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

¹ ETH Zürich, Institut für Astronomie, Wolfgang-Pauli-Strasse 27, 8093 Zürich, Switzerland
e-mail: daemgens@phys.ethz.ch
² Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
³ Department of Physics and Astronomy, University of Hawaii-Hilo, 200 W Kawili St, Hilo, HI 96720, USA
⁴ Lawrence Livermore National Laboratory, 7000 East Ave, Livermore, CA 94550, USA
⁵ Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, USA
⁶ National Astronomical Observatory of Japan, Subaru Telescope, Hilo, HI, USA
⁷ Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ 08544, USA
⁸ Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA
⁹ Institute for Physics/IGAM, NAWI Graz, University of Graz, Universitätsplatz 5/II, 8010 Graz, Austria
¹⁰ Space Telescope Science institute, Baltimore, MD 21218, USA
¹¹ Institut de Recherche sur les Exoplanètes et Université de Montréal, Montréal, QC, Canada
¹² Faculty of Science, York University, 4700 Keele Street, Toronto, ON M3J 1P3, Canada
¹³ Institute for Astronomy, 34 ’Ohi’a Ku St. Pukalani, HI, USA

Received: 24 October 2016
Accepted: 18 February 2017

Abstract

We present new Keck/NIRC2 3–5 μm infrared photometry of the planetary-mass companion to ROXS 42B in L′, and for the first time in Brackett-α (Brff) and in M_s-band. We combine our data with existing near-infrared photometry and K-band (2–2.4 μm) spectroscopy and compare these data with models and other directly imaged planetary-mass objects using forward modeling and retrieval methods in order to characterize the atmosphere of ROXS 42B b. The ROXS 42B b 1.25–5 μm spectral energy distribution most closely resembles that of GSC 06214 B and κ And b, although it has a slightly bluer K_s−M_s color than GSC 06214 B and thus currently lacks evidence of a circumplanetary disk. We cannot formally exclude the possibility that any of the tested dust-free/dusty/cloudy forward models describe the atmosphere of ROXS 42B b well. However, models with substantial atmospheric dust/clouds yield temperatures and gravities that are consistent when fit to photometry and spectra separately, whereas dust-free model fits to photometry predict temperatures/gravities inconsistent with the ROXS 42B b K-band spectrum and vice-versa. Atmospheric retrieval on the 1–5 μm photometry places a limit on the fractional number density of CO₂ of log (n_CO2) < ₂.7, but provides no other constraints so far. We conclude that ROXS 42B b has mid-IR photometric features that are systematically different from other previously observed planetary-mass and field objects of similar temperature. It remains unclear whether this is in the range of the natural diversity of targets at the very young (~2 Myr) age of ROXS 42B b or unique to its early evolution and environment.