Magnetic field measurements of 6 M dwarfs

The analysis is performed on the spectra of 2 partially convective (AD Leo, DS Leo) and 3 fully convective (PM J18482+0741, CN Leo, Barnard star) M dwarfs, along with EV Lac whose mass is close to the fully-convective limit. Our analysis provides measurements of the average small-scale magnetic field, which are compared to longitudinal magnetic field and temperature variation measurements (dTemp) obtained from the same data.

We were able to detect the rotation period in the small-scale magnetic field series for 4 of the 6 stars in our sample. We find that the average magnetic field can vary by up to 0.3kG throughout the year (e.g., CN Leo), or of up to 1kG across rotation phases. The rotation periods retrieved from longitudinal and small-scale magnetic fields are found in agreement within error bars. dTemp measurements are found to anti-correlate with small- scale magnetic field measurements for three stars (EV Lac, DS Leo and Barnard's star).

The results demonstrate our ability to measure rotation periods from high-resolution data through small-scale magnetic field measurements, provided that the inclination of the observed targets is sufficiently large. We observe long-term fluctuations of the average magnetic field that could indicate magnetic cycles in the parent dynamo processes. These long-term variations appear mainly uncorrelated with large-scale magnetic field variations probed through longitudinal field measurements.

Large variations in the amplitude of the rotationally modulated signals, in particular, hint towards a change in the distribution of the surface inhomogeneities accessible to Zeeman broadening measurements.