Proper motions and kinematics of selected bulge globular clusters

TL;DR

This paper measures the proper motions of selected bulge globular clusters over 25 years, providing insights into their kinematics and confirming their association with the bulge bar structure.

Contribution

It introduces a method to derive proper motions and 3D velocities of bulge globular clusters using long-term CCD imaging data.

Findings

Inner bulge clusters have lower transverse velocities than halo clusters.

Most studied clusters are trapped in the bulge bar.

First-time proper-motion-cleaned CMDs for several clusters.

Abstract

We computed proper motions of a selected sample of globular clusters projected on the central bulge, employing CCD images gathered along the last 25 years at the ESO-NTT, ESO-Danish and HST telescopes. We presented a method to derive their proper motions, and a set of coordinate transformations to obtain 3D Galactic velocity vectors of the clusters. We analysed 10 globular clusters, namely Terzan 1, Terzan 2, Terzan 4, Terzan 9, NGC 6522, NGC 6558, NGC 6540, AL~3,ESO456--SC38 and Palomar 6. For comparison purposes we also studied the outer bulge cluster NGC 6652. We discuss the general properties of the proper-motion-cleaned Colour-Magnitude Diagrams, derived for the first time for most of them. A general conclusion is that the inner bulge globular clusters have clearly lower transverse motions (and spatial velocities) than halo clusters, and appear to be trapped in the bulge bar.