Tuning in to the radio environment of HD189733b
R. D. Kavanagh, A. A. Vidotto, D. Ó Fionnagáin, V. Bourrier, R. Fares, M. Jardine, Ch. Helling, C. Moutou, J. Llama, P. J. Wheatley
SSolar and Stellar Magnetic Fields: Origins and ManifestationsProceedings IAU Symposium No. 354, 2020(Editors go here) © 2020 International Astronomical UnionDOI: 00.0000/X000000000000000X
Tuning in to the radio environment of HD189733b
R. D. Kavanagh A. A. Vidotto , D. Ó Fionnagáin , V. Bourrier , R. Fares , ,M. Jardine , Ch. Helling , C. Moutou , J. Llama , P. J. Wheatley School of Physics, Trinity College Dublin, The University of Dublin, Dublin 2, Irelandemail: [email protected] Observatoire de l’Université de Genéve, Chemin des Maillettes 51, Versoix, CH-1290, Switzerland Physics Department, United Arab Emirates University, P.O. Box 15551, Al-Ain, United Arab Emirates University of Southern Queensland, Centre for Astrophysics, Toowoomba, Queensland, 4350, Australia SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife,Scotland, KY16 9SS Centre for Exoplanet Science, University of St Andrews, St Andrews KY16 9SS, UK CNRS/CFHT, 65-1238 Mamalahoa Highway, Kamuela HI 96743, USA Lowell Observatory, 1400 W. Mars Hill Rd, Flagstaff. AZ 86001. USA Department of Physics, University of Warwick, Coventry CV4 7AL, UK
Abstract.
The hot Jupiter HD189733b is expected to be a source of strong radio emission, due to its closeproximity to its magnetically active host star. Here, we model the stellar wind of its host star, based onreconstructed surface stellar magnetic field maps. We use the local stellar wind properties at the planetaryorbit obtained from our models to compute the expected radio emission from the planet. Our findings showthat the planet emits with a peak flux density within the detection capabilities of LOFAR. However, due toabsorption by the stellar wind itself, this emission may be attenuated significantly. We show that the besttime to observe the system is when the planet is near primary transit of the host star, as the attenuation fromthe stellar wind is lowest in this region.
Keywords. stars: individual (HD189733), stars: magnetic fields, stars: winds, outflows, stars: planetarysystems
1. Introduction
Close-in hot Jupiters are expected to be sources of strong auroral radio emission, analogousof what is observed for the magnetised solar system planets (Zarka et al. 2001). This is thoughtto occur due to magnetic interactions between the stellar wind of the host star and the intrinsicmagnetic field of the orbiting planet. However, despite the large number of hot Jupiters detectedto date, along with numerous radio surveys, no sources of exoplanetary radio emission have beendetected (Smith et al. 2009; Lazio et al. 2010; Lecavelier des Etangs et al. 2013; Sirothia et al.2014; O’Gorman et al. 2018)HD189733b is one such exoplanet that is expected to emit strong low frequency radio emission.The planet orbits its host star just 0.03 au. The host star is magnetically active, with its unsignedfield strength observed to vary from 18 to 42 G over a 9 year period (Fares et al. 2017). Here,we model the stellar wind of the host star, and use the stellar wind properties obtained from themodels to predict the flux density and frequency emitted by the planet. This emission is foundto be within the detection limit of LOFAR. However, we also find that the emission may beattenuated significantly by the stellar wind of the host star. A complete description of our work ispublished in Kavanagh et al. (2019). 1 a r X i v : . [ a s t r o - ph . S R ] O c t R. D. Kavanagh et al.
Figure 1.
Top panels:
Radial surface magnetic field maps of the host star reconstructed by Fares et al. (2017),at the epochs 2013 Jun/Jul, 2014 Sep, and 2015 Jul (left to right). These maps are used as boundary conditionsin our stellar wind simulations.
Bottom panels:
Simulated stellar wind of the host star at 2013 Jun/Jul,2014 Sep, and 2015 Jul (left to right). Grey lines show the large-scale structure of the magnetic field of thestar, which is embedded in the stellar wind. Profiles of the radial velocity of the stellar wind in the orbitalplane of the planet are shown. The planetary orbit is shown with a black circle, and Alfvén surfaces areshown in white.
