Atmospheric Characterization and Further Orbital Modeling of $\hat{I}º$ And b

Abstract

We present $\hat{I}º$ 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. $\hat{I}º$ 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 $\hat{I}º$ And b may have lower gravity than those previously reported. Atmospheric models lacking dust/clouds fail to reproduce its entire 1--4.7 $\hat{I}¼m$ spectral energy distribution, cloudy atmosphere models with temperatures of $\sim$ 1700--2000 $K$ better match $\hat{I}º$ 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\,$R_{\mathrm{Jup}}$; the best-fit model (DRIFT-Phoenix) yields the coolest and lowest-gravity values: $T_{\mathrm{eff}}$=1700 K and $\log g$=4.0. An update to $\hat{I}º$ And b's orbit with ExoSOFT using new astrometry spanning seven years reaffirms its high eccentricity ($0.77\pm 0.08$). We consider a scenario where unseen companions are responsible for scattering $\hat{I}º$ And b to a wide separation and high eccentricity. If three planets, including $\hat{I}º$ And b, were born with coplanar orbits and one of them was ejected by gravitational scattering, a potential inner companion with mass $\gtrsim 10M_{\mathrm{Jup}}$ could be located at $\lesssim$ 25 au.

accepted for publication in AJ, typo fixed in ver.2