Magnetic Fields in M Dwarfs: Rapid Magnetic Field Variability in EV Lacertae

We report here our spectropolarimetric observations obtained using the CFHT Espadons high-resolution spectrograph of two M dwarf stars that standard models suggest are fully convective: EV Lac (M3.5) and HH And (M5.5). The difference in their rotational velocity makes them good targets for studying the dependence of the magnetic field topology in M dwarfs on rotation. Our results reveal some aspects of the field topology in EV Lac and HH And. We measured mean longitudinal magnetic field strengths (Bz) in EV Lac ranging from 18 ± 3 to -40 ± 3 G. The Bz variations are seen to occur on a timescale of less than 50 minutes, which is significantly shorter than the rotation period, and are not due to a flaring event. We discuss some formation scenarios of the Zeeman signatures found in EV Lac. For HH And we could not detect circular polarization and thus we place an upper limit on Bz of 5 G.