SF-08-0012

Binary Formation in the Orion Nebula Cluster: Investigating the Low-mass Stellar and Sub-Stellar Population

Matthew De Furio, Michael R Meyer, Megan Reiter, Adam Kraus, Trent Dupuy, John Monnier, Christopher Liu

We present updated results constraining multiplicity demographics for the stellar and substellar population of the Orion Nebula Cluster (ONC), across primary masses 0.012 - 0.7 Msun. Our study utilizes archival Hubble Space Telescope (HST) data obtained with the Advanced Camera for Surveys (ACS) using multiple filters (PID: 10246, PI M. Robberto). Studying the companion populations of young, star-forming regions provides valuable constraints of how the birth environment affects multiple formation and evolution. Previous multiplicity surveys in Taurus (a low-mass, low-density association) identify a companion frequency to low-mass primary stars roughly twice that of the Galactic field, over all mass ratios and for separations of 3-5000 au. Our study investigates the companion population of the ONC (a high-mass, high density star-forming region) where dynamical interactions are more likely to occur and modify the companion population. Our study is sensitive to companions beyond orbital separations of 10 au (0.025”, 0.5λ/D at 555nm) using a double point-spread function (PSF) fitting algorithm with empirical, position-dependent PSF models. We have identified 44 companions to low-mass stars and 7 companions to sub-stellar primaries. We found: 1) the companion frequency (CF) of low-mass stars in the ONC is consistent with the Galactic field over all mass ratios and orbital separations of 10-200 au, in contrast to the higher CF found in low-density regions like Taurus, 2) the power-law fit to the mass ratio distribution of low-mass stellar primaries in the ONC is consistent with that of the field and Taurus, similar to results from N-body simulations and potentially indicative of a universal formation mechanism, and 3) the CF to brown dwarfs for separations > 20 au in the ONC is in excess of the field, requiring further dynamical evolution to destroy binaries with low binding energy to resemble the field.