2026A&A...707A.258R

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.