VLT-UVES analysis of two giants in the bulge metal-poor globular cluster HP-1

Abstract

Context. Metal-poor globular clusters in the bulge are important tracers of early chemical evolution. HP-1 is among the six metal-poor clusters within 5. of the Galactic center, and could be the one closest to the center Aims. The main purpose of this study is the determination of metallicity and elemental ratios. Methods. High resolution spectra of two giants of the bulge globular cluster HP-1 were obtained at the 8m VLT UT2-Kueyen telescope with the UVES spectrograph. This is the second metal-poor globular cluster in the bulge for which a detailed abundance analysis is presented. Multiband V, I, J, H, Ks photometry was used to derive effective temperatures. Results. The present analysis provides a metallicity [Fe/H] = -1.00 ± 0.2. The α-elements oxygen and silicon show [α/Fe] ≈ +0.3, whereas magnesium, calcium and titanium show solar ratios. A proper motion analysis indicates that the two stars are cluster members. Conclusions. The metallicity is unexpected for a blue Horizontal Branch (BHB) cluster. HP-1 is the first known cluster with such a high metallicity combined with a BHB and a steep Red Giant Branch (RGB). Together with NGC 6388 and NGC 6441 of [Fe/H] ∼ −0.6 it would be third with such characteristics, but it differs from them, since these two other clusters have also a populous Red HB, and a normal slope of the RGB for their metallicity, which is not the case of HP-1.