On the AGB stars of M 4: A robust disagreement between spectroscopic observations and theory

TL;DR

This study re-analyzes spectroscopic data of M4's AGB and RGB stars, revealing a persistent discrepancy between observed Na-rich AGB stars and theoretical predictions, challenging current stellar evolution models.

Contribution

It provides a comprehensive re-analysis of existing spectroscopic data and presents new stellar models, highlighting a significant disagreement between observations and theory for M4's AGB stars.

Findings

AGB stars show lower Na and Al than RGB stars in M4

Theoretical models predict all stars should evolve onto the AGB, including Na-rich ones

Systematic uncertainties cannot explain the observed Na-rich AGB deficiency

Abstract

Several recent spectroscopic investigations have presented conflicting results on the existence of Na-rich asymptotic giant branch (AGB) stars in the Galactic globular cluster M4 (NGC6121). The studies disagree on whether or not Na-rich red giant branch (RGB) stars evolve to the AGB. For a sample of previously published HER- MES/AAT AGB and RGB stellar spectra we present a re-analysis of O, Na, and Fe abundances, and a new analysis of Mg and Al abundances; we also present CN band strengths for this sample, derived from low-resolution AAOmega spectra. Following a detailed literature comparison, we find that the AGB samples of all studies consistently show lower abundances of Na and Al, and are weaker in CN, than RGB stars in the cluster. This is similar to recent observations of AGB stars in NGC 6752 and M 62. In an attempt to explain this result, we present new theoretical stellar evolutionary models for M 4; however, these predict that all stars, including Na-rich RGB stars, evolve onto the AGB. We test the robustness of our abundance results using a variety of atmospheric models and spectroscopic methods; however, we do not find evidence that systematic modelling uncertainties can explain the apparent lack of Na- rich AGB stars in M4. We conclude that an unexplained, but robust, discordance between observations and theory remains for the AGB stars in M 4.