2. Modelling the stellar wind of the host star
To model the wind of the host star, we perform 3D magnetohydrodynamic simulations usingthe BATSRUS code developed by Powell et al. (1999), modified by Vidotto et al. (2012). Weuse surface stellar magnetic field maps reconstructed from observations by Fares et al. (2017) asboundary conditions in our simulations, at the epochs 2013 Jun/Jul, 2014 Sep, and 2015 Jul. Inour models, we adopt a coronal base density of 2 MK and number density of 10 cm − . Thesevalues produce a stellar wind with a mass-loss rate of 3 × − M (cid:12) yr − , which is within therange of inferred values for other active K-stars (see Wood 2004; Jardine & Collier Cameron2019; Rodríguez et al. 2019).Figure 1 shows the radial component of each stellar surface magnetic field map, with thecorresponding wind simulation shown below each map. We see that over the modelled timescale,the stellar wind varies in response to the varying surface magnetic field.
3. Predicting radio emission from HD189733b
Using the local stellar wind properties obtained from our simulations, we use the radiometricBode’s law to compute the expected flux density and frequency of emission from the planet, foran assumed planetary magnetic field strength (see Vidotto & Donati 2017). In the model, 0.2% ofthe incident stellar wind’s magnetic power is converted into radio power from the planet (Zarka2010). The planet’s magnetic field is assumed to be dipolar. This is illustrated in Figure 2.Figure 3 shows the predicted peak flux densities received at Earth from HD189733b at2013 Jun/Jul, 2014 Sep, and 2015 Jul, computed using the stellar wind properties at the planet’sorbit obtained from our models. For an assumed planetary magnetic field strength of 10 G, wefind that this emission occurs at a frequency of 25 MHz. Due to the variability of the stellar windover the three modelled epochs, the peak flux densities from the planet also vary. At 25 MHz, theemission predicted from HD189733b place it above the detection limit of LOFAR for a 1 hourintegration time (Grießmeier et al. 2011). he radio environment of HD189733b Figure 2.
Sketch illustrating the stellar wind incident on the magnetic field of the planet. The interactionresults in radio emission from polar cap regions near the surface.
Figure 3.
Peak radio flux densities emitted by the planet at each modelled epoch, for a field strength of 10G.
4. Absorption of the planetary radio emission in the stellar wind of the host star
While we predict that radio emission from HD189733b could be detected with LOFAR, thestellar wind itself can absorb low frequency radio emission (Panagia & Felli 1975). Here we solvethe equations of radiative transfer for the stellar wind, using the numerical code developed by ÓFionnagáin et al. (2019). We find that the planet orbits through regions of the stellar wind that areoptically thick to the predicted frequency emitted from the planet. This is illustrated in Figure 4.As a result, emission from HD189733b may only be observable as the planet approaches and leaveprimary transit of the host star. This could be useful information for timing future radio observingcampaigns in search of exoplanetary radio emission from systems similar to HD189733b.
5. Conclusions
The hot Jupiter HD189733b indeed may be a good target for detecting exoplanetary radioemission. However, as we have shown, the stellar wind can in fact absorb this emission for alarge fraction of the planet’s orbit. The best time to observe the system is when the planet is nearprimary transit of the host star. This is also applicable to other exoplanetary systems similar toHD189733b.
Acknowledgements
RDK acknowledges funding received from the Irish Research Council through the Governmentof Ireland Postgraduate Scholarship Programme. RDK and AAV also acknowledge funding R. D. Kavanagh et al.
Figure 4.
The planet orbits through the radio photosphere of the stellar wind, the region optically thick tothe emitted planetary frequency of 25 MHz. The left panel shows the shape of the radio photosphere at25 MHz in the orbital plane, and the right shows its shape in 3D. received from the Irish Research Council Laureate Awards 2017/2018. VB acknowledges supportby the Swiss National Science Foundation (SNSF) in the frame of the National Centre forCompetence in Research PlanetS, and has received funding from the European Research Council(ERC) under the European Union’s Horizon 2020 research and innovation programme (projectFour Aces; grant agreement No 724427). This work was carried out using the BATSRUS toolsdeveloped at The University of Michigan Center for Space Environment Modeling (CSEM) andmade available through the NASA Community Coordinated Modeling Center (CCMC). Theauthors also wish to acknowledge the SFI/HEA Irish Centre for High-End Computing (ICHEC)for the provision of computational facilities and support.