Chemical analysis of Asymptotic Giant Branch stars in M62

TL;DR

This study analyzes the chemical abundances of AGB and RGB stars in M62, revealing NLTE effects and indicating that AGB stars are predominantly first-generation, with implications for stellar evolution in globular clusters.

Contribution

It provides the first detailed abundance analysis of AGB stars in M62, highlighting NLTE effects and the absence of second-generation AGB stars in this cluster.

Findings

AGB star abundances from neutral lines are underestimated due to NLTE effects.

All AGB stars are consistent with first-generation stars, lacking second-generation counterparts.

A RGB star with extremely high sodium abundance was identified.

Abstract

We have collected UVES-FLAMES high-resolution spectra for a sample of 6 asymptotic giant branch (AGB) and 13 red giant branch (RGB) stars in the Galactic globular cluster M62 (NGC6266). Here we present the detailed abundance analysis of iron, titanium, and light-elements (O, Na, Al and Mg). For the majority (5 out 6) of the AGB targets we find that the abundances, of both iron and titanium, determined from neutral lines are significantly underestimated with respect to those obtained from ionized features, the latter being, instead, in agreement with those measured for the RGB targets. This is similar to recent findings in other clusters and may suggest the presence of Non-Local Thermodynamical Equilibrium (NLTE) effects. In the O-Na, Al-Mg and Na-Al planes, the RGB stars show the typical correlations observed for globular cluster stars. Instead, all the AGB targets are clumped in the regions where first generation stars are expected to lie, similarly to what recently found for the AGB population of NGC6752. While the sodium and aluminum abundance could be underestimated as a consequence of the NLTE bias affecting iron and titanium, the used oxygen line does not suffer from the same effects and the lack of O-poor AGB stars therefore is solid. We can thus conclude that none of the investigated AGB stars belong to the second stellar generation of M62. We also find a RGB star with extremely high sodium abundance ([Na/Fe] = +1.08 dex).