CAPOS: The bulge Cluster APOgee Survey

XI. Unraveling the chemical composition of the bulge globular cluster NGC 6304

¹ Departamento de Astronomía, Facultad de Ciencias, Universidad de La Serena, Raúl Bitrán 1305, La Serena, Chile
² Departamento de Astronomía, Casilla 160-C, Universidad de Concepción, Concepción, Chile
³ Universidad Andres Bello, Facultad de Ciencias Exactas, Departamento de Física y Astronomía – Instituto de Astrofísica, Autopista Concepción-Talcahuano 7100, Talcahuano, Chile

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 26 September 2025
Accepted: 14 November 2025

Abstract

Context. Bulge globular clusters are key to understanding the formation and chemical evolution of the ancient central component of our Galaxy. Thanks to CAPOS, the bulge Cluster APOgee Survey, we can mitigate the observational difficulties limiting access to these objects in the optical and investigate them in more detail in the near-IR.

Aims. Our goal is to perform a rigorous abundance analysis on a large number of member stars in the metal-rich bulge globular cluster NGC 6304, using high-resolution spectra, in order to determine its detailed chemical composition and study the relationship of this globular cluster with its halo counterparts. In addition, we investigate chemical patterns that allow us to identify multiple populations.

Methods. This analysis is based on spectroscopic data obtained by CAPOS, which uses the APOGEE-2S spectrograph to observe in the H band. The abundances of 17 elements (C, N, O, Na, Mg, Al, Si, S, K, Ca, Ti, V, Cr, Mn, Fe, Ni, and Ce) were derived using the BACCHUS code, using atmospheric parameters from both APOGEE’s ASPCAP pipeline and those derived independently from photometry (Gaia and 2MASS) to cross validate our results with the ASPCAP values and provide a consistent chemical analysis.

Results. We derived a mean iron abundance of [Fe/H] = −0.45 ± 0.05 using the ASPCAP stellar parameters, and [Fe/H] = −0.45 ± 0.08 when using photometric stellar parameters, with no evidence of an intrinsic metallicity spread. NGC 6304 shows a typical enhancement in α elements, with [α/Fe] spec = +0.24 ± 0.07 and [α/Fe] phot = +0.23 ± 0.08, similar to what is observed in other globular clusters. We find a significant spread in [N/Fe], with σ_spec = 0.54 and σ_phot = 0.46, along with a clear C–N anticorrelation. Furthermore, we detect a correlation of Ce with both N and Al, consistent with patterns observed in some metal-rich bulge globular clusters but not all.

Conclusions. Our study provides the first comprehensive spectroscopic evidence for multiple populations in NGC 6304. We also find a significant star-to-star variation in Na, but a minimal variation in O, in concordance with trends found in other metal-rich bulge clusters. The absence of the Mg–Al anticorrelation supports the evidence that the MgAl cycle is not active in globular clusters at high metallicity. The observed correlation between Ce and both N and Al suggests that the enrichment of these elements may be driven by asymptotic giant branch stars, positioning Ce as an element involved in the multiple population phenomenon in metal-rich globular clusters. We find generally that abundances are consistent with those of bulge field stars of similar metallicity, suggesting a similar origin and chemical evolution.