiÐcantly di†erent in M15 and M92: [0.1 and [0.4. The large star-to-star scatter (pD0.4) in [Na/Ca] at all giant branch positions in both clusters conÐrms the existence of large proton-capture abundance variations down to stellar luminosities comparable to the horizontal branch.