A stringent upper limit to 18cm radio emission from the extrasolar planet system tau Bootis

TL;DR

This study sets a strict upper limit on 18cm radio emission from tau Bootis, challenging expectations based on stellar rotation and suggesting the system is unusually radio faint or observed during a low emission state.

Contribution

First direct high-frequency radio observation of tau Bootis, providing the most stringent upper limit to date on its radio emission at 1.7 GHz.

Findings

No radio emission detected at 1.7 GHz from tau Bootis.

Tau Bootis is significantly fainter in radio than similar fast-rotating stars.

Results suggest tau Bootis may be underluminous or observed during a low emission phase.

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 tau Bootis 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 tau Bootis system emits strong radio emission at significantly higher frequencies than currently probed, which we aimed to investigate with this work. Methods: We observed tau Bootis with the Westerbork Synthesis Radio Telescope (WSRT) at a frequency of 1.7 GHz. for 12 hours in spectral line mode, reaching a noise level of 42 microJy/beam at the position of the target. Results: No 18cm radio emission is detected from tau Bootis, resulting in a 3 sigma upper limit of 0.13 mJy, corresponding to a 18cm radio luminosity of <3.7e13 erg/s/Hz. We observe tau Bootis 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 tau 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.