Unveiling the Atmosphere of HR 7672 B from the Near-infrared High-resolution Spectrum Using REACH/Subaru

Abstract Characterizing the atmospheres of exoplanets and brown dwarfs is crucial for understanding their atmospheric physics and chemistry, searching for biosignatures, and investigating their formation histories. Recent advances in observational techniques, combining adaptive optics with high-resolution spectrographs, have enabled detailed spectroscopic analysis for directly imaged faint companions. In this paper, we report an atmospheric retrieval on the L-type brown dwarf HR 7672 B using a near-infrared high-contrast high-resolution spectrograph, REACH ( Y , J , H band, R ∼ 100,000), which combines SCExAO with IRD at the Subaru Telescope. Our model, developed based on the ExoJAX spectrum code, simultaneously accounts for several factors, including the presence of clouds in the L dwarf’s atmosphere as well as contamination from the host star’s light and telluric absorption lines in the observed spectra. Our analysis identified H 2 O and FeH as the primary absorbers in the observed J - and H -band spectra. Additionally, the observed features were reproduced with a model that includes cloud opacity, assuming an optically thick cloud at the pressure P top . The resulting temperature at the cloud top pressure suggests the potential formation of clouds composed of TiO 2 , Al 2 O 3 , or Fe. This study is the first science demonstration for faint spectra obtained by REACH, providing a foundation for future investigations into the atmospheres of exoplanets and brown dwarfs.