Space Velocities of Southern Globular Clusters VI. Nine Clusters in the Inner Milky Way

TL;DR

This study measures the proper motions of nine inner Galaxy globular clusters, providing new kinematic data that reveals their possible origins and dynamic states within the Milky Way.

Contribution

First proper-motion measurements for nine low-latitude, inner Galaxy globular clusters, enhancing understanding of their kinematics and origins.

Findings

Two metal-rich clusters likely share a common origin.

Most clusters are confined near the Galactic center, indicating their dynamic states.

Proper motions suggest some clusters are at apocenter, not escaping the inner Galaxy.

Abstract

(abridged) We have measured the absolute proper motions of nine low-latitude, inner Galaxy globular clusters, namely NGC 6273 (M 19), NGC 6284, NGC 6287, NGC 6293, NGC 6333 (M 9), NGC 6342, NGC 6356, NGC 6388 and NGC 6441. These are the first determinations ever made for these clusters. The proper motions are on the ICRS via Hipparcos. The proper-motion errors range between 0.4 and 0.9 mas/yr, and are dominated by the number of measurable cluster members in these regions which are very crowded by the bulge/bar and the thick disk. This samle contains five metal poor ([Fe/H < -1.0) and four metal rich clusters; seven clusters are located within 4 kpc from the Galactic center, while the remaining two, namely NGC 6356 and NGC 6284 are in the background of the bulge at 7.5 kpc from the Galactic center. By combining proper motions with radial velocities and distances from the literature we derive 3D velocities. In a number of cases, distance uncertainties make the kinematical classification ambiguous. The two metal rich clusters NGC 6388 and NGC 6441 have velocities incompatibile with membership in the thick disk or the bar of the Milky Way. They can be though of as members of a kinematically hot system in the inner Galaxy. Curiously, both clusters have similar velocity components. Together with their similar Galactic location and peculiar but similar stellar-population characteristics, these two clusters may share a common origin. Their velocities are also very low indicating that the two clusters are now at/near apocenter, and they will not leave the inner ~4 kpc of the Galaxy.