Dynamics and Multiplicity of Young Star Clusters: Getting the Most Out of Single Epoch Radial Velocity Data

Abstract

The velocity distribution of stellar systems in a cluster and its binary population provide powerful constraints on theories concerning the formation and subsequent evolution of these clusters. Unfortunately radial velocity variations due to binary orbits tend to mask the underlying velocity distribution for clusters with a small velocity dispersion. One possibility to solve this is to identify the spectroscopic binaries through multi-epoch observations. Here we present an alternative procedure. Even with only a single epoch of data, the radial velocity distribution due to the binary orbital motions is expected to be significantly different to the underlying velocity distribution, so it is possible to separate these distributions. This allows the underlying velocity distribution of a cluster, as well as the binary fraction, to be estimated from a single epoch of radial velocity data. We show that the measured velocity distribution depends only weakly on assumptions made about the binary properties. The procedure successfully reproduces the radial velocity dispersion of ∼ 0. 5 km/s in the open cluster NGC 188. A more complete discussion on the mechanics of this procedure, its accuracy, and the test run on NGC 188 can be found in Cottaar et al. (A&A 547:A35, 2012).