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

We present a comprehensive investigation of multiple stellar populations along the asymptotic giant branch (AGB) in 22 Galactic globular clusters (GCs), exploiting APOGEE spectroscopy combined with a robust AGB selection based on Gaia color-magnitude diagrams. Using light-element abundances, including C/Fe , N/Fe , Mg/Fe , and Al/Fe , we successfully disentangled first- (1P) and second- (2P) populations along the AGB. We derived population fractions in the AGB for the largest sample of GCs to date, finding that the 1P fraction decreases with increasing cluster mass, in agreement with other evolutionary phases. By directly comparing AGB and red giant branch (RGB) populations, we defined a quantitative criterion to identify clusters affected by the AGB-manqué phenomenon. We find that in nine GCs the most chemically extreme 2P stars are significantly underrepresented along the AGB, indicating that a fraction of these stars fails to ascend this phase. In the remaining clusters, AGB and RGB abundance distributions are consistent within uncertainties. Our classification is in overall agreement with previous studies and provides the first spectroscopic characterization of AGB multiple populations in eight GCs. Combining our measurements with literature data, we derived, for the first time, the radial distribution of AGB 2P stars in four clusters. While NGC,5024 and ωCentauri show trends consistent with the RGB, NGC,2808 and NGC,7078 may exhibit an unexpected increase of the AGB 2P fraction at large radii, opposite to the RGB 2P fraction. We also present the first detailed spectroscopic characterization of anomalous (i.e., enhanced in heavy elements compared to the bulk of 1P and 2P) AGB populations in NGC,6656 and ωCentauri. In both clusters, anomalous stars show a much more pronounced signature of the AGB-manqué presence than 2P stars, with the fraction of the most Mg-poor and Al-rich AGB dramatically dropping compared to the RGB, possibly due to enhanced helium content and/or increased RGB mass loss. Finally, we report the first detection of intrinsic iron inhomogeneities among 1P AGB stars in NGC,5272, with a spread consistent with that observed along the RGB. This discovery extends the presence of iron variations to the most evolved stellar phase studied so far.