High Resolution Optical Spectroscopy of a Newly Discovered Post-AGB Star with a Surprising Metallicity in the Globular Cluster M79

TL;DR

This study presents a high-resolution spectroscopic analysis of a post-AGB star in globular cluster M79, revealing unexpectedly low iron abundance compared to cluster red giants, possibly due to atmospheric structural differences.

Contribution

It provides the first detailed abundance analysis of this post-AGB star, highlighting a discrepancy in metallicity and proposing a new explanation involving atmospheric structure differences.

Findings

Post-AGB star's iron abundance is 0.6 dex lower than cluster red giants.

Abundances relative to iron are consistent between the star and red giants for 15 elements.

Atmospheric structure differences may explain the metallicity discrepancy.

Abstract

An abundance analysis based on a high-resolution spectrum is presented for a newly discovered post-AGB star in the globular cluster M79. The surprising result is that the iron abundance of the star is apparently about 0.6 dex less than that of the cluster's red giants as reported by published studies including a recent high-resolution spectroscopic analysis by Carretta and colleagues. Abundances relative to iron appear to be the same for the post-AGB star and the red giants for the 15 common elements. It is suggested that the explanation for the lower abundances of the post-AGB star may be that its atmospheric structure differs from that of a classical atmosphere; the temperature gradient may be flatter than predicted by a classical atmosphere.