1 research outputs found
Monitoring the large-scale magnetic field of AD~Leo with SPIRou, ESPaDOnS and Narval. Toward a magnetic polarity reversal?
One manifestation of dynamo action on the Sun is the 22-yr magnetic cycle,
exhibiting a polarity reversal and a periodic conversion between poloidal and
toroidal fields. For M dwarfs, several authors claim evidence of activity
cycles from photometry and analyses of spectroscopic indices, but no clear
polarity reversal has been identified from spectropolarimetric observations.
Our aim is to monitor the evolution of the large-scale field of AD Leo, which
has shown hints of a secular evolution from past dedicated spectropolarimetric
campaigns. We analysed near-infrared spectropolarimetric observations of the
active M dwarf AD Leo taken with SPIRou between 2019 and 2020 and archival
optical data collected with ESPaDOnS and Narval between 2006 and 2019. We
searched for long-term variability in the longitudinal field, the width of
unpolarised Stokes profiles, the unsigned magnetic flux derived from Zeeman
broadening, and the geometry of the large-scale magnetic field using both
Zeeman-Doppler Imaging and Principal Component Analysis. We found evidence of a
long-term evolution of the magnetic field, featuring a decrease in axisymmetry
(from 99% to 60%). This is accompanied by a weakening of the longitudinal field
(-300 to -50 G) and a correlated increase in the unsigned magnetic flux (2.8 to
3.6 kG). Likewise, the width of the mean profile computed with selected
near-infrared lines manifests a long-term evolution corresponding to field
strength changes over the full time series, but does not exhibit modulation
with the stellar rotation of AD Leo in individual epochs. The large-scale
magnetic field of AD Leo manifested first hints of a polarity reversal in late
2020 in the form of a substantially increased dipole obliquity, while the
topology remained predominantly poloidal and dipolar. This suggests that
low-mass M dwarfs with a dipole-dominated magnetic field can undergo magnetic
cycles.Comment: 26 pages, 18 figures, 8 table