A Detailed Study of Giants and Horizontal Branch Stars in M68: Atmospheric Parameters and Chemical Abundances

TL;DR

This study provides a comprehensive high-resolution spectroscopic analysis of various evolutionary stages of stars in globular cluster M68, introducing a hybrid method for accurate atmospheric parameter determination and confirming known chemical abundance patterns.

Contribution

It develops a hybrid approach for deriving stellar atmospheric parameters that resolves inconsistencies in spectroscopic and photometric methods, enabling consistent chemical abundance analysis across different star types.

Findings

Derived a mean metallicity of [Fe/H] = -2.42 with low scatter.

Confirmed the Na-O anti-correlation characteristic of globular clusters.

Observed consistent chemical abundance patterns across evolutionary stages.

Abstract

In this paper, we present a detailed high-resolution spectroscopic study of post main sequence stars in the Globular Cluster M68. Our sample, which covers a range of 4000 K in $T_{eff}$, and 3.5 dex in $log(g)$, is comprised of members from the red giant, red horizontal, and blue horizontal branch, making this the first high-resolution globular cluster study covering such a large evolutionary and parameter space. Initially, atmospheric parameters were determined using photometric as well as spectroscopic methods, both of which resulted in unphysical and unexpected $T_{eff}$, $log(g)$, $\xi_{t}$, and [Fe/H] combinations. We therefore developed a hybrid approach that addresses most of these problems, and yields atmospheric parameters that agree well with other measurements in the literature. Furthermore, our derived stellar metallicities are consistent across all evolutionary stages, with $\langle$[Fe/H]$\rangle$ = $-$2.42 ($\sigma$ = 0.14) from 25 stars. Chemical abundances obtained using our methodology also agree with previous studies and bear all the hallmarks of globular clusters, such as a Na-O anti-correlation, constant Ca abundances, and mild $r$-process enrichment.