Multiple stellar populations in Asymptotic Giant Branch stars of Galactic Globular Clusters

TL;DR

This study investigates multiple stellar populations in AGB stars of 58 Galactic Globular Clusters using Hubble data, revealing that helium-enriched stars often do not evolve into the AGB phase, supporting the AGB-manqué scenario.

Contribution

It provides the first comprehensive analysis of MPs along the AGB in a large sample of GCs, highlighting differences from RGB populations and supporting theoretical models.

Findings

AGB stars show larger light-element and helium variation spread than photometric errors.

The fraction of metal-enhanced AGB stars is generally lower than in RGB.

Helium-enriched stars tend to avoid the AGB phase, supporting the AGB-manqué scenario.

Abstract

Multiple stellar populations (MPs) are a distinct characteristic of Globular Clusters (GCs). Their general properties have been widely studied among main sequence, red giant branch (RGB) and horizontal branch (HB) stars, but a common framework is still missing at later evolutionary stages. We studied the MP phenomenon along the AGB sequences in 58 GCs, observed with the Hubble Space Telescope in ultraviolet (UV) and optical filters. By using UV-optical color-magnitude diagrams, we selected the AGB members of each cluster and identified the AGB candidates of the metal-enhanced population in type II GCs. We studied the photometric properties of AGB stars and compared them to theoretical models derived from synthetic spectra analysis. We observe the following features: i) the spread of AGB stars in photometric indices sensitive to variations of light-elements and helium is typically larger than that expected from photometric errors; ii) the fraction of metal-enhanced stars in the AGB is lower than in the RGB in most of the type II GCs; iii) the fraction of 1G stars derived from the chromosome map of AGB stars in 15 GCs is larger than that of RGB stars; v) the AGB/HB frequency correlates with the average mass of the most helium-enriched population. These findings represent a clear evidence of the presence of MPs along the AGB of Galactic GCs and indicate that a significant fraction of helium-enriched stars, which have lower mass in the HB, does not evolve to the AGB phase, leaving the HB sequence towards higher effective temperatures, as predicted by the AGB-manqu\'e scenario.