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

Research output: Contribution to journalArticlepeer-review

26 Citations (Scopus)


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 S images were collected with an exquisite spatial resolution (FWHM 1/40.1 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 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 1/40.12 and 1/43 kpc, respectively.

Original languageEnglish
Pages (from-to)5530-5550
Number of pages21
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
Publication statusPublished - 21 Apr 2019
Externally publishedYes


  • Galaxy: bulge
  • globular clusters: individual: HP 1
  • infrared: stars
  • instrumentation: adaptive optics

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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