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
Monthly Notices of the Royal Astronomical Society, Volume 484, Issue 4, April 2019, Pages 5530–5550, https://doi.org/10.1093/mnras/stz003
Published: 05 January 2019
Article history
Received: 08 May 2018 Revision Received: 14 November 2018
Accepted: 10 December 2018
Source/Fonte: Cosmos
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 KS images were collected with an exquisite spatial resolution (FWHM ∼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 KS 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 ∼0.12 and ∼3 kpc, respectively.
instrumentation: adaptive optics, Galaxy: bulge, globular clusters: individual: HP 1, infrared: stars
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