Globular Clusters: Absolute Proper Motions and Galactic Orbits

Abstract

We cross-match objects from several different astronomical catalogs to determine the absolute proper proper motions of stars within the 30-arcmin radius fields of 115 Milky-Way globular clusters with the accuracy of 1-2 mas/yr. The proper motions are based on positional data recovered from the USNO-B1, 2MASS, URAT1, ALLWISE, UCAC5, and GAIA DR1 surveys with up to 10 positions spanning an epoch difference of up to ∼ 65 years, and reduced to GAIA DR1 TGAS frame using UCAC5 as the reference catalog. Cluster members are photometrically identified by selecting horizontal-and red-giant branch stars on color-magnitude diagrams, and the mean absolute proper motions of the clusters with a typical formal error of ∼ 0.4 mas/yr are computed by averaging the proper motions of selected members. The inferred absolute proper motions of clusters are combined with available radial-velocity data and heliocentric distance estimates to compute the cluster orbits in terms of the Galactic potential models based on Miyamoto and Nagai disk, Hernquist spheroid, and modified isothermal dark-matter halo (axisymmetric model without a bar) and the same model + rotating Ferre's bar (non-axisymmetric). Five distant clusters have higher-than-escape velocities, most likely due to large errors of computed transversal velocities, whereas the computed orbits of all other clusters remain bound to the Galaxy. Unlike previously published results, we find the bar to affect substantially the orbits of most of the clusters, even those at large Galactocentric distances, bringing appreciable chaotization, especially in the portions of the orbits close to the Galactic center, and stretching out the orbits of some of the thick-disk clusters.

Chemical abundances in the metal-intermediate GC NGC 6723

“…Located at R = 8.7 kpc, it is an old (≈ 12.50 Gyr, Dotter et al 2010) GC that lies at a region of low foreground interstellar reddening, with E(B −V ) = 0.063 mag (Lee et al 2014, hereafter L14), making it an excellent candidate for the study of GCs in the densest regions of the Milky Way. It has been frequently assumed to belong to the Bulge due to its proximity to it at about R GC ≈ 2.6 kpc, however its dynamics point out to a inner Halo membership since the work of Dinescu et al (2003), a conclusion now confirmed by brand new proper motion estimates from Gaia 1 (Vasiliev 2018;Chemel et al 2018).…”

Formation Imprints in the Kinematics of the Milky Way Globular Cluster System

“…More recently, Pérez-Villegas et al (2018) analyzed 9 bulge GCs and concluded that they move on rather eccentric prograde or retrograde orbits that are strongly influenced by the Galactic bar. A chaotization of the cluster orbits due to the MW bar was also found by Chemel et al (2018) from the analysis of the motions of 115 GCs in a non-axisymmetric MW potential with a bar.…”

Globular Clusters of the Galaxy: Chemical Composition vs Kinematics

“…Differences in the abundances of α-elements in the stellar objects of the Galaxy and the surrounding low-mass dwarf satellite galaxies confirm the wellknown conclusion that all globular clusters and field stars of the accreted halo are remnants of galaxies of higher mass than the current environment of the Galaxy. A possible exception is a distant low-metal cluster with low relative abundance of α-elements Rup 106. stars and dwarf satellite galaxies, which we use for comparison.For all 157 globular clusters, we calculated the rectangular coordinates from positions and distances from [7], and for 115 of them-from [8]. We supplemented the data for the last 115 clusters with cylindrical velocity components, taking them from the authors of [8], and for the clusters Ter 4, Pal 3, Pal 5, Pal 13, NGC6528 and NGC7006 calculated them from their proper motions, radial velocities and distances from [7].…”

“…A possible exception is a distant low-metal cluster with low relative abundance of α-elements Rup 106. stars and dwarf satellite galaxies, which we use for comparison.For all 157 globular clusters, we calculated the rectangular coordinates from positions and distances from [7], and for 115 of them-from [8]. We supplemented the data for the last 115 clusters with cylindrical velocity components, taking them from the authors of [8], and for the clusters Ter 4, Pal 3, Pal 5, Pal 13, NGC6528 and NGC7006 calculated them from their proper motions, radial velocities and distances from [7]. As a result, the number of clusters with known velocities increased by more than two-thirds compared to our previous work MKG19 and totals 121 objects.…”

“…The average internal error declared by the authors for determining the components of spatial velocities is approximately 17 km s −1 . Elements of galactic orbits calculated from these velocities are also taken from [8]. We used the latter here solely to distinguish clusters that have orbital points further than 15 kpc from the galactic center (see below for more details).…”