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

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 as boundary conditions. We find that the mass-loss rate, angular momentumloss 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 µJy, 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 ∼ 10 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.

Radio eclipses of exoplanets by the winds of their host stars

“…Towards higher frequencies however, this effect is less pronounced. This was shown by Kavanagh et al (2019), in that the planet is easier to detect both near primary transit of the host star and at higher emitted frequencies. Figure 1.…”

“…The value of F ν,0 chosen here is purely for illustrative purposes. The thermal spectrum of the stellar wind is computed as per the method laid out inÓ Fionnagáin et al (2019) and Kavanagh et al (2019). We compute the spectrum placing the system at 1 pc from the observer.…”

“…In addition to this, it is likely that the emitted flux density varies over the orbit. This has been shown to be the case if the emission mechanism powering the planetary radio emission is the interaction between the stellar wind and planetary magnetosphere, as the wind of the host star is not uniform (see Figure 4 of Kavanagh et al 2019;Nicholson et al 2016). For instance, Jupiter is observed to experience active periods where the flux density emitted increases (Zarka et al 2004).…”

Different types of star-planet interactions

“…However, in the super-Alfvenic regime, this direct connection no longer exists. This implies that, as the stellar magnetism evolves (e.g., through an activity cycle), so does the Alfven surface and the star-planet connectivity (Nicholson et al 2016;Kavanagh et al 2019). This evolution might, for example, affect when anomalous activity through star-planet interaction is triggered, giving rise to an 'on/off' nature (Shkolnik et al 2008).…”

“…In the case of the warm-Neptune GJ436b, models of the asymmetric distribution of material around the planet can constrain local stellar wind conditions (Bourrier et al 2016), which can then be used to probe the global properties of stellar winds . Winds of cool dwarfs are very rarefied and often challenging to be observed (Wood 2004;Fichtinger et al 2017;Ó Fionnagáin et al 2019;Jardine & Collier Cameron 2019).…”