The Hubble Missing Globular Cluster Survey. II. Survey membership tools and kinematic analysis of NGC 6749

TL;DR

This paper presents advanced tools and analysis for studying poorly known globular clusters, demonstrating improved proper motion measurements and kinematic profiles for NGC 6749 using combined HST and Gaia data.

Contribution

It introduces a new method combining HST and Gaia data for precise cluster membership and kinematic analysis, with publicly available tools and data.

Findings

Proper motions improved by a factor of 10 at faint magnitudes.

Successfully determined the velocity dispersion and anisotropy profiles of NGC 6749.

Decontaminated the cluster's color-magnitude diagram effectively.

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 positional measurements, this data set 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 here showcase how the combined Gaia HST proper motions have a quality 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 are determined using the public code GAIAHUB, which for NGC 6749 combines data sets 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 enable recovering the proper motion for 662 stars for which Gaia could only measure the positions. These proper motions are efficient in decontaminating the colour magnitude diagram of NGC 6749, and make it possible to compare the efficacy of a method of statistical decontamination that relies only on the photometric information extracted from the HST parallel fields. Finally, using the sample of best measured proper motions we determine the velocity dispersion and anisotropy profiles of NGC 6749, that reveal an isotropic behaviour in the cluster inner regions and a slight radial anisotropy outside 1.5 half light radii. The proper motions and the code to statistically decontaminate the clusters color magnitude diagram are made available as public products of the survey.