Journal Article

A Clear View of a Cloudy Brown Dwarf Companion from High-resolution Spectroscopy

Mollière, Paul, Wang, Ji, Ruffio, Jean-Baptiste, Kolecki, Jared, Cetre, Sylvain, Schofield, Tobias, Blake, Geoffrey A, Ruane, Garreth, Finnerty, Luke, Xuan, Jerry W, Echeverri, Daniel, Martin, Emily C, Ragland, Sam, Wallack, Nicole, Inglis, Julie, Pezzato, Jacklyn, Venenciano, Taylor, Delorme, Jacques-Robert, Morris, Evan, Baker, Ashley, Mukherjee, Sagnick, Jovanovic, Nemanja, Wang, Jason, Calvin, Benjamin, Bryan, Marta, Chun, Mark, Bartos, Randall, Fitzgerald, Michael P, Sappey, Ben, Wallace, J. Kent, Horstman, Katelyn, Wizinowich, Peter, Knutson, Heather, Vigan, Arthur, Skemer, Andrew, Bond, Charlotte Z, López, Ronald, Mawet, Dimitri, Doppmann, Greg, Ren, Bin

2022

Overview

Direct imaging studies have mainly used low-resolution spectroscopy (R ∼ 20–100) to study the atmospheres of giant exoplanets and brown dwarf companions, but the presence of clouds has often led to degeneracies in the retrieved atmospheric abundances (e.g., carbon-to-oxygen ratio, metallicity). This precludes clear insights into the formation mechanisms of these companions. The Keck Planet Imager and Characterizer (KPIC) uses adaptive optics and single-mode fibers to transport light into NIRSPEC (R ∼ 35,000 in the K band), and aims to address these challenges with high-resolution spectroscopy. Using an atmospheric retrieval framework based on petitRADTRANS, we analyze the KPIC high-resolution spectrum (2.29–2.49 μm) and the archival low-resolution spectrum (1–2.2 μm) of the benchmark brown dwarf HD 4747 B (m = 67.2 ± 1.8 M Jup, a = 10.0 ± 0.2 au, T eff ≈ 1400 K). We find that our measured C/O and metallicity for the companion from the KPIC high-resolution spectrum agree with those of its host star within 1σ–2σ. The retrieved parameters from the K-band high-resolution spectrum are also independent of our choice of cloud model. In contrast, the retrieved parameters from the low-resolution spectrum are highly sensitive to our chosen cloud model. Finally, we detect CO, H2O, and CH4 (volume-mixing ratio of log(CH4) = −4.82 ± 0.23) in this L/T transition companion with the KPIC data. The relative molecular abundances allow us to constrain the degree of chemical disequilibrium in the atmosphere of HD 4747 B, and infer a vertical diffusion coefficient that is at the upper limit predicted from mixing length theory.

• Publisher: IOP Publishing
• Subject: Abundance / Adaptive optics / Astrochemistry / Brown dwarf stars / Clouds / Diffusion coefficient / Extrasolar planets / High resolution / Image resolution / Metallicity / Methane / Mixing length / Mixing ratio / Optics / Parameter sensitivity / Planet detection / Spectroscopic analysis / Spectroscopy / Spectrum analysis / Vertical diffusion