A Search for Intermediate Separation Low Mass Binaries in the Orion Nebula Cluster

Abstract

We present the results of a binary population study in the Orion Nebula Cluster (ONC) using archival Hubble Space Telescope (HST) data obtained with the Advanced Camera for Surveys (ACS) in Johnson V filter (HST Proposal 10246, PI M. Robberto). Young clusters and associations hold clues to the origin and properties of multiple star systems. Binaries with separations $< 100 $ AU are useful as tracers of the initial binary population since they are not as likely to be destroyed through dynamical interactions. Low mass, low stellar density star-forming regions such as Taurus-Auriga, reveal an excess of multiples compared to the Galactic Field. Studying the binary population of higher mass, higher stellar density star-forming regions like the ONC provides useful information concerning the origin of the Galactic Field star population. In this survey, we characterize the previously unexplored (and incomplete) separation parameter space of binaries in the ONC (15 - 160 AU) by fitting a double-PSF model built from empirical PSFs. We identified 14 candidate binaries (11 new detections) and find that 8$_{-2\\%}^{+4\\%}$ of our observed sample are in binary systems, complete over mass ratios and separations of 0.6 $< $ q $< $ 1.0 and 30 $< $ a $< $ 160 AU. This is consistent with the Galactic Field M-dwarf population over the same parameter ranges, 6.5\\% $\\pm$ 3\\%. Therefore, high mass star forming regions like the ONC would not require further dynamical evolution for their binary population to resemble the Galactic Field, as some models have hypothesized for young clusters.