τ Boötis b: Hunting for reflected starlight^*

¹ Instituto de Astrofísica de Canarias, C/vía Láctea s/n, 38205 La Laguna, Spain e-mail: frodler@iac.es
² Formerly at the Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
³ Formerly at the Institut für Astronomie, Universität Wien, Türkenschanzstrasse 17, 1180 Vienna, Austria

Received: 9 November 2009
Accepted: 8 February 2010

Abstract

Aims. We attempt to detect starlight reflected from the hot Jupiter orbiting the main-sequence star τ Boo, in order to determine the albedo of the planetary atmosphere, the orbital inclination of the planetary system and the exact mass of the planetary companion.

Methods. We analyze high-precision, high-resolution spectra, collected over two half nights using UVES at the VLT/UT2, by way of data synthesis. We interpret our data using two different atmospheric models for hot Jupiters.

Results. Although a weak candidate signal appears near the most probable radial velocity amplitude, its statistical significance is insufficient for us to claim a detection. However, this feature agrees very well with a completely independently obtained result by another research group, which searched for reflected light from τ Boo b. As a consequence of the non-detection of reflected light, we place upper limits to the planet-to-star flux ratio at the 99.9% significance level. For the most probable orbital inclination around i = , we can limit the relative reflected radiation to be less than ϵ = 5.7 × 10^-5 for grey albedo. This implies a geometric albedo smaller than 0.40, assuming a planetary radius of 1.2 R_Jup.