On the Binary Frequency of the Lowest Mass Members of the Pleiades with Hubble Space Telescope Wide Field Camera 3

Abstract: We present the results of a Hubble Space Telescope Wide Field Camera 3 imaging survey of 11 of the lowest mass brown dwarfs in the Pleiades known (25-40 Mjup). These objects represent the predecessors to T dwarfs in the field. Using a semi-empirical binary PSF-fitting technique, we are able to probe to 0.03" (0.75 pixel), better than 2x the WFC3/UVIS diffraction limit. We did not find any companions to our targets. From extensive testing of our PSF-fitting method on simulated binaries, we compute detection limits which rule out companions to our targets with mass ratios of $q\gtrsim0.7$ and separations $a\gtrsim4$ AU. Thus, our survey is the first to attain the high angular resolution needed to resolve brown dwarf binaries in the Pleiades at separations that are most common in the field population. We constrain the binary frequency over this range of separation and mass ratio of 24-40 Mjup Pleiades brown dwarfs to be <11% for 1$\sigma$ (<26% at 2$\sigma$). This binary frequency is consistent with both younger and older brown dwarfs in this mass range.