Near-infrared long-term monitoring of AD Leo with SPIRou Towards a magnetic polarity reversal?

Abstract

A clear manifestation of dynamo action operating in the interior of stars is given by magnetic cycles. The Sun is an important benchmark as we observe a polarity reversal over a time scale of 11 yr. For M dwarfs, numerous studies on photometry and spectroscopic indices reported evidence of activity cycles, but no polarity reversal has been observed so far. Long-term spectropolarimetric surveys can reveal these changes and therefore provide constraints on dynamo theories. In particular, we can shed more light on the variety of magnetic field geometries observed with tomographic techniques: strong, axisymmetric and simple against weak, non-axisymmetric and complex. This is also of fundamental interest to understand the impact of magnetic fields on planet formation, characterisation and habitability around active stars. We present the preliminary analysis of SPIRou spectropolarimetric observations of the active M dwarf AD Leo, covering 2019 and 2020. We examined the long-term behaviour of the longitudinal magnetic field and we recovered the magnetic field geometry via Zeeman-Doppler Imaging. Including the optical data sets investigated in previous studies, we found evidence of a secular evolution of the magnetic field in the form of a varying intensity and reduced axisymmetry (from 90% to 50%), while the topology remained predominantly poloidal and dipolar. This is a first hint that low-mass M dwarfs with a dipole dominated magnetic field can undergo magnetic cycles.