A First-Look Atmospheric Modeling Study of the Young Directly-Imaged Planet-Mass Companion, ROXs 42Bb

Abstract

We present and analyze $J$$K_s$$L^{\prime }$ photometry and our previously published $H$-band photometry and $K$-band spectroscopy for ROXs 42Bb, an object Currie et al. (2014) first reported as a young directly imaged planet-mass companion. ROXs 42Bb exhibits IR colors redder than field L dwarfs but consistent with other planet-mass companions. From the H${}_2$O-2 spectral index, we estimate a spectral type of L0 $\pm$ 1; weak detections/non-detections of the $CO$ bandheads, $NaI$, and $CaI$ support evidence for a young, low surface gravity object primarily derived from the $H_2$(K) index. ROXs 42Bb's photometry/K-band spectrum are inconsistent with limiting cases of dust-free atmospheres ($COND$) and marginally inconsistent with the AMES/DUSTY models and the BT-SETTL models. However, ROXS 42Bb data are simultaneously fit by atmosphere models incorporating several micron-sized dust grains entrained in thick clouds, although further modifications are needed to better reproduce the $K$-band spectral shape. ROXs 42Bb's best-estimated temperature is $T_{eff}$ $\sim$ 1950--2000 $K$, near the low end of the empirically-derived range in Currie et al. (2014). For an age of $\sim$ 1--3 $Myr$ and considering the lifetime of the protostar phase, ROXs 42Bb's luminosity of log($L$/$L_{\odot }$) $\sim$ -3.07 $\pm$ 0.07 implies a mass of 9${}_{-3}^{+3}$ $M_J$, making it one of the lightest planetary mass objects yet imaged.

13 pages, 3 tables, 9 figures. Accepted for publication in ApJ

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