NGC 6791: A Probable Bulge Cluster without Multiple Populations

NGC 6791 is a unique stellar cluster, key to our understanding of both the multiple stellar population phenomenon and the evolution and assembly of the Galaxy. However, despite many investigations, its nature is still very controversial. Geisler et al. found evidence suggesting that it was the first open cluster to possess multiple populations, but several subsequent studies did not corroborate this. It has also been considered a member of the thin or thick disk or even the bulge, and either as an open or globular cluster or even the remnant of a dwarf galaxy. Here we present and discuss detailed abundances derived from high-resolution spectra obtained with UVES at VLT and HIRES at Keck of 17 evolved stars of this cluster. We obtained a mean [Fe/H] = +0.313 ± 0.005, in good agreement with recent estimates, and with no indication of star-to-star metallicity variation, as expected. We also did not find any variation in Na, in spite of having selected the very same stars as in Geisler et al., where an Na variation was claimed. This points to the presence of probable systematics in the lower-resolution spectra of this very high metallicity cluster analyzed in that work. In fact, we find no evidence for an intrinsic spread in any element, corroborating recent independent APOGEE data. The derived abundances indicate that NGC 6791 very likely formed in the Galactic bulge and that the proposed association with the thick disk is unlikely, despite its present Galactic location. We confirm the most recent hypothesis suggesting that the cluster could have formed in the bulge and radially migrated to its current location, which appears to be the best explanation for this intriguing object.