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Large Closed-Field Corona of WX Uma Evidenced from Radio Observations

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Large Closed-Field Corona of WX Uma Evidenced from Radio Observations Astronomy & Astrophysics manuscript no. WXUMa-large-corona ©ESO 2021 May 4, 2021 Letter to the Editor Large closed-field corona of WX UMa evidenced from radio observations I. Davis1; 2, H. K. Vedantham1; 3, J. R. Callingham1; 4, T. W. Shimwell1; 4, A. A. Vidotto5, P. Zarka6, T. P. Ray7, and A. Drabent8 1 ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, Dwingeloo, 7991 PD, The Netherlands 2 Department of Physics and Astronomy, University of New Mexico, Albuquerque, USA 3 Kapteyn Astronomical Institute, University of Groningen, PO Box 72, 97200 AB, Groningen, The Netherlands 4 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA, Leiden, The Netherlands 5 School of Physics, Trinity College Dublin, the University of Dublin, Dublin-2, Ireland 6 LESIA, CNRS – Observatoire de Paris, PSL 92190, Meudon, France 7 Dublin Institute for Advanced Studies, Ireland 8 Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany Received XXX; accepted YYY ABSTRACT The space-weather conditions that result from stellar winds significantly impact the habitability of exoplanets. The conditions can be calculated from first principles if the necessary boundary conditions– namely on the plasma density in the outer corona and the radial distance at which the plasma forces the closed magnetic field into an open geometry– are specified. Low frequency radio observations (ν . 200 MHz) of plasma and cyclotron emission from stars probe these magneto-ionic conditions. Here we report the detection of low-frequency (120 − 167 MHz) radio emission associated with the dMe6 star WX UMa. If the emission originates in WX UMa’s corona, we show that the closed field regions extends to at least ≈ 10 stellar radii, that is about a factor of a few larger than the solar value, and possibly to & 20 stellar radii. Our results suggest that the magnetic-field structure of M dwarfs is in between Sun- like and planet-like configurations, where compact over-dense coronal loops with X-ray emitting plasma co-exist with a large-scale magnetosphere with lower plasma density and closed magnetic geometry. Key words. Stars: coronae – Radio continuum: stars – Plasmas – Radiation mechanisms: non-thermal 1. Introduction volume that expands as the stellar wind. The density contrast be- tween plasma in short coronal loops and the bulk of the corona M-dwarfs often host multiple terrestrial temperate exoplanets could, in principle, be orders of magnitude on M-dwarfs because (Dressing & Charbonneau 2015; Gillon et al. 2017). Since M- their intense magnetic fields can support such a pressure differ- dwarfs display heightened levels of magnetic activity over gi- ence (see Pneuman 1968, for a theoretical analysis of a generic gayear timescales, their high energy photon and plasma ejection open-closed field boundary region). As a result, existing MHD could adversely affect the habitability of exoplanets (Kopparapu simulations have been hampered by a lack of empirically deter- et al. 2013). mined boundary conditions for the coronal gas properties. Magnetohydrodynamical (MHD) simulations of exoplanet space-weather have been used to model the interplanetary Polarised lower frequency radio emission, due to the plasma plasma density and magnetic field strength around low-mass or cyclotron maser mechanism, is expected to originate at much stars (Vidotto 2021; Vidotto et al. 2013; Cohen et al. 2014; Gar- higher coronal layers (many stellar radii as we demonstrate here) raffo et al. 2017). These models must necessarily ascribe bound- instead of from short coronal loops close to the stellar surface ary conditions on the plasma density and velocity, as well as where the densest X-ray emitting gas is thought to be confined. the magnetic field strength and topology on some closed sur- Such radio observations can therefore provide a crucial con- arXiv:2105.01021v1 [astro-ph.SR] 3 May 2021 face. The large-scale magnetic field at the stellar surface can be straint on the density of the outflowing plasma and significantly well constrained by Zeeman Doppler imaging maps (Morin et al. improve the reliability of exoplanet space-weather simulations. 