Multiple Stellar Populations in Asymptotic Giant Branch Stars of Galactic Globular Clusters

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 asymptotic giant branch (AGB) sequences in 58 GCs, observed with the Hubble Space Telescope in UV and optical filters. 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 the AGB stars and compared them to theoretical models derived from synthetic spectral analysis. We observed 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 that 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; and (v) the AGB/HB frequency correlates with the average mass of the most helium-enriched population. These findings represent 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, do not evolve to the AGB phase, leaving the HB sequence toward higher effective temperatures, as predicted by the AGB manqué scenario.