Mid-infrared characterization of the planetary-mass companion ROXs 42B b

Abstract

We present new Keck/NIRC2 3–5μm infrared photometry of the planetary-mass companion to ROXS 42B inL′, and for the first timein Brackett-α(Brα) and inMs-band. We combine our data with existing near-infrared photometry andK-band (2–2.4μm) spectroscopyand compare these data with models and other directly imaged planetary-mass objects using forward modeling and retrieval methods inorder to characterize the atmosphere of ROXS 42B b. The ROXS 42B b 1.25–5μm spectral energy distribution most closely resemblesthat of GSC 06214 B andκAnd b, although it has a slightly bluerKs−Mscolor than GSC 06214 B and thus currently lacks evidence of acircumplanetary disk. We cannot formally exclude the possibility that any of the tested dust-free/dusty/cloudy forward models describethe atmosphere of ROXS 42B b well. However, models with substantial atmospheric dust/clouds yield temperatures and gravities thatare consistent when fit to photometry and spectra separately, whereas dust-free model fits to photometry predict temperatures/gravitiesinconsistent with the ROXS 42B bK-band spectrum and vice-versa. Atmospheric retrieval on the 1–5μm photometry places a limiton the fractional number density of CO2of log(nCO2)<−2.7, but provides no other constraints so far. We conclude that ROXS 42B bhas mid-IR photometric features that are systematically different from other previously observed planetary-mass and field objects ofsimilar temperature. It remains unclear whether this is in the range of the natural diversity of targets at the very young (∼2 Myr) ageof ROXS 42B b or unique to its early evolution and environment.