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

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 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 a lower plasma density and closed magnetic geometry.