The enigmatic globular cluster UKS 1 obscured by the bulge: H-band discovery of nitrogen-enhanced stars

¹ Instituto de Astronomía y Ciencias Planetarias, Universidad de Atacama, Copayapu 485, Copiapó, Chile
e-mail: jose.fernandez@uda.cl
² Institut Utinam, CNRS-UMR 6213, Université Bourgogne-Franche-Comté, OSU THETA Franche-Comté, Observatoire de Besançon, BP 1615, 251010 Besançon Cedex, France
³ Depto. de Cs. Físicas, Facultad de Ciencias Exactas, Universidad Andrés Bello, Av. Fernández Concha 700, Las Condes, Santiago, Chile
⁴ Vatican Observatory, 00120 Vatican City State, Italy
⁵ Department of Physics and JINA Center for the Evolution of the Elements, University of Notre Dame, Notre Dame, IN 46556, USA
⁶ Departamento de Astronomía, Casilla 160-C, Universidad de Concepción, Concepción, Chile
⁷ Departamento de Astronomía, Universidad de La Serena, Avenida Juan Cisternas 1200, La Serena, Chile
⁸ Instituto de Investigación Multidisciplinario en Ciencia y Tecnología, Universidad de La Serena, Benavente 980, La Serena, Chile
⁹ Universidade de São Paulo, IAG, Rua do Matão 1226, Cidade Universitária, São Paulo 05508-900, Brazil
¹⁰ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹¹ NSF’s Optical-Infrared Astronomy Research Laboratory, Tucson, AZ 85719, USA
¹² Universidade Federal do Rio Grande do Sul, Instituto de Física, Av. Bento Gonçalves 9500, Porto Alegre, RS, Brazil
¹³ Instituto de Astronomía, Universidad Católica del Norte, Av. Angamos 0610, Antofagasta, Chile
¹⁴ Centro de Astronomía (CITEVA), Universidad de Antofagasta, Av. Angamos 601, Antofagasta, Chile
¹⁵ Millennium Institute of Astrophysics, Santiago, Chile
¹⁶ School of Physics and Astronomy, Sun Yat-sen University, Zhuhai 519082, PR China
¹⁷ Universidad Nacional de Córdoba, Observatorio Astronómico de Córdoba, Laprida 854, 5000 Córdoba, Argentina
¹⁸ Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Godoy Cruz, 2290 Ciudad Autónoma de Buenos Aires, Argentina

Received: 2 September 2020
Accepted: 22 September 2020

Abstract

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 (μ_αcos(δ), μ_δ) = (−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_−1.29^+0.93 Gyr, suggesting that UKS 1 is a fossil relic in the Galactic bulge.