A sudden change of the global magnetic field of the active M dwarf AD Leo revealed by full Stokes spectropolarimetric observations

TL;DR

This study presents the first high-resolution spectropolarimetric observations of the active M dwarf AD Leo, revealing a sudden change in its global magnetic field over a 4-year period, indicating long-term magnetic variability.

Contribution

It provides the first evidence of long-term global magnetic variation in an active M dwarf with a dipole-dominated field topology.

Findings

Circular polarisation profiles weakened significantly after 2012.

Magnetic maps show a 20% decrease in total magnetic energy from 2012 to 2016.

The magnetic field became more concentrated with a smaller surface area in recent observations.

Abstract

In this paper we present an analysis of the first high-resolution full Stokes vector spectropolarimetric observations of the active M dwarf AD Leo. Based on observations collected in 2016 with the ESPaDOnS instrument at CFHT, we derived the least-squares deconvolved Stokes profiles and detected linear polarisation signatures in spectral lines. At the same time, we discovered that the circular polarisation profiles corresponding to our data set are significantly weaker compared to all archival spectra of AD Leo, which exhibited approximately constant profiles over the timescale of at least 6 years until 2012. Magnetic maps obtained using Zeeman Doppler imaging confirm the sudden change in the surface magnetic field. Although the total magnetic field energy decreased by about 20% between 2012 and 2016, the field component responsible for the observed circular polarisation signatures corresponds to a stronger field occupying a smaller fraction of the stellar surface in the more recent map. These results represent the first evidence that active M dwarfs with dipole-dominated axisymmetric field topologies can undergo a long-term global magnetic variation.