tau Boo b: Not so bright, but just as heavy

Abstract

We present new results derived from high-resolution optical spectra of the tau Boo system, secured in March-May 2000. The results do not show the same feature reported by Cameron et al (1999) as a candidate reflected-light signature from the planet. Together with earlier results from the 1998 and 1999 seasons, the new data yield a 99.9 percent upper limit on the opposition planet/star flux ratio epsilon<3.5E-5 between 387.4 and 586.3 nm, a factor 3 deeper than the upper limit of Charbonneau et al (1999). For an assumed planet radius Rp= 1.2 RJup, the upper limit on the mean geometric albedo is p < 0.22, 40 percent that of Jupiter. We find new evidence that the star&#39;s rotation is synchronised with the planet&#39;s orbital motion. Using a Monte Carlo analysis we infer that the planet&#39;s mass must lie in the range 5.5 to 10 times the mass of Jupiter.