Phenomenology and periodicity of radio emission from the stellar system AU Microscopii^★

¹ ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, Dwingeloo, 7991 PD, The Netherlands
e-mail: bloot@astron.nl
² Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV, Groningen, The Netherlands
³ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA, Leiden, The Netherlands
⁴ School of Mathematics and Physics, The University of Queensland, St Lucia, QLD 4072, Australia
⁵ Centre for Astrophysics, University of Southern Queensland, West Street, Toowoomba, QLD 4350, Australia
⁶ Departament d’Astronomia i Astrofísica, Universitat de València, Burjassot, 46100, Spain
⁷ Universidad Internacional de Valencia (VIU), Valencia, 46002, Spain
⁸ Observatori Astronòmic, Universitat de València, Parc Científic, Paterna, 46980, Spain
⁹ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008, Granada, Spain
¹⁰ Facultad de Ciencias, Universidad de Zaragoza, Pedro Cerbuna 12, 50009, Zaragoza, Spain
¹¹ School of Sciences, European University Cyprus, Diogenes street, Engomi, 1516 Nicosia, Cyprus

Received: 25 September 2023
Accepted: 13 December 2023

Abstract

Stellar radio emission can measure a star’s magnetic field strength and structure, plasma density, and dynamics, and the stellar wind pressure impinging on exoplanet atmospheres. However, properly interpreting the radio data often requires temporal baselines that cover the rotation of the stars, orbits of their planets, and any longer-term stellar activity cycles. Here we present our monitoring campaign on the young, active M dwarf AU Microscopii with the Australia Telescope Compact Array between 1.1 and 3.1 GHz. With over 250 h of observations, these data represent the longest radio monitoring campaign on a single main-sequence star to date. We find that AU Mic produces a wide variety of radio emission, for which we introduce a phenomenological classification scheme predicated on the polarisation fraction and time-frequency structure of the emission. Such a classification scheme is applicable to radio emission from other radio-bright stars. The six types of radio emission detected on AU Mic can be broadly categorised into five distinct types of bursts, and broadband quiescent emission. We find that the radio bursts are highly circularly polarised and periodic with the rotation period of the star, implying that the emission is beamed. It is therefore most likely produced by the electron cyclotron maser instability. We present a model to show that the observed pattern of emission can be explained by emission from auroral rings on the magnetic poles. The total intensity of the broadband emission is stochastic, but we show that its circular polarisation fraction is also periodic with the rotation of the star. Such a periodicity in the polarised fraction of emission has not been observed on an M dwarf before. We present a qualitative model to describe the periodicity in the polarisation fraction of the broadband emission, using low-harmonic gyromagnetic emission. Using a simple qualitative model, we infer a magnetic obliquity of at least 20° from the observed variation in polarisation fraction. Finally, we show that the radio emission might be evolving on long timescales, hinting at a potential stellar magnetic activity cycle.