# MOVES II. Tuning in to the radio environment of HD189733b

## TL;DR

This study models the stellar wind of HD189733 and predicts radio emissions from both the star and a hot Jupiter exoplanet, revealing conditions for detectability and the influence of stellar wind properties.

## Contribution

The paper introduces a combined stellar wind and planetary radio emission model for HD189733, incorporating magnetic field data and radiative transfer calculations to assess radio detectability.

## Key findings

- Planetary radio emission peaks at 2-25 MHz with ~100 mJy flux.
- Planetary radio signals are only observable during 67% of the orbit.
- Stellar wind plasma frequency exceeds 21 MHz, hindering low-frequency radio propagation.

## Abstract

We present stellar wind modelling of the hot Jupiter host HD189733, and predict radio emission from the stellar wind and the planet, the latter arising from the interaction of the stellar wind with the planetary magnetosphere. Our stellar wind models incorporate surface stellar magnetic field maps at the epochs Jun/Jul 2013, Sep 2014, and Jul 2015 as boundary conditions. We find that the mass-loss rate, angular momentum-loss rate, and open magnetic flux of HD189733 vary by 9%, 40%, and 19% over these three epochs. Solving the equations of radiative transfer, we find that from 10 MHz-100 GHz the stellar wind emits fluxes in the range of $10^{-3}$-$5$ $\mu$Jy, and becomes optically thin above 10 GHz. Our planetary radio emission model uses the radiometric Bode's law, and neglects the presence of a planetary atmosphere. For assumed planetary magnetic fields of 1-10 G, we estimate that the planet emits at frequencies of 2-25 MHz, with peak flux densities of $\sim10^2$ mJy. We find that the planet orbits through regions of the stellar wind that are optically thick to the emitted frequency from the planet. As a result, unattenuated planetary radio emission can only propagate out of the system and reach the observer for 67% of the orbit for a 10 G planetary field, corresponding to when the planet is approaching and leaving primary transit. We also find that the plasma frequency of the stellar wind is too high to allow propagation of the planetary radio emission below 21 MHz. This means a planetary field of at least 8 G is required to produce detectable radio emission.