Atmosphere Retrieval of Planetary Mass Companions with the APOLLO Code A Case Study of HD 106906b and Prospects for JWST

We present retrieved atmospheric properties for the very low mass companion, HD 106906b, which is spatially resolved from its host star, using APOLLO, a spectral retrieval code designed for flexibility of atmosphere models. APOLLO allows retrieval on both transit and emission spectra of planets to determine molecular abundances as well as comparison of different parameterizations of the thermal structure and cloud properties. We compare fits of our models to high signal-to-noise spectra of HD 106906b, a 10-20 Jupiter mass early L-type companion orbiting a 10-15 Myr old binary system at 730 AU projected separation, obtained with SINFONI on the ESO VLT (Daemgen et al. 2017). Using these models, we measure molecular abundances and present constraints on cloud properties. Comparing the retrieved abundances of volatile species (e.g. C/O) to those of the host star, we can speculate on whether this object may have formed much closer to its host star through a core accretion-like mechanism and was later dynamically ejected to larger orbital separation. Because HD 106906b is a prime target for JWST, we present a similar analysis of model JWST spectra for this object to demonstrate the performance of APOLLO with data obtained over a broader wavelength regime (e.g. 0.6 to 5 microns).