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arXiv · 1901.03721

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

L. O. Kerber·M. Libralato·S. O. Souza·R. A. P. Oliveira·S. Ortolani·A. Pérez-Villegas·B. Barbuy·B. Dias·E. Bica·D. Nardiello
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Abstract

HP$\,$1 is an $α$-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.

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L. O. Kerber, M. Libralato, S. O. Souza, R. A. P. Oliveira, S. Ortolani, A. Pérez-Villegas, B. Barbuy, B. Dias, E. Bica, D. Nardiello. 2019-01-11. A Deep View of a Fossil Relic in the Galactic Bulge: The Globular Cluster HP$\,$1. https://doi.org/10.1093/mnras%2Fstz003

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