Planetpol polarimetry of the exoplanet systems 55 Cnc and tau Boo

Planetpol polarimetry of the exoplanet systems 55 Cnc and tau Boo
http://arxiv.org/abs/0807.2568

Abstract wrote:

We present very sensitive polarimetry of 55 Cnc and tau Boo in an attempt to detect the partially polarised reflected light from the planets orbiting these two stars. 55 Cnc is orbited by a hot Neptune planet (55 Cnc e) at 0.038 AU, a hot Jupiter planet (55 Cnc b) at 0.11 AU, and at least 3 more distant planets. The fractional polarisation of this star is very stable, with a standard deviation in the nightly averaged Stokes Q/I and U/I parameters of 2.2x10^{-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 polarisation behaviour (scaled by the maximum polarisation, p_m at 90 degrees) and pressure insensitive extinction. Atmospheres in which multiple scattering plays only a small role have an almost linear relation between polarisation and A_G. In this case, the 4 sigma upper limits are A_G<0.13(R/1.2 R_{Jup})^{-2}p_m^{-1} for 55 Cnc e and A_G<1.21(R/1.2 R_{Jup})^{-2}p_m^{-1} for 55 Cnc b. This is most easily explained if 55 Cnc e is relatively small, like GJ436b, and therefore not a pure H-He planet. tau Boo 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 polarisation parameters of 5.1x10^{-6}. The 4 sigma upper limit is A_G<0.37(R/1.2 R_{Jup})^{-2}p_m^{-1} for tau Boo b. This extends the similar upper limits reported previously for this planet to longer wavelengths. The relatively large scatter in the tau Boo data may be due to the spot activity detected photometrically by the MOST satellite. These results contrast markedly with the recent claim of a 3 sigma detection of a periodic polarisation signal from HD189733 with amplitude P=2x10^{-4}, attributed to the planet HD189733 b.

The paper writes that 55 Cancri e and b have inclinations of 55 degrees, and Tau Bootis has an inclination of 40 degrees.

Paper wrote:

The inclination of the stellar rotation axis can be determined from the measured photometric period and the vsin(i) Doppler broadening of stellar lines as i =~ 40 degrees, e.g. Leigh et al.(2003). Recent mapping of the magnetic field using (circular) spectropolarimetry (Catala et al. 2007) also finds a similar intermediate inclination. Assuming that the planetary orbit shares this inclination, this allows the mass of the planet to be determined as ~ 6 - 7 M_jup.

The inclination of 55 Cnc comes from astrometric detection of the outermost planet 55 Cnc d, see this paper for details. One thing to bear in mind about this astrometric result is that the orbit of 55 Cnc d has been revised since then: at the time, results suggested an eccentric orbit for the planet, whereas the five planet solution with more data puts it in a circular one, so perhaps some caution is required regarding the astrometric result.

The inclination of Tau Boötis comes from assuming coplanarity with the stellar equator - since the planet has apparently managed to tidally-lock the star, this is a reasonable assumption.