CAPOS: The bulge Cluster APOgee Survey

Abstract

We performed a detailed abundance analysis of ten red giant members of the heavily obscured bulge globular cluster HP 1 using high-resolution, high S/N, near-infrared spectra collected with the Apache Point Observatory Galactic Evolution Experiment II survey (APOGEE-2), obtained as part of the bulge Cluster APOgee Survey (CAPOS). We investigated the chemical abundances for a variety of species, including the light (C, N), odd-Z (Al), α (O, Mg, Si, S, Ca, and Ti), Fe-peak (Ni, Fe), and neutron-capture (Ce) elements. The derived mean cluster metallicity is [Fe/H] = −1.15 ± 0.03, with no evidence of an intrinsic metallicity spread. HP 1 exhibits a typical α enrichment that follows the trend for similar metallicity Galactic globular clusters (GCs), such as NGC 288 and NGC 5904, although our [Si/Fe] abundances are relatively high. We find a significant nitrogen spread (~1 dex) and a large fraction of nitrogen-enhanced ([N/Fe] > +0.7) stars that populate the cluster. We also detect intrinsic star-to-star spreads in [C/Fe], [O/Fe], [Al/Fe], and [Ca/Fe], which are (anti)correlated with several chemical species, indicating the prevalence of the multiple-population phenomenon in HP 1. We uncover for the first time a possible correlation between Ca and Al, although the sample is small. The mean ⟨[Mg/Fe]⟩ = +0.29 and ⟨[Al/Fe]⟩ = +0.46 place HP 1 in the region dominated by in situ GCs, supporting the in situ nature of this cluster.