The enigmatic globular cluster UKS~1 obscured by the bulge:
  \textit{H}-band discovery of nitrogen-enhanced stars
  The presence of nitrogen-enriched stars in globular clusters provides key
evidence for multiple stellar populations (MPs), as has been demonstrated with
globular cluster spectroscopic data towards the bulge, disk, and halo. In this
work, we employ the VVV Infrared Astrometric Catalogue (VIRAC) and the DR16
SDSS-IV release of the APOGEE survey to provide the first detailed
spectroscopic study of the bulge globular cluster UKS~1. Based on these data, a
sample of six selected cluster members was studied. We find the mean
metallicity of UKS~1 to be [Fe/H]$=-0.98\pm0.11$, considerably more metal-poor
than previously reported, and a negligible metallicity scatter, typical of that
observed by APOGEE in other Galactic globular clusters. In addition, we find a
mean radial velocity of $66.1\pm12.9$ km s$^{-1}$, which is in good agreement
with literature values, within 1$\sigma$. By selecting stars in the VIRAC
catalogue towards UKS~1, we also measure a mean proper motion of
($\mu_{\alpha}\cos(\delta)$, $\mu_{\delta}$) $=$
($-2.77\pm0.23$,$-2.43\pm0.16$) mas yr$^{-1}$. We find strong evidence for the
presence of MPs in UKS~1, since four out of the six giants analysed in this
work have strong enrichment in nitrogen ([N/Fe]$\gtrsim+0.95$) accompanied by
lower carbon abundances ([C/Fe]$\lesssim-0.2$). Overall, the light- (C, N),
$\alpha$- (O, Mg, Si, Ca, Ti), Fe-peak (Fe, Ni), Odd-Z (Al, K), and the
\textit{s}-process (Ce, Nd, Yb) elemental abundances of our member candidates
are consistent with those observed in globular clusters at similar metallicity.
Furthermore, the overall star-to-star abundance scatter of elements exhibiting
the multiple-population phenomenon in UKS~1 is typical of that found in other
global clusters (GCs), and larger than the typical errors of some [X/Fe]
abundances. Results from statistical isochrone fits in the VVV colour-magnitude
diagrams indicate an age ...
