Dynamical and Atmospheric Characterization of the Substellar Companion HD 33632 Ab from Direct Imaging, Astrometry, and Radial-velocity Data*

Abstract We present follow-up SCExAO/CHARIS H- and K -band ( R ∼ 70) high-contrast integral field spectroscopy and Keck/NIRC2 photometry of directly imaged brown dwarf companion HD 33632 Ab and new radial-velocity data for the system from the SOPHIE spectrograph, complemented by Hipparcos and Gaia astrometry. These data enable more robust spectral characterization compared to lower-resolution spectra from the discovery paper and more than double the available astrometric and radial-velocity baseline. HD 33632 Ab’s spectrum is well reproduced by a field L8.5–L9.5 dwarf. Using the Exo-REM atmosphere models, we derive a best-fit temperature, surface gravity, and radius of T eff = 1250 K, log( g ) = 5, and R = 0.97 R J , respectively, as well as a solar C/O ratio. Adding the SOPHIE radial-velocity data enables far tighter constraints on the companion’s orbital properties (e.g., i = <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>47.5</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>4.7</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>2.5</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> o ) and dynamical mass ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>52.8</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>2.4</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>2.6</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> M J ) than derived from imaging data and Gaia eDR3 astrometry data alone. HD 33632 Ab should be a prime target for multiband imaging and spectroscopy with the James Webb Space Telescope and the Roman Space Telescope’s Coronagraphic Instrument, shedding detailed light on HD 33632 Ab’s clouds and chemistry and providing a key reference point for understanding young exoplanet atmospheres.