Atmospheric Characterization and Further Orbital Modeling of \(\kappa\) And b

We present \(\kappa\) 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. \(\kappa\) And b's Y/Y-K colors 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. (2018) 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 \(\kappa\) And b may have lower gravity than those previously reported. Atmospheric models lacking dust/clouds fail to reproduce its entire 1--4.7 \(\mu m\) spectral energy distribution, cloudy atmosphere models with temperatures of \(\sim\) 1700--2000 \(K\) better match \(\kappa\) And b data. Most well-fitting model comparisons favor 1700--1900 \(K\), a surface gravity of log(g) \(\sim\) 4--4.5, and a radius of 1.3--1.6\,\(R_{\rm Jup}\); the best-fit model (DRIFT-Phoenix) yields the coolest and lowest-gravity values: \(T_{\rm eff}\)=1700 K and \(\log g\)=4.0. An update to \(\kappa\) And b's orbit with ExoSOFT using new astrometry spanning seven years reaffirms its high eccentricity (\(0.77\pm0.08\)). We consider a scenario where unseen companions are responsible for scattering \(\kappa\) And b to a wide separation and high eccentricity. If three planets, including \(\kappa\) And b, were born with coplanar orbits and one of them was ejected by gravitational scattering, a potential inner companion with mass \(\gtrsim10M_{\rm Jup}\) could be located at \(\lesssim\) 25 au.