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تسجيل مستخدم جديدLooking for a Pulse: A search for Rotationally Modulated Radio Emission from the Hot Jupiter, Tau Bootis b
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Hot Jupiters have been proposed as a likely population of low frequency radio sources due to electron cyclotron maser emission of similar nature to that detected from the auroral regions of magnetized solar system planets. Such emission will likely be confined to specific ranges of orbital/rotational phase due to a narrowly beamed radiation pattern. We report on GMRT 150 MHz radio observations of the hot Jupiter Tau Bootis b, consisting of 40 hours carefully scheduled to maximize coverage of the planets 79.5 hour orbital/rotational period in an effort to detect such rotationally modulated emission. The resulting image is the deepest yet published at these frequencies and leads to a 3-sigma upper limit on the flux density from the planet of 1.2 mJy, two orders of magnitude lower than predictions derived from scaling laws based on solar system planetary radio emission. This represents the most stringent upper limits for both quiescent and rotationally modulated radio emission from a hot Jupiter yet achieved and suggests that either a) the magnetic dipole moment of Tau Bootis b is insufficient to generate the surface field strengths of > 50 Gauss required for detection at 150 MHz or b) Earth lies outside the beaming pattern of the radio emission from the planet.
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The helium absorption line at 10830 {AA}, originating from the metastable triplet state 2$^3$S, has been suggested as an excellent probe for the extended atmospheres of hot Jupiters and their hydrodynamic escape processes, and has recently been detec
ted in the transmission spectra of a handful of planets. The isotropic re-emission will lead to helium airglow that may be observable at other orbital phases. The goal of this paper is to investigate the detectability of He I emission at 10830 {AA} in the atmospheres of exoplanets using high-resolution spectroscopy, providing insights into the properties of the upper atmospheres of close-in gas giants. We estimated the expected strength of He I emission in hot Jupiters based on their transmission signal. We searched for the He I 10830 {AA} emission feature in tau Boo b in three nights of high-resolution spectra taken by CARMENES at the 3.5m Calar Alto telescope. The spectra from each night were corrected for telluric absorption, sky emission lines, and stellar features, and were shifted to the planetary rest frame to search for the emission. The He I emission is not detected in tau Boo b, reaching a 5 sigma contrast limit of 4$times$10$^{-4}$ for emission line widths above 20 km/s. This is roughly a factor of 8 above the expected level of emission (assuming a typical He I transit absorption of 1% for hot Jupiters). This suggests that targeting the He I emission with well-designed observations using upcoming instruments such as VLT/CRIRES+ and E-ELT/HIRES is possible.
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with 5.84 Jupiter masses and 1.06 Jupiter radii), but located at different orbital distances (between its actual orbit of 0.046 AU and 0.2 AU). Due to the strong gravity of such planets and efficient radiative cooling, the upper atmosphere is (almost) hydrostatic and the exobase remains very close to the planet, which makes it a good candidate for radio observations. We expect similar conditions as for Jupiter, i.e. a region between the exobase and the magnetopause that is filled with a depleted plasma density compared with cases where the whole magnetosphere cavity is filled with hydrodynamically outward flowing ionospheric plasma. Thus, unlike classical hot Jupiters like the previously studied planets HD 209458b and HD 189733b, supermassive hot Jupiters should be in general better targets for radio observations.
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 syst
em 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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