Hubble Space Telescope Astrometry in the Orion Nebula Cluster: Census of Low-mass Runaways

Abstract

We present a catalog of high-precision proper motions in the Orion Nebula Cluster (ONC), based on Treasury Program observations with the Hubble Space Telescope's (HST) ACS/WFC camera. Our catalog contains 2454 objects in the magnitude range of 14.2 < m_(F775W) < 24.7, thus probing the stellar masses of the ONC from ~0.4 M☉ down to ~0.02 M☉ over an area of ~550 arcmin². We provide a number of internal velocity dispersion estimates for the ONC that indicate a weak dependence on stellar location and mass. There is good agreement with the published velocity dispersion estimates, although nearly all of them (including ours at σ_(v,x) = 0.94 and σ_(v,y) = 1.25 mas yr⁻¹) might be biased by the overlapping young stellar populations of Orion A. We identified four new ONC candidate runaways based on HST and the Gaia DR 2 data, all with masses less than ~1 M☉. The total census of known candidate runaway sources is 10—one of the largest samples ever found in any Milky Way open star cluster. Surprisingly, none of them have tangential velocities exceeding 20 km s⁻¹. If most of them indeed originated in the ONC, it may compel the re-examination of dynamical processes in very young star clusters. It appears that the mass function of the ONC is not significantly affected by the lost runaways.