Planetpol polarimetry of the exoplanet systems 55Cnc and τBoo

We present very sensitive polarimetry of 55Cnc and τBoo in an attempt to detect the partially polarized reflected light from the planets orbiting these two stars. 55Cnc is orbited by a hot Neptune planet (55Cnce) at 0.038au, a hot Jupiter planet (55Cncb) at 0.11au and at least three more distant planets. The polarization of this system is very stable, showing no sign of the periodic variations that would be expected if a short-period planet were detected. The measured standard deviation of the nightly averaged Stokes Q/I and U/I parameters is 2.2 × 10^-6. We derive upper limits on the geometric albedo, A_G, and planetary radius using Monte Carlo multiple scattering simulations of a simple model atmosphere. We assume Rayleigh-like scattering and polarization behaviour (scaled by the maximum polarization, p_m, at 90°) and pressure insensitive extinction. Atmospheres in which multiple scattering plays only a small role have an almost linear relation between polarization and A_G. In this case, the 4σ upper limit is A_G < 0.13(R/1.2R_Jup)^-2p^-1_m for 55Cnce. This is most easily explained if 55Cnce is relatively small, like GJ436b, and therefore not a pure H-He planet. The data do not provide a useful upper limit for 55Cncb. τBoo is orbited by an unusually massive hot Jupiter planet. The data show a standard deviation in the night-to-night average Stokes Q/I and U/I polarization parameters of 5.1 × 10^-6. The 4σ upper limit is A_G < 0.37(R/1.2R_Jup)^-2p^-1_m for τBoob, adopting the fairly well established orbital inclination i ~ 40°. This extends the similar upper limits reported previously for this planet to longer wavelengths. The fact that the τBoo data show more scatter, despite the smaller photon noise for this bright star, may be due to the spot activity detected photometrically by the Microvariability & Oscillations of Stars (MOST) satellite. These results contrast markedly with the recent claim of a 3σ detection of a periodic polarization signal from HD189733 with amplitude P = 2 × 10^-4, attributed to the planet HD189733b.

Based on observations made with the William Herschel Telescope operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatory del Roque de los Muchachos of the Instituto de Astrofisica de Canarias.

E-mail: p.w.lucas@herts.ac.uk