HST 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 2,454 objects in the magnitude range of $14.2<m_{F775W}<24.7$, thus probing the stellar masses of the ONC from $\sim$0.4 $M_{\odot }$ down to $\sim$0.02 $M_{\odot }$ over an area of $\sim$550 arcmin${}^2$. We provide a number of internal velocity dispersion estimates for the ONC that indicate a weak dependence on the stellar location and mass. There is good agreement with the published velocity dispersion estimates, although nearly all of them (including ours at ${\sigma }_{v,x}=0.94$ and ${\sigma }_{v,y}=1.25$ mas yr${}^{-1}$) might be biased by the overlapping young stellar populations of Orion A. We identified 4 new ONC candidate runaways based on HST and the Gaia DR2 data, all with masses less than $\sim$1 $M_{\odot }$. 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 has the tangential velocity exceeding 20 km s${}^{-1}$. If most of them indeed originated in the ONC, it may compel 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.

arXiv comment16 pages, 10 figures, 5 tables. Accepted for publication in AJ

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