A stringent upper limit to 18 cm radio emission from the extrasolar planet system τ Boötis

Abstract

Context. It has been speculated for many years that some extrasolar planets may emit strong cyclotron emission at low radio frequencies in the range 10-100 MHz. Despite several attempts no such emission has yet been seen. Aims. The hot Jupiter system τ Boötis is one of the nearest (d = 15 pc) exoplanets known to date. The gravitational influence of this massive hot Jupiter (M = 6 M jup) has locked the star-planet system, making the star rotate in P ∼ 3.3 days, 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 Boötis system emits strong radio emission at significantly higher frequencies than currently probed, which we aimed to investigate with this work. Methods. We observed τ Boötis with the Westerbork Synthesis Radio Telescope (WSRT) at a frequency of 1.7 GHz. for 12 h in spectral line mode, reaching a noise level of 42 μJy/beam at the position of the target. Results. No 18 cm radio emission is detected from τ Boötis, resulting in a 3σ upper limit of 0.13 mJy, corresponding to a 18 cm radio luminosity of <3.7 × 10 13 erg s -1 Hz -1. We observe τ Boötis to be two orders of magnitude fainter than expected from the stellar relation between radio luminosity and rotation velocity. Conclusions. This implies that either the τ Boötis 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. © 2012 ESO.