Supergiant GCIRS 22 in the Milky Way nuclear star cluster: Revised alpha abundances

¹ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, 06000 Nice, France
² Division of Astrophysics, Department of Physics, Lund University, Box 118, S22100 Lund, Sweden
³ Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
⁴ MAUCA – Master track in Astrophysics, Université Côte d’Azur, Observatoire de la Côte d’Azur, Parc Valrose, 06100 Nice, France
⁵ Department of Physics and Astronomy, UCLA, 430 Portola Plaza, Box 951547, Los Angeles, CA 90095-1547, USA
⁶ Department of Physics, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan
⁷ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁸ Theoretical Astrophysics, Department of Physics and Astronomy, Uppsala University, 751 20 Uppsala, Sweden
⁹ Aryabhatta Research Institute of Observational Sciences, Manora Peak, Nainital 263002, India
¹⁰ Miyagi University of Education, 149 Aramaki-Aza-Aoba, Aoba-ku, Sendai, Miyagi 980-0845, Japan

^★ Corresponding author: brian.thorsbro@oca.eu

Received: 27 July 2025
Accepted: 21 September 2025

Abstract

Context. The chemical abundances of alpha elements in Galactic Centre (GC) supergiants provide key insights into the chemical enrichment and star formation history of the Milky Way’s nuclear star cluster. Previous studies have reported enhanced alpha-element abundances, which raises questions about the chemical evolution of this unique region.

Aims. We aim to reassess the alpha-element abundances in the GC supergiant GCIRS 22 using updated spectral modelling and non-local thermodynamic equilibrium (NLTE) corrections to resolve discrepancies from earlier abundance analyses.

Methods. We analysed high-resolution near-infrared spectra of GCIRS 22 using contemporary line lists and precise stellar parameters derived from scandium line diagnostics. We applied comprehensive NLTE corrections to accurately determine the abundances of silicon and calcium.

Results. Our analysis reveals solar-scale alpha abundances ([Ca/Fe] = 0.06 ± 0.07; [Si/Fe] = than −0.08 ± 0.20) for GCIRS 22, which are significantly lower than previous local thermodynamic equilibrium (LTE) based findings. NLTE corrections reduce the calcium abundance by approximately 0.3 dex compared to LTE estimates; this aligns our results with recent studies and highlights the importance of accurate NLTE modelling.

Conclusions. The solar-scale alpha-element abundances observed in GCIRS 22 suggest that recent star formation in the region has not been dominated by Type II supernovae, such as those expected from a recent starburst. Our findings support a scenario of episodic star formation, characterized by intermittent bursts separated by extended quiescent phases, or potentially driven by gas inflows from the inner disc, funnelled by the Galactic bar. Future comprehensive NLTE studies of additional GC stars will be essential for refining our understanding of the region’s chemical evolution and star formation history.