The enigmatic globular cluster UKS 1 obscured by the bulge: 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 ± 0.11, 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 ± 12.9 km s−1 , which is in good agreement with literature values, within 1σ. By selecting stars in the VIRAC catalogue towards UKS 1, we also measure a mean proper motion of (µα cos(δ), µδ) = (−2.77 ± 0.23,−2.43 ± 0.16) 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]& +0.95) accompanied by lower carbon abundances ([C/Fe]. −0.2). Overall, the light- (C, N), α- (O, Mg, Si, Ca, Ti), Fe-peak (Fe, Ni), Odd-Z (Al, K), and the 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 of 13.10+0.93 −1.29 Gyr, suggesting that UKS 1 is a fossil relic in the Galactic bulge.Fil: Fernández Trincado, José G.. Universidad de Atacama.; Chile. Universite de Bourgogne; Francia. Centre National de la Recherche Scientifique; FranciaFil: Minniti, Dante. Universidad Andrés Bello; Chile. Vatican Observatory; ItaliaFil: Beers, Timothy C.. University of Notre Dame; Estados UnidosFil: Sandro, Villanova. Universidad de Concepción; ChileFil: Geisler, Doug. Universidad de Concepción; Chile. Universidad de La Serena; ChileFil: Souza, Stefano O.. Universidade de Sao Paulo; BrasilFil: Smith, Leigh C.. University of Cambridge; Reino UnidoFil: Placco, Vinicius M.. National Optical-Infrared Astronomy Research Laboratory; Estados UnidosFil: Vieira, Katherine. Universidad de Atacama; ChileFil: Pérez Villegas, Angeles. Universidade de Sao Paulo; BrasilFil: Barbuy, Beatriz. Universidade de Sao Paulo; BrasilFil: Alves Brito, Alan. Universidade Federal do Rio Grande do Sul; BrasilFil: Bidin, Christian Moni. Universidad Católica del Norte; ChileFil: Alonso García, Javier. Universidad de Antofagasta; Chile. Millennium Institute of Astrophysics; ChileFil: Tang, Baitian. Sun Yat-sen University; ChinaFil: Palma, Tali. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Astronomía Teórica y Experimental. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba. Instituto de Astronomía Teórica y Experimental; Argentin