来自太阳系外行星系统Tau Bootis的18厘米射电发射的严格上限
A stringent upper limit to 18cm radio emission from the extrasolar planet system tau Bootis
摘要：
内容：多年来，人们一直推测，一些太阳系外行星可能会在10-100 MHz的低射频范围内发出强烈的回旋辐射。尽管进行了几次尝试，但尚未看到这种排放。目标：热木星系统牧夫座τ是迄今已知的最近的（d=15 pc）系外行星之一。这颗巨大的热木星（M=6 M_jup）的引力影响锁定了恒星-行星系统，使恒星的自转周期为P~3.3天，与行星的轨道周期相似。从恒星自转和射电光度之间的良好相关性来看，可以想象，牧夫座τ系统发射的强射电辐射频率比目前探测的频率高得多，我们的目标是通过这项工作进行调查。方法：我们用Westerbork综合射电望远镜（WSRT）在1.7 GHz的频率上观测了牧夫座τ星。在谱线模式下持续12小时，在目标位置处达到42微焦/束的噪声水平。结果如下：牧夫座τ没有探测到18厘米射电辐射，因此3 σ的上限为0.13 mJy，相当于18厘米射电光度<3.7e13 erg/s/Hz。我们观察到牧夫座τ是两个数量级比预期的恒星之间的关系，无线电光度和旋转速度。结论：这意味着，与类似的快速旋转恒星相比，牧夫座τ系统的无线电亮度不足，或者我们碰巧在无线电发射的低状态下观察到目标。
英文摘要：
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.
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