CAPOS: The bulge Cluster APOgee Survey XI. Unraveling the chemical composition of the bulge globular cluster NGC 6304

TL;DR

This study conducts a detailed chemical abundance analysis of the bulge globular cluster NGC 6304 using near-IR spectra, revealing its metallicity, element variations, and potential enrichment processes, and compares its composition to bulge field stars.

Contribution

First detailed near-IR abundance analysis of NGC 6304, identifying multiple populations and chemical enrichment patterns in a metal-rich bulge globular cluster.

Findings

No intrinsic metallicity spread in NGC 6304.

Detection of N-C anticorrelation and Ce-N/Al correlations.

Minimal Mg-Al anticorrelation, indicating inactive MgAl cycle at high metallicity.

Abstract

Context. With CAPOS, we can mitigate the observational difficulties limiting access to bulge globular clusters in the optical and investigate them in more detail in the near-IR. Aims. To perform a rigorous abundance analysis of the metal-rich bulge globular cluster NGC 6304, in order to determine its detailed chemical composition and identify multiple populations. Methods. We analyzed APOGEE-2 near-IR spectra of 27 giant members. 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 ASPCAP and photometry (Gaia and 2MASS). Results. We derived $[{\rm Fe/H}] = -0.45\pm0.05$ using the ASPCAP parameters, and $[{\rm Fe/H}] = -0.45\pm0.08$ when using photometric parameters, with no evidence of an intrinsic metallicity spread. NGC 6304 shows $[\alpha/{\rm Fe}]_{\rm spec} = 0.24\pm0.07$ and $[\alpha/{\rm Fe}]_{\rm phot} = 0.23\pm0.08$. We find a significant spread in $[{\rm N/Fe}]$, with $\sigma_{\rm spec} = 0.54$ and $\sigma_{\rm phot} = 0.46$, along with a 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. Conclusions. We find a significant star-to-star variation in Na, but a minimal variation in O. 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 multiple populations in metal-rich globular clusters. We find that abundances are consistent with those of bulge field stars of similar metallicity, suggesting a similar origin and chemical evolution.