$r$-process Abundance Dispersion in the Globular Cluster M5 using Keck Archival Data

Abstract

We studied $28$ RGB stars in the mildly metal-rich globular cluster M5 ([Fe/H] $= -1.29$) using archival high-resolution spectra from the Keck Observatory archive (KOA) to better understand the $r$-process in globular clusters. Previous studies (M15, M92, and NGC 2298) have shown $r$-process dispersion in varying amounts, hinting at the source of the $r$-process in those clusters. We extend these dispersion studies to the more metal-rich cluster M5 by studying the rare-earth peak, specifically the elements Ba, Nd, and Eu. We separately analyze the different stellar generations, as traced by the abundance of Na and O. Based on the Nd and Eu abundances, we report a tenuous detection of $r$-process dispersion that is dependent on the generation and element. Based on a log-likelihood dispersion study accounting for measurement errors, Nd has an intrinsic first generation abundance spread of $\sigma_{1G}(\text{Nd}) = 0.15_{-0.07}^{+0.10}$ and an $2\sigma$ upper limit on the second generation spread of $\sigma_{2G}(\text{Nd}) < 0.28$. The upper limits on the Eu intrinsic spread are $\sigma_{1G}(\text{Eu}) < 0.34$ and $\sigma_{2G}(\text{Eu}) < 0.16$. A potential dispersion implies the cluster gas was inhomogeneously polluted, either due to an event concurrent with the formation of the cluster or due to clouds of disparate composition that coalesced to form the cluster.