NICMOS Kernel-Phase Interferometry I: Catalogue of Brown Dwarfs Observed in F110W and F170M

TL;DR

This paper introduces Argus, a new pipeline analyzing kernel phases in HST/NICMOS images to detect close-in brown dwarf companions, refining astrometry and providing contrast curves for population studies.

Contribution

The paper presents a novel kernel-phase interferometry pipeline, Argus, applied to HST/NICMOS data, enabling detection of faint companions at separations below classical imaging limits.

Findings

Detected no new companions but refined astrometry of 19 known ones.

Achieved detection of companions with flux ratios of ~100 at half the diffraction limit.

Binary fraction consistent with previous surveys despite higher resolution sensitivity.

Abstract

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 $\lambda/D$ 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 \emph{HST/NICMOS} F110W and F170M image archive of 114 nearby brown dwarfs (observed in 7 different programs). Our pipeline is able to detect companions down to flux ratios of $\sim10^2$ at half the classical diffraction limit. We discover no new companions but recover and refine astrometry of 19 previous imaging companions (two with multiple epochs) and confirm two previous kernel-phase detections. We discuss the limitations of this technique with respect to non-detections of previously confirmed or proposed companions. We present contrast curves to enable population studies to leverage non-detections and to demonstrate the strength of this technique at separations inaccessible to classical imaging techniques. The binary fraction of our sample ($\epsilon_b=14.4^{+4.7}_{-3.0}$%) is consistent with previous binary surveys, even with sensitivity to much tighter separation companions.