Binary Formation in the Orion Nebula Cluster: Exploring the Sub-stellar Limit

The binary populations of young star-forming regions contain crucial information about the origins of multiple star systems, and some are likely sources of the Galactic field star population. Low-mass stellar associations, such as Taurus-Auriga, reveal an excess of low-mass binaries with projected separations > 3 AU relative to the Galactic field. High-mass, high density star clusters like the Orion Nebula Cluster (ONC) may contain dissimilar binary populations compared to low-mass associations and provide valuable constraints of environmental effects on binary formation (cf. Tokovinin & Briceno, 2020; De Furio et al. 2019). However, the binary population of sub-stellar primaries, which may be highly susceptible to dynamical interactions and feedback from on-going star formation, is sparsely explored in star-forming regions of various properties, e.g. stellar mass and density. We present the results of our investigation into the very low mass binary population of the ONC for primary masses 0.015 - 0.1 M_⊙ using archival Hubble Space Telescope data obtained with the Advanced Camera for Surveys (PID: 10246). Our method (De Furio et al. 2019, in prep.) implements a double point-spread function (PSF) fit using the empirical, position dependent PSF models of Anderson & King (2006). With this technique, we identify 8 binaries with projected separations > 10 AU (0.025”) for sub-stellar M-type primaries, 6 of which are new detections. Compared to the sub-stellar binary population of the Galactic field, we have found a potential excess (2.2σ difference) in the ONC sub-stellar binary population over mass ratios of 0.5 - 1.0 and projected separations of 20 - 200 AU. Although the stellar M-type population of the ONC does not require further dynamical evolution for the binary population to resemble that of the Galactic field (De Furio et al. 2019), the sub-stellar population of the ONC may require destruction of low-binding energy binaries to resemble the field population.