Neutron-Capture Elements in the Metal-poor Globular Cluster M15

Abstract

We report on observations of six giants in the globular cluster M15 (NGC 7078) using the Subaru Telescope to measure neutron-capture elemental abundances. Our abundance analyses, based on high-quality blue spectra, confirm the star-to-star scatter in the abundances of heavy neutron-capture elements (e.g., Eu), and we found no significant s-process contribution to them, as was found in previous studies. We have found that, for the first time, there are anticorrelations between the abundance ratios of light to heavy neutron-capture elements ([Y/Eu] and [Zr/Eu]) and the heavy neutron-capture elements (e.g., Eu). This indicates that the light neutron-capture elements in these stars cannot be explained by only a single r-process. Another process that contributed significantly to the light neutron-capture elements is required in M15. Our results suggest a complicated enrichment history for M15 and its progenitor.