Radial Velocities of Three Poorly Studied Clusters and the Kinematics of Open Clusters

TL;DR

This study measures radial velocities of three open clusters and analyzes the kinematics of 134 clusters, revealing age-related changes in rotational velocity and velocity dispersion indicative of dynamical heating.

Contribution

It provides the first radial velocity measurements for three intermediate-age clusters and analyzes the kinematic evolution of open clusters with age.

Findings

Older clusters rotate more slowly than younger ones.

Velocity dispersion increases with cluster age.

Kinematic heating affects open cluster dynamics over time.

Abstract

We present radial velocities for stars in the field of the open star clusters Berkeley 44, Berkeley 81, and NGC 6802 from spectra obtained using the Wisconsin-Indiana-Yale-NOAO (WIYN) 3.5 m telescope. These clusters are of intermediate age (1-3 Gyr), located within the solar Galactocentric radius, and have no previous radial velocity measurements. We find mean radial velocities of -9.6 +/- 3.0 km s^-1, 48.1 +/- 2.0 km s^-1, and 12.4 +/- 2.8 km s^-1 for Be 44, Be 81, and NGC 6802, respectively. We present an analysis of radial velocities of 134 open clusters of a wide range of ages using data obtained in this study and the literature. Assuming the system of clusters rotates about the Galactic center with a constant velocity, we find older clusters exhibit a slower rotation and larger line-of-sight (LOS) velocity dispersion than younger clusters. The gradual decrease in rotational velocity of the cluster system with age is accompanied by a smooth increase in LOS velocity dispersion, which we interpret as the effect of heating on the open cluster system over time.