Atmospheric Characterization and Further Orbital Modeling of κ Andromeda b

Abstract

We present κ Andromeda b’s photometry and astrometry taken with Subaru/SCExAO+HiCIAO and Keck/NIRC2, combined with recently published SCExAO/CHARIS low-resolution spectroscopy and published thermal infrared photometry to further constrain the companion’s atmospheric properties and orbit. The Y / Y − K colors of κ And b are redder than field dwarfs, consistent with its youth and lower gravity. Empirical comparisons of its Y -band photometry and CHARIS spectrum to a large spectral library of isolated field dwarfs reaffirm the conclusion from Currie et al. that it likely has a low gravity but admit a wider range of most plausible spectral types (L0–L2). Our gravitational classification also suggests that the best-fit objects for κ And b may have lower gravity than those previously reported. Atmospheric models lacking dust/clouds fail to reproduce its entire 1–4.7 μ m spectral energy distribution (SED), and cloudy atmosphere models with temperatures of ∼1700–2000 K better match κ And b data. Most well-fitting model comparisons favor 1700–1900 K, a surface gravity of log( g ) ∼ 4–4.5, and a radius of 1.3–1.6 R Jup ; the best-fit model (D rift -P hoenix ) yields the coolest and lowest-gravity values: T eff  = 1700 K and log g  = 4.0. An update to κ And b’s orbit with ExoSOFT using new astrometry spanning 7 yr reaffirms its high eccentricity (0.77 ± 0.08). We consider a scenario where unseen companions are responsible for scattering κ And b to a wide separation and high eccentricity. If three planets, including κ And b, were born with coplanar orbits, and one of them was ejected by gravitational scattering, a potential inner companion with mass ≳10 M Jup could be located at ≲25 au.