# Direct Imaging and Spectroscopy of a Candidate Companion Below/Near the Deuterium-Burning Limit In The Young Binary Star System, ROXs 42B [SSA]

We present near-infrared high-contrast imaging photometry and integral field spectroscopy of ROXs 42B, a binary M0 star member of the 1–3 Myr-old $\rho$ Ophiuchus star-forming region from data collected over a 7 year span. Each data set reveals a faint companion (hereafter ROXs 42Bb) located $\sim$ 1.16″ ($r_{proj}$ $\approx$ 150 $AU$) from the primaries. ROXs 42Bb’s astrometry is inconsistent with a background star but consistent with a bound companion, possibly one with detected orbital motion. The most recent data set reveals a second candidate companion at $\sim$ 0.5″ of roughly equal brightness. ROXs 42Bb’s $H$ and $K_{s}$ band photometry is similar to dusty/cloudy young late M/early L dwarfs with masses at or below the deuterium-burning limit. $K$-band VLT/SINFONI spectroscopy shows ROXs 42Bb to be a cool substellar object (M8–L0; $T_{eff}$ $\approx$ 1800–2600 $K$), not a background dwarf star, with a spectral shape indicative of young, low surface gravity companions with masses at/below the deuterium-burning limit. We estimate ROXs 42Bb’s mass to be 6–15 $M_{J}$, either below the deuterium burning limit and thus planet mass or straddling the deuterium-burning limit nominally separating planet-mass companions from other substellar objects. Given ROXs 42Bb’s mass with respect to the primaries and its projected separation, it may represent the lowest mass objects formed like binary stars or a class of planet-mass objects formed by protostellar disk fragmentation/disk instability, the latter slightly blurring the distinction between non-deuterium burning planets like HR 8799 bcde and low-mass, deuterium-burning brown dwarfs.

Date added: Mon, 21 Oct 13