Revealing the Atmosphere of a Planetary Mass Companion

We present our latest atmospheric modeling results for HD 106906b, a cloudy L dwarf companion to a 15 Myr old F dwarf binary star system. Characterizing the atmospheres of directly imaged planetary-mass companions allows us to probe their possible formation histories, and can distinguish planets from companion brown dwarfs on wide orbits. One way is to chemically characterize the companion’s atmosphere, comparing its composition with its host. The carbon-to-oxygen (C/O) ratio is a key piece of evidence needed to establish one formation pathway over another. Our work represents the first “retrieval” of an L dwarf’s atmosphere using the APOLLO atmospheric retrieval framework, equipped to model the cloudy photospheres expected for HD 106906 b’s ~1800 K effective temperature, with a high-resolution (R~1500), ground-based emission spectrum (Daemgen et al. 2017). We are able to constraint the C/O ratio, but uncertainties remain in the accuracy of our best-fit atmospheric thermal profile. While the greater infrared wavelength coverage of the James Webb Space Telescope should better characterize clouds in L dwarf atmospheres, it will also crucially improve constraints on the temperature structures of these companions.