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We test current atmospheric retrieval frameworks with HD 4747 B, a benchmark brown dwarf companion near the L/T transition where cloud modeling is important. HD 4747 B orbits an old G-type star with a semi-major axis of 10 AU, and has a precise dynamical mass from long-term monitoring with radial velocity and direct imaging. Our data consists of near-IR low-resolution spectra (R~50) from GPI and high-resolution (R~35,000) spectra from Keck/NIRSPEC with the newly commissioned single-mode fiber mode, which is part of the Keck Planet Imager and Characterizer (KPIC). KPIC allows us to obtain high-resolution spectra of brown dwarfs and exoplanets at high contrast to complement existing low-res data. We developed a likelihood-based framework to combine high and low-res spectra in a joint retrieval with petitRADTRANS. We test several cloud models with various levels of complexity, and use both statistical metrics (e.g. chi-squared, Bayes factor) and checking that the retrieved composition matches that of the host star to assess how sensitive we are to cloud assumptions. We also consider the effect of disequilibrium chemistry by including a quench pressure, and explore different PT parametrizations, ensuring our retrieved PT profile is in agreement with that from self-consistent models. Our preliminary results highlight the importance of including clouds in the retrieval to get a physically plausible radius, and that the low- and high-res data have nicely complementary power in constraining different model parameters.

Cloud Nine Con | Jerry Xuan | Joint retrievals of low-resolution and high-resolution spectra for a cloudy brown dwarf companion near the L/T transition