An extreme paucity of second population AGB stars in the normal globular cluster M4

TL;DR

This study reveals an extreme scarcity of second population AGB stars in the globular cluster M4, suggesting that SP2 stars may avoid the AGB phase, challenging current understanding of stellar evolution in such clusters.

Contribution

First high-resolution spectroscopic analysis of AGB stars in M4 using HERMES, showing all AGB stars belong to the primordial subpopulation, indicating SP2 stars may not evolve through the AGB.

Findings

AGB stars in M4 are predominantly from SP1.

No AGB stars with [Na/O] > -0.17 found.

Contrasts with RGB abundance distribution.

Abstract

Galactic Globular clusters (GCs) are now known to harbour multiple stellar populations, which are chemically distinct in many light element abundances. It is becoming increasingly clear that asymptotic giant branch (AGB) stars in GCs show different abundance distributions in light elements compared to those in the red giant branch (RGB) and other phases, skewing toward more primordial, field-star-like abundances, which we refer to as subpopulation one (SP1). As part of a larger program targeting giants in GCs, we obtained high-resolution spectra for a sample of 106 RGB and 15 AGB stars in Messier 4 (NGC 6121) using the 2dF+HERMES facility on the Anglo-Australian Telescope. In this Letter we report an extreme paucity of AGB stars with [Na/O] > -0.17 in M4, which contrasts with the RGB that has abundances up to [Na/O] =0.55. The AGB abundance distribution is consistent with all AGB stars being from SP1. This result appears to imply that all subpopulation two stars (SP2; Na-rich, O-poor) avoid the AGB phase. This is an unexpected result given M4's horizontal branch morphology -- it does not have an extended blue horizontal branch. This is the first abundance study to be performed utilising the HERMES spectrograph.