Predicting radio emission from the newborn hot Jupiter V830 Tauri b and its host star

¹ School of Physics, Trinity College Dublin, University of Dublin, Ireland
e-mail: Aline.Vidotto@tcd.ie
² Université de Toulouse, UPS-OMP, IRAP, 14 avenue É. Belin, 31400 Toulouse, France
³ CNRS, IRAP/UMR 5277, 14 avenue É. Belin, 31400 Toulouse, France

Received: 12 September 2016
Accepted: 4 March 2017

Abstract

Magnetised exoplanets are expected to emit at radio frequencies analogously to the radio auroral emission of Earth and Jupiter. Here, we predict the radio emission from V830 Tau b, the youngest (2 Myr) detected exoplanet to date. We model the wind of its host star using three-dimensional magnetohydrodynamics simulations that take into account the reconstructed stellar surface magnetic field. Our simulations allow us to constrain the local conditions of the environment surrounding V830 Tau b that we use to then compute its radio emission. We estimate average radio flux densities of 6 to 24 mJy, depending on the assumption of the radius of the planet (one or two Jupiter radii). These radio fluxes are not constant along one planetary orbit, and present peaks that are up to twice the average values. We show here that these fluxes are weakly dependent (a factor of 1.8) on the assumed polar planetary magnetic field (10 to 100 G), opposed to the maximum frequency of the emission, which ranges from 18 to 240 MHz. We also estimate the thermal radio emission from the stellar wind. By comparing our results with the Karl G. Jansky Very Large Array and the Very Long Baseline Array observations of the system, we constrain the stellar mass-loss rate to be ≲ 3 × 10^-9M_⊙ yr^-1, with likely values between ~ 10^-12 and 10^-10M_⊙ yr^-1. With these values, we estimate that the frequency-dependent extension of the radio-emitting wind is around ~ 3 to 30 stellar radii (R_⋆) for frequencies in the range of 275 to 50 MHz, implying that V830 Tau b, at an orbital distance of 6.1 R_⋆, could be embedded in the regions of the host star’s wind that are optically thick to radio wavelengths, but not deeply so. We also note that planetary emission can only propagate in the stellar wind plasma if the frequency of the cyclotron emission exceeds the stellar wind plasma frequency. In other words, we find that for planetary radio emission to propagate through the host star wind, planetary magnetic field strengths larger than ~ 1.3 to 13 G are required. Since our radio emission computations are based on analogies with solar system planets, we caution that our computations should be considered as estimates. Nevertheless, the V830 Tau system is a very interesting system for conducting radio observations from both the perspective of radio emission from the planet as well as from the host star’s wind.