A first-look atmospheric modeling study of the young directly imaged planet-mass companion, ROXS 42Bb

We present and analyze JK_sL′ photometry and our previously published H-band photometry and K-band spectroscopy for ROXs 42Bb, an object Currie et al. 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 H₂O-2 spectral index, we estimate a spectral type of L0 ± 1; weak detections/non-detections of the CO bandheads, Na I, and Ca I support evidence for a young, low surface gravity object primarily derived from the H₂ (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 ∼ 1950-2000 K, near the low end of the empirically derived range in Currie et al. For an age of 1-3 Myr and considering the lifetime of the protostar phase, ROXs 42Bb's luminosity of log(L/L _⊙) 3.07 ± 0.07 implies a mass of 9 M_J , making it one of the lightest planetary-mass objects yet imaged.