Chemical Abundances for Evolved Stars in M5: Lithium through Thorium

TL;DR

This study analyzes high-resolution spectra of stars in globular cluster M5 to determine chemical abundances across different evolutionary stages, revealing consistent neutron-capture signatures and effects of analysis offsets.

Contribution

It provides detailed chemical abundance measurements for stars in M5, highlighting the importance of accounting for analysis effects when comparing different stellar evolutionary branches.

Findings

Detected abundance offsets between AGB and RGB due to analysis effects.

Confirmed signatures of hot hydrogen burning cycle products across all stellar types.

Identified a predominantly r-process neutron-capture signature with a small s-process contribution.

Abstract

We present analysis of high-resolution spectra of a sample of stars in the globular cluster M5 (NGC 5904). The sample includes stars from the red giant branch (seven stars), the red horizontal branch (two stars), and the asymptotic giant branch (eight stars), with effective temperatures ranging from 4000 K to 6100 K. Spectra were obtained with the HIRES spectrometer on the Keck I telescope, with a wavelength coverage from 3700 to 7950 angstroms for the HB and AGB sample, and 5300 to 7600 angstroms for the majority of the RGB sample. We find offsets of some abundance ratios between the AGB and the RGB branches. However, these discrepancies appear to be due to analysis effects, and indicate that caution must be exerted when directly comparing abundance ratios between different evolutionary branches. We find the expected signatures of pollution from material enriched in the products of the hot hydrogen burning cycles such as the CNO, Ne-Na, and Mg-Al cycles, but no significant differences within these signatures among the three stellar evolutionary branches especially when considering the analysis offsets. We are also able to measure an assortment of neutron-capture element abundances, from Sr to Th, in the cluster. We find that the neutron-capture signature for all stars is the same, and shows a predominately r-process origin. However, we also see evidence of a small but consistent extra s-process signature that is not tied to the light-element variations, pointing to a pre-enrichment of this material in the protocluster gas.