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Galactic globular clusters (GCs) are known to host multiple stellarpopulations: a first generation with a chemical pattern typical of halo fieldstars and a second generation (SG) enriched in Na and Al and depleted in O andMg. Both stellar generations are found at different evolutionary stages (e.g.,the main-sequence turnoff, the subgiant branch, and the red giant branch). Thenon detection of SG asymptotic giant branch (AGB) stars in several metal-poor([Fe/H] < -1) GCs suggests that not all SG stars ascend the AGB phase, and thatfailed AGB stars may be very common in metal-poor GCs. This observationrepresents a serious problem for stellar evolution and GC formation/evolutiontheories. We report fourteen SG-AGB stars in four metal-poor GCs (M 13, M 5, M3, and M 2) with different observational properties: horizontal branch (HB)morphology, metallicity, and age. By combining the H-band Al abundancesobtained by the APOGEE survey with ground-based optical photometry, we identifySG Al-rich AGB stars in these four GCs and show that Al-rich RGB/AGB GC starsshould be Na-rich. Our observations provide strong support for present,standard stellar models, i.e., without including a strong mass-loss efficiency,for low-mass HB stars. In fact, current empirical evidence is in agreement withthe predicted distribution of FG and and SG stars during the He-burning stagesbased on these standard stellar models.

Clear evidence for the presence of second-generation asymptotic giant branch stars in metal-poor Galactic globular clusters