Letter to the Editor

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

¹ ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, Dwingeloo 7991 PD, The Netherlands
e-mail: idavis@caltech.edu
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, Dublin, Ireland
⁸ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany

Received: 9 March 2021
Accepted: 29 April 2021

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 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.