Stellar Proper Motions in the Orion Nebula Cluster

Abstract: The Orion Nebula Cluster (ONC) is the nearest site of ongoing massive star formation, which allows us to study the kinematics and dynamics of the region in detail and constrain star formation theories. Using HST ACS/WFPC2/WFC3IR and Keck II NIRC2 data, we have measured the proper motions of 701 stars within an $\sim6'\times6'$ field of view around the center of the ONC. We have found more than 10 escaping star candidates, concentrated predominantly at the core of the cluster. The proper motions of the bound stars are consistent with a normal distribution, albeit elongated North-South along the Orion filament, with proper motion dispersions of $(\sigma_{\mu,\alpha^*}, \sigma_{\mu,\delta}) = (0.83\pm0.02,\,1.12\pm0.03)$ mas yr$^{-1}$ or intrinsic velocity dispersions of $(\sigma_{v,\alpha^*}, \sigma_{v,\delta}) = (1.57\pm0.04,\,2.12\pm0.06)$ km s$^{-1}$ assuming a distance of 400 pc to the ONC. The cluster shows no evidence for tangential-to-radial anisotropy. Our velocity dispersion profile agrees with the prediction from the observed stellar + gas density profile from Da Rio et al. (2014), indicating that the ONC is in virial equilibrium. This finding suggests that the cluster was formed with a low star formation efficiency per dynamical timescale based on comparisons with current star formation theories. Our survey also recovered high-velocity IR sources BN, x, and n in the BN/KL region. The estimated location of the first two sources $\sim500$ years ago agrees with that of the radio source I, consistent with their proposed common origin from a multi-stellar disintegration. However, source n appears to have a small proper motion and is unlikely to have been involved in the event.