Large closed-field corona of WX Ursae Majoris evidenced from radio observations

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 are specified, namely the plasma density in the outer corona and the radial distance at which the plasma forces the closed magnetic field into an open geometry. 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 region extends to at least ≈ 10 stellar radii, which 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 Sunlike and planet-like configurations, where compact over-dense coronal loops with X-ray emitting plasma co-exist with a large-scale magnetosphere with a lower plasma density and closed magnetic geometry.

Radio masers on WX UMa: hints of a Neptune-sized planet, or magnetospheric reconnection?

“…The polarisation of ECMI emission depends on the magnetic polarity of the point on the field line where it is generated relative to the observer, as well as the magnetoionic mode. For LOFAR, the convention for emission in the o-mode is that the Stokes V flux is observed with a positive sign for a positive polarity (𝐵 𝑟 > 0), and negative for a negative polarity (𝐵 𝑟 < 0) (Davis et al 2021). For x-mode emission, the reverse is true.…”

“…Between 2014 and 2016, WX UMa was detected nearly continuously in the radio over three 8-hour intervals (Davis et al 2021;Callingham et al 2021). The observed emission exhibits a high degree of circular polarisation ( 70%), with a peak flux density of ∼ 1.2 mJy.…”

“…Recent observations with the LOw Frequency ARray (LOFAR, Van Haarlem et al 2013) have begun to shed light on the coherent radio emission mechanisms at play in the coronae of M dwarfs (Vedantham et al 2020;Davis et al 2021;Callingham et al 2021). Many of these observations could be due to interactions between the star's magnetic field and a planet orbiting in the sub-Alfvénic region of the stellar wind, which can extend out to tens of stellar radii in the case of M dwarfs (Davis et al 2021;Kavanagh et al 2021). Within 0.1 au, M dwarfs are expected to host numerous rocky exoplanets (Burn et al 2021), the same region where the habitable zones of these stars are thought to lie (Kopparapu et al 2013).…”

Peculiar radio$-$X-ray relationship in active stars

“…We are presently conducting a systematic survey of the northern sky for stellar, brown dwarf, and exoplanetary radio emissions (see for e.g. Davis et al 2021;Callingham et al 2021b,a;Vedantham et al 2020b,a). We primarily identify our targets in the pipeline-processed data from the LOFAR Two-Metre Sky Survey (LoTSS; Shimwell et al 2017Shimwell et al , 2019Tasse et al 2021) as circularly-polarised sources in the Stokes V component of the survey (V-LoTSS; Callingham et al in prep).…”

The population of M dwarfs observed at low radio frequencies

“…We note that one of our detections, WX UMa, provides direct evidence for a distant source site. WX UMa has a large-scale dipolar magnetic field with a strength of ∼ 3.5 kG at the stellar surface 32 (cyclotron frequency of ≈ 10 GHz) which means that the 120 MHz emission from WX UMa must originate at a distance of at least 5.5 stellar radii from the star 33 . Such a conclusion has the important implication that M dwarf coronae can be structurally similar to planetary magnetosphere in that they are dominated by a global dipole with a closed field topology.…”