Multiple populations along the asymptotic giant branch: a Gaia+APOGEE study of 22 Galactic globular clusters

Abstract

We present an investigation of multiple stellar populations along the asymptotic giant branch (AGB) in 22 globular clusters (GCs), exploiting APOGEE spectroscopy combined with AGB selection based on Gaia color-magnitude diagrams. Using light-element abundances ([C/Fe], [N/Fe], [Mg/Fe], and [Al/Fe]), we disentangle first- (1P) and second- (2P) populations along the AGB. We derive their fractions in the AGB for the largest sample of GCs to date, finding that the 1P fraction decreases with cluster mass, as in other evolutionary phases. By comparing AGB and red giant branch (RGB), we define a quantitative criterion to identify clusters affected by the AGB-manque phenomenon. We find that in nine GCs the most chemically extreme 2P stars are underrepresented along the AGB, indicating that they fail to ascend this phase. Our classification is in agreement with previous studies and provides the first spectroscopic characterization of AGB multiple populations in eight GCs. We derive, for the first time, the radial distribution of AGB 2P stars in four clusters. While NGC5024 and omegaCentauri show trends consistent with the RGB, NGC2808 and NGC7078 may exhibit an unexpected increase of the AGB 2P fraction at large radii, opposite to the RGB stars. We present the first detailed spectroscopic characterization of anomalous AGB populations in NGC6656 and omegaCentauri, i.e. the populations enhanced in heavy elements compared to the bulk of 1P and 2P. In both clusters, anomalous stars show a more pronounced AGB-manque signature than 2P stars, with the fraction of the most Mg-poor and Al-rich AGB dropping compared to the RGB, possibly due to enhanced He and/or increased RGB mass loss. We report the first detection of iron inhomogeneities among 1P AGB stars in NGC5272, with a spread consistent with the RGB one. This extends the presence of iron variations to the most evolved stellar phase studied so far.