DANCING IN THE DARK: NEW BROWN DWARF BINARIES FROM KERNEL PHASE INTERFEROMETRY

Abstract

This paper revisits a sample of ultracool dwarfs in the Solar neighborhood\npreviously observed with the Hubble Space Telescope's NICMOS NIC1 instrument.\nWe have applied a novel high angular resolution data analysis technique based\non the extraction and fitting of kernel phases to archival data. This was found\nto deliver a dramatic improvement over earlier analysis methods, permitting a\nsearch for companions down to projected separations of $\\sim$1 AU on NIC1\nsnapshot images. We reveal five new close binary candidates and present revised\nastrometry on previously-known binaries, all of which were recovered with the\ntechnique. The new candidate binaries have sufficiently close separation to\ndetermine dynamical masses in a short-term observing campaign. We also present\nfour marginal detections of objects which may be very close binaries or high\ncontrast companions. Including only confident detections within 19 parsecs, we\nreport a binary fraction of at least $\\epsilon_b = 17.2^{+5.7}_{-3.7}%$. The\nresults reported here provide new insights into the population of nearby\nultracool binaries, while also offering an incisive case study of the benefits\nconferred by the kernel phase approach in the recovery of companions within a\nfew resolution elements of the PSF core.\n