HST Kernel-Phase Interferometry: Field Age Brown Dwarf Population Demographics

Filling out the dearth of detections between direct imaging and radial velocity surveys will test theories of planet formation and (sub)stellar binarity across the full range of semi-major axes, connecting formation of close to wide separation gas giants and substellar companions. Direct detection of close-in companions is notoriously difficult: coronagraphs and point spread function (PSF) subtraction techniques fail near the λ/D diffraction limit. We present a new faint companion detection pipeline called Argus which analyzes kernel phases, an interferometric observable analogous to closure phases from non-redundant aperture masking but  utilizing the full unobscured telescope aperture. We demonstrate the pipeline, and the power of interferometry, by performing a companion search on the entire HST/NICMOS F110W and F170M image archive of 114 nearby brown dwarfs (observed in 7 different programs). We discover no new companions but recover and refine astrometry of 18 previous imaging companions (two  with multiple epochs). We confirm two previous kernel-phase detections and do not recover a number of other candidates. We  present contrast curves showing that our pipeline is able to detect companions down to flux ratios of ∼10² at half the classical diffraction limit. We then conduct a demographic study of our detections and detection limits using a Bayesian framework. When  incorporating a limit on unresolved binaries (based on an RV survey) we find a companion frequency of 25% which, even with   sensitivity to much tighter separation companions, is consistent with previous studies of substellar multiplicity. We find a slightly wider distribution (log-normal width of 0.7 dex) centered at a tighter separation (2.1 au) than previous studies with lower sensitivity to tight companions. We also find a steep mass ratio powerlaw index of 5.5-14 depending on the assumed field age of the sample (0.9-3.0 Gyr). We also present results of our current work applying this pipeline to HST/ACS observations of the young star-forming regions of Taurus and Upper Scorpius, searching for lower mass companions at tighter separations than previously accessible with classical PSF fitting techniques.