2012A&A...546A.116S

It has been speculated for many years that some extrasolar planets may emit strong cyclotron emission at low radio frequencies in the range 10-100MHz. Despite several attempts no such emission has yet been seen. The hot Jupiter system τ Bootis is one of the nearest (d=15pc) exoplanets known to date. The gravitational influence of this massive hot Jupiter (M=6M_jup) has locked the star-planet system, making the star rotate in P∼3.3days, similar to the orbital period of the planet. From the well established correlation between stellar rotation and radio luminosity, it is conceivable that the τ Bootis system emits strong radio emission at significantly higher frequencies than currently probed, which we aimed to investigate with this work. We observed τ Bootis with the Westerbork Synthesis Radio Telescope (WSRT) at a frequency of 1.7GHz. for 12h in spectral line mode, reaching a noise level of 42µJy/beam at the position of the target. No 18cm radio emission is detected from τ Bootis, resulting in a 3σ upper limit of 0.13mJy, corresponding to a 18cm radio luminosity of <3.7x10¹³erg/s/Hz. We observe τ Bootis to be two orders of magnitude fainter than expected from the stellar relation between radio luminosity and rotation velocity. This implies that either the τ Bootis system is underluminous in the radio compared to similar fast-rotating stars, or that we happened to observe the target during a low state of radio emission.