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

Abstract

We present new Keck/NIRC2 3$-$5 $\mu$m infrared photometry of the planetary-mass companion to ROXS 42B in $L^\prime$, and for the first time in Brackett-$\alpha$ (Br$\alpha$) and in $M_\mathrm{s}$-band. We combine our data with existing near-infrared photometry and $K$-band (2$-$2.4 $\mu$m) spectroscopy and compare these 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. ROXS 42B b's 1.25$-$5 $\mu$m spectral energy distribution most closely resembles that of GSC 06214 B and $\kappa$ And b, although it has a slightly bluer $K_{\rm s}$$-$$M_{\rm s}$ color than GSC 06214 B and thus so far lacks evidence for a circumplanetary disk. We cannot formally exclude the possibility that any of the tested dust-free/dusty/cloudy forward models describe 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 ROXS 42B b's $K$-band spectrum and vice-versa. Atmospheric retrieval on the 1$-$5 $\mu$m photometry places a limit on the fractional number density of CO$_2$ of $\log(n_{\rm CO_2})<-2.7$ 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 ($\sim$2 Myr) age of ROXS 42B b, or unique to its early evolution and environment.