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

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.

Figures (10)

• Figure 1: Colored image of NGC 6749 using the new HST (ACS/WFC) observations. The white cross shows the center of the cluster baumgardt21, while the white circle has a radius equal to the half-light radius.
• Figure 2: Comparison between the Gaia and GH PMs. The top panels show the VPDs of the PMs while the two bottom panels show the PM errors along $\alpha\cos\delta$ and $\delta$, as a function of Gaia G magnitude. The black dots refer to the Gaia PMs, while blue and red dots show stars in our catalog with and without a corresponding PM in the Gaia catalog, respectively.
• Figure 3: VPD (left panel) and CMDs (middle and right panels) of NGC 6749 using the new HST data of the MGCS survey. The red circle with radius 1 $\mathrm{mas}\,\mathrm{yr}^{-1}$ in the VPD is used to select likely cluster stars (those included within the circle). In the central CMD, the gray dots represents all stars in our HST photometric catalog, while black and red dots are, respectively, foreground and cluster stars for which we have a PM measurements. The horizontal dashed line corresponds to the Gaia limiting magnitude. Finally, the right panel shows the location of cluster member stars in the CMD.
• Figure 4: Distribution of the NGC 6749 GH PMs. The central scatter plot shows the 2D VPD, where the points are color-coded by the membership probability obtained from the MCMC chains (color bar). The top and right histograms show the normalized marginal distributions of the PMs, together with the posterior distributions of the GMM components, the target in red, the field in blue and their sum in gray.
• Figure 5: Result of the statistical decontamination. ( top-left panel) Contaminated target CMD, ( top-right panel) parallel field CMD, ( bottom-left panel) target CMD color-coded for the membership probability, ( bottom-right panel) membership probability distribution. The target CMDs shown are corrected for the differential reddening.