A Deep View of a Fossil Relic in the Galactic Bulge: The Globular Cluster HP1

Abstract

HP1 is an $\alpha$-enhanced and moderately metal-poor bulge globular cluster with a blue horizontal branch. These combined characteristics make it a probable relic of the early star formation in the innermost Galactic regions. Here we present a detailed analysis of a deep near-infrared (NIR) photometry of HP1 obtained with the NIR GSAOI+GeMS camera at the Gemini-South telescope. $J$ and $K_{\mathrm{S}}$ images were collected with an exquisite spatial resolution (FWHM $\sim 0.1$ arcsec), reaching stars at two magnitudes below the MSTO. We combine our GSAOI data with archival F606W-filter $HST$ ACS/WFC images to compute relative proper motions and select bona fide cluster members. Results from statistical isochrone fits in the NIR and optical-NIR colour-magnitude diagrams indicate an age of $12.8_{-0.8}^{+0.9}$ Gyr, confirming that HP1 is one of the oldest clusters in the Milky Way. The same fits also provide apparent distance moduli in the $K_{\mathrm{S}}$ and $V$ filters in very good agreement with the ones from 11 RR Lyrae stars. By subtracting the extinction in each filter, we recover a heliocentric distance of $6.59_{-0.15}^{+0.17}$ kpc. Furthermore, we refine the orbit of HP1 using this accurate distance and update and accurate radial velocities (from high resolution spectroscopy) and absolute proper motions (from Gaia DR2), reaching mean perigalactic and apogalactic distances of $\sim$0.12 and $\sim$3 kpc respectively.

22 pages, 22 figures, accepted for publication in MNRAS

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