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.

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

References in corpus (7)

• Gaia Data Release 1. Summary of the astrometric, photometric, and survey properties
• The distance to the Orion Nebula
• The Structure, Dynamics and Star Formation Rate of the Orion Nebula Cluster
• Radio Measurements of the stellar proper motions in the core of the Orion Nebula Cluster
• The Proper Motions of the Double Radio Source n in the Orion BN/KL Region
• New Evidence for the Dynamical Decay of a Multiple System in the Orion Kleinmann-Low Nebula
• On the dynamical evolution of the Orion Trapezium

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• Constraining the initial conditions of NGC 2264 using ejected stars found in Gaia DR2
• Photoionized Herbig-Haro objects in the Orion Nebula through deep high-spectral resolution spectroscopy III: HH514
• The origin of a distributed stellar population in the star-forming region W4
• A VLBA Survey of radio stars in the Orion Nebula Cluster: II. Astrometry
• SIRIUS Project. IV. The formation history of the Orion Nebula Cluster driven by clump mergers
• Faint Stars in a Faint Galaxy: II. The Low Mass Stellar Initial Mass Function of the Boötes I Ultrafaint Dwarf Spheroidal Galaxy
• Searching for young runaways across the sky
• SIRIUS Project. V. Formation of off-center ionized bubbles associated with Orion Nebula Cluster
• Analysis of Selected Runaway Stars in the Orion Nebula Based on Data from the Gaia EDR3 Catalogue
• A Gaia view on the star formation in the Monoceros OB1 and R1 associations