High-resolution abundance analysis of red giants in the metal-poor bulge globular cluster HP 1

Context. The globular cluster HP 1 is projected at only 3°.33 from the Galactic center. Together with its distance, this makes it one of the most central globular clusters in the Milky Way. It has a blue horizontal branch (BHB) and a metallicity of [Fe/H] ≈ -1.0. This means that it probably is one of the oldest objects in the Galaxy. Abundance ratios can reveal the nucleosynthesis pattern of the first stars as well as the early chemical enrichment and early formation of stellar populations. Aims. High-resolution spectra obtained for six stars were analyzed to derive the abundances of the light elements C, N, O, Na, and Al, the alpha-elements Mg, Si, Ca, and Ti, and the heavy elements Sr, Y, Zr, Ba, La, and Eu. Methods. High-resolution spectra of six red giants that are confirmed members of the bulge globular cluster HP 1 were obtained with the 8 m VLT UT2-Kueyen telescope with the UVES spectrograph in FLAMES-UVES configuration. The spectroscopic parameter derivation was based on the excitation and ionization equilibrium of Fe i and Fe ii. Results. We confirm a mean metallicity of [Fe/H] = -1.06 ± 0.10, by adding the two stars that were previously analyzed in HP 1. The alpha-elements O and Mg are enhanced by about +0.3 ≲ [O,Mg/Fe] ≲ +0.5 dex, Si is moderately enhanced with +0.15 ≲ [Si/Fe] ≲ +0.35 dex, while Ca and Ti show lower values of -0.04 ≲ [Ca,Ti/Fe] ≲ +0.28 dex. The r-element Eu is also enhanced with [Eu/Fe] ≈ +0.4, which together with O and Mg is indicative of early enrichment by type II supernovae. Na and Al are low, but it is unclear if Na-O are anticorrelated. The heavy elements are moderately enhanced, with -0.20 < [La/Fe] < +0.43 dex and 0.0 < [Ba/Fe] < +0.75 dex, which is compatible with r-process formation. The spread in Y, Zr, Ba, and La abundances, on the other hand, appears to be compatible with the spinstar scenario or other additional mechanisms such as the weak r-process.