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. (2012) found evidence suggesting 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 both 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. (2012), where a Na variation was claimed. This points to the
presence of probable systematics in the lower resolution spectra of this very
high metallicity cluster analysed 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 the best explanation
for this intriguing object.