2010) but the radial evolution of the field, and the radius at which The vast majority of stellar radio emission has only been it transitions from a closed topology to radially distended open studied at frequencies greater than 1 GHz (Güdel 2002) ow- topology, cannot be determined without explicit knowledge of ing mainly to sensitivity constraints. The advent of sensitive the plasma density. The plasma density in these simulations is radio telescopes at metre wavelengths has opened up the low- usually assumed ad-hoc or estimated from the X-ray luminos- frequency window to stars and brown dwarfs (Lynch et al. 2017; ity of the star. However, the X-ray measurements are likely bi- Villadsen & Hallinan 2019; Vedantham et al. 2020c; Callingham ased towards over-dense gas that is held confined in short coronal et al. 2021a; Vedantham et al. 2020a). As part of our ongoing ef- loops (smaller than the stellar radius typically) since X-ray emis- fort to constrain coronal parameters using low-frequency emis- sivity depends on the square of the electron density. Instead, it is sion, here we focus on WX UMa— a system blindly detected the lower density plasma that occupies the bulk of the coronal (Callingham et al. 2021b) in the ongoing Low-Frequency Ar- Article number, page 1 of 10 A&A proofs: manuscript no. WXUMa-large-corona ray (LOFAR) Two-metre Sky Survey (LoTSS) (Shimwell et al. Further details of the radio data processing and extraction of 2017, 2019) with the LOFAR telescope (van Haarlem et al. dynamic spectra are given in AppendixA. 2013). We have chosen WX UMa from our LoTSS detected stars because its magnetic field strength and topology are well known (Morin et al. 2010). Here we use the LOFAR radio data to show that, if the emission originates in the corona of WX UMa, then 3. Discussion the low frequency of emission, despite the strong magnetic field of WX UMa, requires the star to have a closed-field configura- The known properties of WX UMa which will aid in our inter- tion out to a radial distance of & 10 stellar radii and possibly pretation of the radio emission are summarised in Table2. Be- & 20 stellar radii. This is substantially larger than radial dis- cause the emission is persistent, we only consider cases where tances to the open-closed field boundary of 3−5 stellar radii that the emitting plasma is trapped within a closed magnetic geome- has been derived from existing MHD simulations of M-dwarfs try. This is because plasma that unbinds from the star is expected stellar winds (Vidotto et al. 2013, 2014; Kavanagh et al. 2021), to expand on a timescale that is much shorter than the duration of but comparable to the size of ‘slingshot prominences’ (Collier our observations. For example, solar radio emission from plasma Cameron & Robinson 1989) proposed to exist on fast-rotating flowing along open field lines typically lasts seconds to min- cool stars (Jardine et al. 2020). utes as seen in solar Type-II and Type-III bursts (Wild & Smerd 1972). Magnetically trapped plasma in stellar coronae can emit 2. Observations & Results via different mechanisms depending on the energy of the emit- 2.1. Low-frequency radio detection ting electrons and the ambient magneto-ionic parameters (Dulk 1985). The high circular polarisation fraction rules out all known WX UMa was discovered as a radio source in the 120-168 MHz incoherent emission mechanisms. Coherent emission mecha- LoTSS survey (Callingham et al. 2021b) as part of our on-going nisms that can generate up to 100% circularly polarised emission blind survey for stellar, brown dwarf and planetary radio emis- are fundamental plasma emission and fundamental and second sion (Vedantham et al. 2020c,b; Callingham et al. 2019, 2021a). harmonic cyclotron emission. While cyclotron emission occurs LoTSS surveys the sky with a tessellation of partially overlap- at the harmonics of the electron cyclotron frequency (Treumann ping pointings (Shimwell et al. 2017, 2019). WX UMa was ob- 2006; Melrose & Dulk 1982), plasma emission in a magnetic served in four such pointings (see Table1 and Fig.1). How- trap is thought to occur at the harmonics of the upper hybrid fre- ever, two of the four pointings were observed simultaneously quency (Zaitsev & Stepanov 1983; Stepanov et al. 2001) which using LOFAR’s multi-beaming capability and therefore have re- is the quadrature sum of the plasma and electron cyclotron fre- dundant information between them. The source is detected in all quencies. Both of these mechanisms can account for the long three independent pointings and has a consistently high (& 70%) duration of the observed emission. Plasma emission will persist circular polarisation fraction when averaged over the full ≈ 8 h 00 for as long as the turbulent Langmuir wave spectrum that pow- of each observation. Its companion star, GJ 412 A that is ≈ 30 ers the emission is maintained. CMI from ultracool dwarfs is away, is not detected in any pointing. commonly seen to be pulsed at the rotation period due to beam- ing, but for conducive geometries, it can be visible at all rotation 2.2. Spectro-temporal properties phases. CMI’s beaming geometry is discussed in more detail in §3.6 and AppendixC. The low frequency radio emission from WX UMa is broad-band and persistent. The emission persists for most of the 8 hr obser- vation block in each exposure, as seen in Fig.2. The spectrum, 3.1. Emission site shown in Fig.3 is particularly interesting, as it has a consis- tent shape across the observations: it is bright at the low-end of The magnetic field of WX UMa is largely dipolar with a polar the observational band, and rapidly declines within a few MHz surface magnetic field strength of ≈ 4:5 kG (Morin et al.
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