[P-EM11] Exoplanet

[PEM11-P18] Subaru/IRD high-resolution spectroscopy of a T-type brown dwarf and investigation of its atmospheric properties with high-resolution spectrum model ExoJAX

*Yui Kawashima¹, Hajime Kawahara², Yui Kasagi³, Hiroyuki Ishikawa^4,5, Kento Masuda⁶, Takayuki Kotani^4,5, Tomoyuki Kudo⁵, Teruyuki Hirano^4,5, Masayuki Kuzuhara^4,5, Stevanus Nugroho^4,5, John Livingston^4,5, Motohide Tamura^4,5,7, IRD team (1.RIKEN, 2.Japan Aerospace Exploration Agency, 3.The Graduate University for Advanced Studies, 4.Astrobiology Center, 5.National Astronomical Observatory of Japan, 6.Osaka University, 7.The University of Tokyo)

Keywords:Atmospheres, Brown Dwarfs, Exoplanets, High-resolution Spectroscopy, Spectra, Composition

Understanding the atmospheres of exoplanets and brown dwarfs holds the key to revealing their formation and evolutionary processes. Brown dwarf atmospheres share composition and temperature with those of extrasolar gas giant planets. In general, brown dwarfs are observable with a higher signal-to-noise ratio when compared to exoplanets. Thus, the observation of brown dwarf atmospheres helps us establish our understanding of various processes in the atmospheres of such temperature and composition, including chemistry, thermal structure, dynamics, and cloud formation. Also, their high-resolution spectra serve as excellent templates for the observational validation of the molecular line lists at such high temperatures. Molecular line lists play a critical role in the detection of chemical species in exoplanet atmospheres, which are often observed with a lower signal-to-noise ratio.

Recently, we observed a high-resolution spectrum of a T6.5-type brown dwarf Gl 229B with the InfraRed Doppler (IRD, R ~ 70,000) spectrograph mounted on the Subaru Telescope. We have constrained its atmospheric properties, such as the molecular abundances and thermal structure, using an inverse-problem approach with ExoJAX. ExoJAX is an auto-differentiable high-resolution spectrum model for exoplanets and brown dwarfs we recently developed (Kawahara, Kawashima et al. 2022). In addition, we have revealed that in some wavelength regions, specific molecular line lists do not match the observed absorption features.