Space Velocities of Southern Globular Clusters. V. A Low Galactic Latitude Sample

TL;DR

This study measures the proper motions of six southern globular clusters, revealing their orbital characteristics and associations, and provides new data for understanding their dynamics and origins.

Contribution

It presents the first proper motion measurements for these low Galactic latitude clusters using the Hipparcos system, and analyzes their orbits and associations.

Findings

NGC 5927 belongs to the thick disk.

Most clusters have high-eccentricity, low-energy orbits.

NGC 3201 has a highly retrograde, energetic orbit.

Abstract

We have measured the absolute proper motions of globular clusters NGC 2808, 3201, 4372, 4833, 5927 and 5986. The proper motions are on the Hipparcos system and they are the first determinations ever made for these low Galactic latitude clusters. The proper motion uncertainties range from 0.3 to 0.5 mas/yr. The inferred orbits indicate that 1) the single metal rich cluster in our sample, NGC 5927, dynamically belongs to the thick disk, 2) the remaining metal poor clusters have rather low-energy orbits of high eccentricity; among these, there appear to be two "pairs" of dynamically associated clusters, 3) the most energetic cluster in our sample, NGC 3201 is on a highly retrograde orbit -- which had already been surmised from its radial velocity alone -- with an apocentric distance of 22 kpc, and 4) none of the metal poor clusters appear to be associated with the recently detected SDSS streams, or with the Monoceros structure. These are the first results of the Southern Proper-Motion Program (SPM) where the second-epoch observations are taken with the CCD camera system recently installed on the double astrograph at El Leoncito, Argentina.