The stellar wind cycles and planetary radio emission of the τ Boo system

Abstract

τ Boo is an intriguing planet‐host star that is believed to undergo magnetic cycles similar to the Sun, but with a duration that is about one order of magnitude smaller than that of the solar cycle. With the use of observationally derived surface magnetic field maps, we simulate the magnetic stellar wind of τ Boo by means of three‐dimensional magnetohydrodynamics numerical simulations. As the properties of the stellar wind depend on the particular characteristics of the stellar magnetic field, we show that the wind varies during the observed epochs of the cycle. Although the mass‐loss rates we find (∼2.7 × 10−12 M⊙ yr−1) vary less than 3 per cent during the observed epochs of the cycle, our derived angular‐momentum‐loss rates vary from 1.1 to 2.2 × 1032 erg. The spin‐down times associated with magnetic braking range between 39 and 78 Gyr. We also compute the emission measure from the (quiescent) closed corona and show that it remains approximately constant through these epochs at a value of ∼1050.6 cm−3. This suggests that a magnetic cycle of τ Boo may not be detected by X‐ray observations. We further investigate the interaction between the stellar wind and the planet by estimating radio emission from the hot Jupiter that orbits at 0.0462 au from τ Boo. By adopting reasonable hypotheses, we show that, for a planet with a magnetic field similar to Jupiter (∼14 G at the pole), the radio flux is estimated to be about 0.5–1 mJy, occurring at a frequency of 34 MHz. If the planet is less magnetized (field strengths roughly smaller than 4 G), detection of radio emission from the ground is unfeasible due to the Earth’s ionospheric cut‐off. According to our estimates, if the planet is more magnetized than that and provided the emission beam crosses the observer line‐of‐sight, detection of radio emission from τ Boo b is only possible by ground‐based instruments with a noise level of ≲1 mJy, operating at low frequencies.

Time-scales of close-in exoplanet radio emission variability

“…the time-scale of the field evolution. The predicted radio fluxes are of the same order of magnitude as those predicted by Vidotto et al (2012) despite the differing models and assumptions used. The radio flux variations in Fig.…”

“…They argue that emissions above their detection threshold may exist but, due to emission variability, it was not detectable at the time of their observations. With few exceptions (Fares et al 2010;Vidotto et al 2012Vidotto et al , 2015, emission variability has been neglected in radio emission models to date. Most assume stellar magnetic fields and winds that are both steady and isotropic.…”

Activity and magnetic field structure of the Sun-like planet-hosting star HD 1237

“…CME events, Cohen et al 2011b) or quiescent phenomena (e.g. planetary radio emission, Stevens 2005;Vidotto et al 2012) can also be considered in future work. Fig.…”

The effects of stellar winds on the magnetospheres and potential habitability of exoplanets

“…The most up to date models are fully three dimensional and self-consistent (Vidotto et al 2009(Vidotto et al , 2012 and use more physically motivated arguments to determine mass loss rates (Holzwarth & Jardine 2007;Cohen et al 2007;Cranmer & Saar 2011). However many assumptions are used and scaling relations are still present in some models.…”