Barium and sodium abundances in the globular clusters M15 and M92

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 fiber-fed spectrometer. Medium-resolution spectra (R = lambda/delta lambda similar to 20000) in the wavelength range 5740-5980 Angstrom were obtained for 31 stars in M15 and 34 stars in M92. The stellar samples range over the top three magnitudes of the cluster color-magnitude diagrams; 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 lambda 5857.4, the Ba II lambda 5853.7, and the Na I D lambda lambda 5889.9, 5895.9 lines, and Fe, Si, and Ti abundances have also been derived for the coolest, most luminous 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 [[Ca/H]] abundance in M15 is about 0.05 dex larger than in M92, consistent with earlier suggestions of a very small overall metallicity difference between these two clusters, Using the Ca abundance as a surrogate for cluster metallicity, the overall relative abundances of Ba indicated by the [[Ba/Ca]] ratios are significantly different in M15 and M92: -0.1 and -0.4. The large star-to-star scatter (sigma similar to 0.4) in [Na/Ca] at all giant branch positions in both clusters confirms the existence of large proton-capture abundance variations down to stellar luminosities comparable to the horizontal branch.