DIRECT IMAGING AND SPECTROSCOPY OF A CANDIDATE COMPANION BELOW/NEAR THE DEUTERIUM-BURNING LIMIT IN THE YOUNG BINARY STAR SYSTEM, ROXs 42B

Abstract

We present near-infrared high-contrast imaging photometry and integral field\nspectroscopy of ROXs 42B, a binary M0 member of the 1--3 Myr-old $\\rho$\nOphiuchus star-forming region, from data collected over 7 years. Each data set\nreveals a faint companion -- ROXs 42Bb -- located $\\sim$ 1.16" ($r_{proj}$\n$\\approx$ 150 $AU$) from the primaries at a position angle consistent with a\npoint source identified earlier by Ratzka et al. (2005). ROXs 42Bb's astrometry\nis inconsistent with a background star but consistent with a bound companion,\npossibly one with detected orbital motion. The most recent data set reveals a\nsecond candidate companion at $\\sim$ 0.5" of roughly equal brightness, though\npreliminary analysis indicates it is a background object. ROXs 42Bb's $H$ and\n$K_{s}$ band photometry is similar to dusty/cloudy young, low-mass late M/early\nL dwarfs. $K$-band VLT/SINFONI spectroscopy shows ROXs 42Bb to be a cool\nsubstellar object (M8--L0; $T_{eff}$ $\\approx$ 1800--2600 $K$), not a\nbackground dwarf star, with a spectral shape indicative of young, low surface\ngravity planet-mass companions. We estimate ROXs 42Bb's mass to be 6--15\n$M_{J}$, either below the deuterium burning limit and thus planet mass or\nstraddling the deuterium-burning limit nominally separating planet-mass\ncompanions from other substellar objects. Given ROXs 42b's projected separation\nand mass with respect to the primaries, it may represent the lowest mass\nobjects formed like binary stars or a class of planet-mass objects formed by\nprotostellar disk fragmentation/disk instability, the latter slightly blurring\nthe distinction between non-deuterium burning planets like HR 8799 bcde and\nlow-mass, deuterium-burning brown dwarfs.\n