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Register a new userThe search for radio emission from the exoplanetary systems 55 Cancri, $upsilon$ Andromedae, and $tau$ Bo{o}tis using LOFAR beam-formed observations
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Observing planetary auroral radio emission is the most promising method to detect exoplanetary magnetic fields, the knowledge of which will provide valuable insights into the planets interior structure, atmospheric escape, and habitability. We present LOFAR-LBA circularly polarized beamformed observations of the exoplanetary systems 55 Cancri, $upsilon$ Andromedae, and $tau$ Bo{o}tis. We tentatively detect circularly polarized bursty emission from the $tau$ Bo{o}tis system in the range 14-21 MHz with a flux density of $sim$890 mJy and with a significance of $sim$3$sigma$. For this detection, no signal is seen in the OFF-beams, and we do not find any potential causes which might cause false positives. We also tentatively detect slowly variable circularly polarized emission from $tau$ Bo{o}tis in the range 21-30 MHz with a flux density of $sim$400 mJy and with a statistical significance of $>$8$sigma$. The slow emission is structured in the time-frequency plane and shows an excess in the ON-beam with respect to the two simultaneous OFF-beams. Close examination casts some doubts on the reality of the slowly varying signal. We discuss in detail all the arguments for and against an actual detection. Furthermore, a $sim$2$sigma$ marginal signal is found from the $upsilon$ Andromedae system and no signal is detected from the 55 Cancri system. Assuming the detected signals are real, we discuss their potential origin. Their source probably is the $tau$ Bootis planetary system, and a possible explanation is radio emission from the exoplanet $tau$ Bootis b via the cyclotron maser mechanism. Assuming a planetary origin, we derived limits for the planetary polar surface magnetic field strength, finding values compatible with theoretical predictions. Further low-frequency observations are required to confirm this possible first detection of an exoplanetary radio signal. [Abridged]
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We present archival Giant Metrewave Radio Telescope (GMRT) observations of two exoplanetary systems, $tau$ Bootis, and 55 Cancri, at 610 MHz and 150 MHz, respectively. Theoretical models predict these systems to have some of the highest expected flux densities at radio wavelengths. Both $tau$ Bootis and 55 Cancri have been previously observed at low frequency ($sim$ 30 MHz) with Low-Frequency Array (LOFAR) (Turner et al. 2020). $tau$ Bootis shows tentative signatures of circularly polarized emission at 30 MHz, while no emission was detected from 55 Cancri. We do not detect radio emission from both the systems, but the GMRT observations set $3sigma$ upper limits of 0.6 mJy at 610 MHz for $tau$ Bootis and 4.6 mJy at 150 MHz for 55 Cancri. The sensitivity achieved at 610 MHz in these observations is comparable to some of the deepest images of an exoplanet field.
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.
Using the CHARA Array and the Palomar Testbed Interferometer, the chemically peculiar star $lambda$ Bo{o}tis has been spatially resolved. We have measured the limb darkened angular diameter to be $theta_{LD} = 0.533pm0.029$ mas, corresponding to a linear radius of $R_{star} = 1.70 pm 0.10 R_odot$. The measured angular diameter yields an effective temperature for $lambda$ Boo of $T_{eff} = 8887 pm 242$ K. Based upon literature surface gravity estimates spanning $log{(g)} = 4.0-4.2$ $[rm{cm s}^{-rm{2}}]$, we have derived a stellar mass range of $M_{star} = 1.1 - 1.7$ $M_odot$. For a given surface gravity, the linear radius uncertainty contributes approximately $sigma(M_star) = 0.1-0.2 M_odot$ to the total mass uncertainty. The uncertainty in the mass (i.e., the range of derived masses) is primarily a result of the uncertainty in the surface gravity. The upper bound of our derived mass range ($log(g)=4.2, M_star = 1.7pm0.2 M_odot$) is consistent with 100-300 MYr solar-metallicity evolutionary models. The mid-range of our derived masses ($log(g)=4.1, M_star = 1.3pm0.2 M_odot$) is consistent with 2-3 GYr metal-poor evolutionary models. A more definitive surface gravity determination is required to determine a more precise mass for $lambda$ Boo.
The bright star 55 Cancri is known to host five planets, including a transiting super-Earth. We use the CHARA Array to directly determine the following of 55 Cncs stellar astrophysical parameters: $R=0.943 pm 0.010 R_{odot}$, $T_{rm EFF} = 5196 pm 24$ K. Planet 55 Cnc f ($M sin i = 0.155 M_{Jupiter}$) spends the majority of the duration of its elliptical orbit in the circumstellar habitable zone (0.67--1.32 AU) where, with moderate greenhouse heating, it could harbor liquid water. Our determination of 55 Cancris stellar radius allows for a model-independent calculation of the physical diameter of the transiting super-Earth 55 Cnc e ($simeq 2.1 R_{earth}$), which, depending on the assumed literature value of planetary mass, implies a bulk density of 0.76 $rho_{earth}$ or 1.07 $rho_{earth}$.
The naked-eye star 55 Cancri hosts a planetary system with five known planets, including a hot super-Earth (55 Cnc e) extremely close to its star and a farther out giant planet (55 Cnc b), found in milder irradiation conditions with respect to other known hot Jupiters. This system raises important questions on the evolution of atmospheres for close-in exoplanets, and the dependence with planetary mass and irradiation. These questions can be addressed by Lyman-alpha transit observations of the extended hydrogen planetary atmospheres, complemented by contemporaneous measurements of the stellar X-ray flux. In fact, planet `e has been detected in transit, suggesting the system is seen nearly edge-on. Yet, planet `b has not been observed in transit so far. Here, we report on Hubble Space Telescope STIS Lyman-alpha and Chandra ACIS-S X-ray observations of 55 Cnc. These simultaneous observations cover two transits of 55 Cnc e and two inferior conjunctions of 55 Cnc b. They reveal the star as a bright Lyman-alpha target and a variable X-ray source. While no significant signal is detected during the transits of 55 Cnc e, we detect a surprising Lyman-alpha absorption of 7.5 +/- 1.8% (4.2 sigma) at inferior conjunctions of 55 Cnc b. The absorption is only detected over the range of Doppler velocities where the stellar radiation repels hydrogen atoms towards the observer. We calculate a false-alarm probability of 4.4%, which takes into account the a-priori unknown transit parameters. This result suggests the possibility that 55 Cnc b has an extended upper H I atmosphere, which undergoes partial transits when the planet grazes the stellar disc. If confirmed, it would show that planets cooler than hot Jupiters can also have extended atmospheres.
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