Supergiant GCIRS 22 in the Milky Way Nuclear Star Cluster: Revised alpha abundances

TL;DR

This study revisits the alpha-element abundances of the Galactic Centre red supergiant GCIRS 22 using updated NLTE spectral modelling and modern line data. High-resolution near-infrared spectra from Keck/NIRSPEC are analyzed with departure-coefficient NLTE corrections and SME-based synthesis on MARCS atmospheres, yielding near-solar [Ca/Fe] and [Si/Fe] values: $[Ca/Fe] = 0.06 \pm 0.07$ and $[Si/Fe] = -0.08 \pm 0.20$, with NLTE reducing Ca by ~0.3 dex compared to LTE. The results, validated by Bayesian isochrone fitting and an independent grid-based approach, indicate GCIRS 22 is a young, $\sim 100$ Myr star with $[Fe/H] \approx -0.13$, suggesting GC star formation in a chemically matured ISM and challenging previous claims of strong $\alpha$-enhancement in the NSC. These findings imply a more nuanced chemical evolution for the GC, compatible with episodic star formation or bar-driven gas inflows, and motivate broader NLTE analyses across GC stellar populations to refine the NSC's formation history.

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, raising 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: High-resolution near-infrared spectra of GCIRS 22 were analysed 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 $\pm$ 0.07; [Si/Fe] = $-$0.08 $\pm$ 0.20) for GCIRS 22, significantly lower than previous LTE-based findings. NLTE corrections reduce the calcium abundance by approximately 0.3,dex compared to LTE estimates, aligning our results with recent studies and highlighting 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 disk, 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.

Figures (4)

• Figure 1: Four fine structure energy level transitions in neutral scandium 3d$^2$4s – 3d4s4p. In the top panel is the observed spectra of GCIRS 22. In the bottom panel: Model spectra showing the sensitivity of the scandium lines to varying the carbon and nitrogen abundances, showing that the scandium lines are not affected by blending of CN molecular line features. The sensitivity analysis was based on a model using an effective temperature of 3600 K and solar abundances. We have included a model using an effective temperature of 4500 K to illustrate the temperature sensitivity of the scandium lines as the lines almost vanishes at this temperature due to ionisation.
• Figure 2: Metallicity determination from individual Fe I Lines. The mean [Fe/H] is found to be $-0.08 \pm 0.17$
• Figure 3: [Si/Fe], [Ca/Fe] and [Mg/Fe] abundance ratios from individual lines. The average abundance of alpha elements was found to be $-0.03 \pm 0.07$. Excluding the magnesium, which can be argued for due to the high scatter, yields and average of $-0.01 \pm 0.09$.
• Figure 4: Left panel silicon over iron abundance ratio vs iron abundance, plotting the value obtained for GCIRS 22 and comparing to literature results for the Milky Way Nuclear Star Cluster thorsbro:2020ryde:2025. On the right panel similarly for calcium, however, the gray dots are the results from cunha:2007 with an arrow indicated the changed value for GCIRS 22. The primary reason for the new value of calcium abundance for GCIRS 22 is due to accounting for NLTE effects, and thus likely to be similar reduction of calcium abundance in the remaining set from cunha:2007. The gray line is an "alpha-knee" model for a simplified Milky Way Bulge chemical evolution scenario over-plotted for convenience matteucci:12.