Upsilon Andromedae b in polarized light: New constraints on the planet size, density and albedo

Abstract

Polarimetry is a novel tool to detect and characterize exoplanets and their atmospheres. Polarized scattered light from the non-transiting hot Jupiter $\upsilon$~And~b is measured to further constrain its orbit, mass, density, and geometrical albedo. We obtained polarimetric measurements in the $UBV$ bands over the orbital period and deduce an average peak-to-peak amplitude of $(49 \pm 5)\times10^{-6}$ in both Stokes $q$ and $u$. From our data we evaluate the orbit inclination $i=111\degr\pm11\degr$, longitude of the ascending node $Ω=236\degr\pm12\degr$ (or equivalently 56\degr), the effective size of the scattering atmosphere in the optical blue of $1.36\pm0.20$\,$R_{\rm J}$. These combined with spectroscopic measurements result in the planet mass $0.74\pm0.07$\,$M_{\rm J}$, mean density $0.36\pm0.08$\,g\,cm$^{-3}$, and surface gravity $\sim10^3$\,cm\,s$^{-2}$, which favor a close similarity of $\upsilon$~And~b to other inflated hot Jupiters. We also significantly improved the periastron epoch $T_{\rm p}={\rm JD}2,450,032.451$, interior conjunction epoch $T_{\rm t}={\rm JD}2,450,034.668$, and periastron longitude $ω=279\degr\pm14\degr$. The latter indicates that the apsidal resonance known for planets c and d includes also planet b. Obtained limits on the wavelength dependent geometrical albedo (average 0.35) indicate its similarity to Neptune with peak reflectivity in the blue. Combining all available measurements at various passbands, we construct a unified wavelength dependent albedo of an average hot Jupiter. It appears to be largely shaped by Rayleigh scattering in the blue and atomic and molecular absorption in the optical and near infrared. Our findings demonstrate the power of polarimetry for studying non-transiting exoplanets.