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

Hirschi

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 vrhel = -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.

Citation

Hirschi. (2021). UVES analysis of red giants in the bulge globular cluster NGC 6522. Astronomy & Astrophysics, https://doi.org/10.1051/0004-6361/202140815

• Acceptance Date: Jul 3, 2021
• Publication Date: Oct 5, 2021
• Journal: Astronomy & Astrophysics
• Print ISSN: 0004-6361
• Publisher: EDP Sciences
• DOI: https://doi.org/10.1051/0004-6361/202140815
• Keywords: abundances, globular clusters, NGC 6522, bulge
• Publisher URL: https://www.aanda.org/articles/aa/abs/2021/10/aa40815-21/aa40815-21.html