The Abundances of Neutron Capture Species in the Very Metal-Poor Globular Cluster M15: An Uniform Analysis of RGB and RHB Stars

TL;DR

This study provides a self-consistent analysis of neutron-capture element abundances in RGB and RHB stars of globular cluster M15, revealing a metallicity discrepancy, anti-correlated element behavior, and insights into the cluster's nucleosynthetic history.

Contribution

It presents the first uniform abundance analysis of RGB and RHB stars in M15 using an updated line transfer code, addressing previous metallicity discrepancies and exploring neutron-capture element patterns.

Findings

Discovered a 0.1 dex metallicity difference between RHB and RGB stars.

Confirmed anti-correlation of light neutron-capture elements with Ba and Eu.

Found no conclusive Pb detection, indicating minimal s-process contribution.

Abstract

The globular cluster M15 is unique in its display of star-to-star variations in the neutron-capture elements. Comprehensive abundance surveys have been previously conducted for handfuls of M15 red giant branch (RGB) and red horizontal branch (RHB) stars. No attempt has been made to perform a single, self-consistent analysis of these stars, which exhibit a wide range in atmospheric parameters. In the current effort, a new comparative abundance derivation is presented for three RGB and six RHB members of the cluster. The analysis employs an updated version of the line transfer code MOOG, which now appropriately treats coherent, isotropic scattering. The apparent discrepancy in the previously reported values for the metallicity of M15 RGB and RHB stars is addressed and a resolute disparity of $\Delta(RHB-RGB) \approx 0.1$ dex in the iron abundance was found. The anti-correlative behavior of the light neutron capture elements (Sr, Y, Zr) is clearly demonstrated with both Ba and Eu, standard markers of the {\it s}- and {\it r}-process, respectively. No conclusive detection of Pb was made in the RGB targets. Consequently for the M15 cluster, this suggests that the main component of the {\it s}-process has made a negligible contribution to those elements normally dominated by this process in solar system material. Additionally for the M15 sample, a large Eu abundance spread is confirmed, which is comparable to that of the halo field at the same metallicity. These abundance results are considered in the discussion of the chemical inhomogeneity and nucleosynthetic history of M15.