# A Deep View of a Fossil Relic in the Galactic Bulge: The Globular Cluster HP$\,$1

## TL;DR

This study provides a detailed analysis of the globular cluster HP1, confirming its status as one of the oldest in the Milky Way, through deep near-infrared and optical data, precise proper motions, and orbit calculations.

## Contribution

It offers a comprehensive multi-wavelength analysis of HP1, including age, distance, and orbit, utilizing advanced imaging and proper motion data to refine its properties and history.

## Key findings

- Age of 12.8 Gyr confirmed for HP1.
- Distance of approximately 6.6 kpc established.
- Refined orbit with perigalactic distance of 0.12 kpc.

## Abstract

HP$\,$1 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 HP$\,$1 obtained with the NIR GSAOI+GeMS camera at the Gemini-South telescope. $J$ and $K_{\rm S}$ 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 $12.8^{+0.9}_{-0.8}$ Gyr, confirming that HP$\,$1 is one of the oldest clusters in the Milky Way. The same fits also provide apparent distance moduli in the $K_{\rm S}$ 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 $6.59^{+0.17}_{-0.15}$ kpc. Furthermore, we refine the orbit of HP$\,$1 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.