The Hubble Missing Globular Cluster Survey

II. Survey membership tools and kinematic analysis of NGC 6749

¹ INAF – Astrophysics and Space Science Observatory of Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
² Department of Physics and Astronomy, University of Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
³ INAF, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, Padova 35122, Italy
⁴ School of Mathematics and Physics, University of Queensland, Brisbane, Qld 4072, Australia
⁵ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁶ atlanTTic, Universidade de Vigo, Escola de Enxeñaría de Telecomunicación, 36310 Vigo, Spain
⁷ Universidad de La Laguna, Avda. Astrofísico Fco. Sánchez, 38205 La Laguna, Tenerife, Spain
⁸ INAF – Osservatorio Astronomico di Abruzzo, Via M. Maggini, 64100 Teramo, Italy
⁹ INFN – Sezione di Pisa, Università di Pisa, Largo Pontecorvo 3, 56127 Pisa, Italy
¹⁰ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone, Roma, Italy
¹¹ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
¹² Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
¹³ Institute for Computational Cosmology & Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 1 December 2025
Accepted: 27 January 2026

Abstract

The Hubble Missing Globular Cluster Survey has secured high-quality astro-photometric data in two bands for 34 clusters never observed with HST. When combined with Gaia astrometry, this dataset enables the investigation of the bulk motion and the internal kinematics of these poorly studied clusters to an unprecedented level of detail. Focusing on the case of NGC 6749, we show that the quality of the combined Gaia-HST proper motions is sufficient to accurately assess the cluster stellar membership, determine its absolute proper motion with a precision superior to Gaia, and to investigate its kinematic profile for the first time. Proper motions were determined using the public code GAIAHUB, which for NGC 6749 combined datasets separated in time by ~8 years.The resulting measurements improve the precision of Gaia proper motions by a factor of 10 at the faint end and enabled us to recover the proper motion for 662 stars for which Gaia was only able to measure the positions. These proper motions efficiently decontaminate the colormagnitude diagram of NGC 6749 and made it possible to compare the efficacy of a method of statistical decontamination that only relies on the photometric information extracted from the HST parallel fields. Finally, using the sample of best-measured proper motions, we determined the velocity dispersion and anisotropy profiles of NGC 6749, which reveal an isotropic behavior in the inner cluster regions and a slight radial anisotropy outside 1.5 half-light radii. The proper motions and the code for statistically decontaminating the cluster’s color-magnitude diagram are made available as public products of the survey.