Metallicity Discrepancy between RGB and RHB Stars of the Globular Cluster M15

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The surveys of Sneden et al. (1997, 2000) found star-to-star abundance scatter of the light elements and the neutron-capture elements in M15 Red Giant Branch (RGB) stars. Further examination of three select RGB tip stars revealed a distinct r-process nucleosynthetic signature (consistent with a scaled solar system r-process abundance distribution). As part of a general survey of metal-poor red horizontal branch (RHB) stars, Preston et al. (2006) observed six RHB members of M15. They detected some star-to-star abundance scatter in both the light and n-capture elements. Notably, Preston et al. found that the mean metallicity of these stars was significant lower (by roughly 0.2 dex) than their RGB counterparts. We present a new comparative abundance derivation for three RGB and six RHB stars of the globular cluster M15. We make a considerable effort to understand the apparent discrepancy in the metallicity of RGB and RHB stars. We then perform a detailed examination of the n-capture elements. We will discuss the abundance results and consider the chemical inhomogeneity of M15. ©2008 American Institute of Physics