UVES analysis of red giants in the bulge globular cluster NGC 6522^★

¹ Universidade de São Paulo, IAG, Rua do Matão 1226, Cidade Universitária, São Paulo 05508-900, Brazil
e-mail: barbuy@astro.iag.usp.br
² Astrophysikalisches Institut Potsdam, An der Sternwarte 16, Potsdam, 14482, Germany
³ Astrophysics Group, Keele University, Keele, Staffordshire ST5 5BG, UK
⁴ Kavli Institute for the Physics and Mathematice of the Universe (WPI), University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa 277-8583, Japan
⁵ INAF, Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, 34143 Trieste, Italy
⁶ IFPU, Istitute for the Fundamental Physics of the Universe, Via Beirut, 2, 34151, Grignano, Trieste, Italy
⁷ E.A. Milne Centre for Astrophysics, Department of Physics & Mathematics, University of Hull, Hull HU6 7RX, UK
⁸ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, Konkoly Thege Miklos ut 15-17, 1121 Budapest, Hungary
⁹ Joint Institute for Nuclear Astrophysics – Center for the Evolution of the Elements, USA
¹⁰ Università di Padova, Dipartimento di Astronomia, Vicolo dell’Osservatorio 2, 35122 Padova, Italy
¹¹ INAF-Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹² Universidade Estadual de Santa Cruz, Depto. de Ciências Exatas e Tecnológicas, Rodovia Jorge Amado km 16, 45662-900, Ilhéus, Brazil
¹³ Universidade Federal do Rio de Janeiro, Instituto de Física, Av. Athos da Silveira Ramos, 149, 21941-972, Rio de Janeiro, Brazil
¹⁴ Universidade Federal do Rio Grande do Sul, Departamento de Astronomia, CP 15051, Porto Alegre 91501-970, Brazil
¹⁵ South African Astronomical Observatory (SAAO), PO Box 9, Observatory 7935, South Africa

Received: 15 March 2021
Accepted: 3 July 2021

Abstract

Context. NGC 6522 is a moderately metal-poor bulge globular cluster ([Fe/H] ~ −1.0), and it is a well-studied representative among a number of moderately metal-poor blue horizontal branch clusters located in the bulge. The NGC 6522 abundance pattern can give hints on the earliest chemical enrichment in the central Galaxy.

Aims. The aim of this study is to derive abundances of the light elements C and N; alpha elements O, Mg, Si, Ca, and Ti; odd-Z elements Na and Al; neutron-capture elements Y, Zr, Ba, La, and Nd; and the r-process element Eu. We verify if there are first- and second-generation stars: we find clear evidence of Na–Al, Na–N, and Mg–Al correlations, while we cannot identify the Na–O anti-correlation from our data.

Methods. High-resolution spectra of six red giants in the bulge globular cluster NGC 6522 were obtained at the 8m VLT UT2-Kueyen telescope with both the UVES and GIRAFFE spectrographs in FLAMES+UVES configuration. In light of Gaia data, it turned out that two of them are non-members, but these were also analysed. Spectroscopic parameters were derived through the excitation and ionisation equilibrium of Fe I and Fe II lines from UVES spectra. The abundances were obtained with spectrum synthesis. Comparisons of abundances derived from UVES and GIRAFFE spectra were carried out.

Results. The present analysis combined with previous UVES results gives a mean radial velocity of v_r^hel = −15.62±7.7 km s⁻¹ and a metallicity of [Fe/H] = −1.05 ± 0.20 for NGC 6522. Mean abundances of alpha elements for the present four member stars are enhanced with [O/Fe] = +0.38, [Mg/Fe] = ≈+0.28, [Si/Fe] ≈ +0.19, and [Ca/Fe] ≈ +0.13, together with the iron-peak element [Ti/Fe] ≈ +0.13, and the r-process element [Eu/Fe] = +0.40. The neutron-capture elements Y, Zr, Ba, and La show enhancements in the +0.08 < [Y/Fe] < +0.90, 0.11 < [Zr/Fe] < +0.50, 0.00 < [Ba/Fe] < +0.63, 0.00 < [La/Fe] < +0.45, and −0.10 < [Nd/Fe] < +0.70 ranges. We also discuss the spread in heavy-element abundances.