Detection of strong magnetic fields on M dwarfs

We have detected Zeeman splitting of the Fe I line (g(eff) = 2.5) at 8468.40 Angstrom in the very active M4.5 Ve stars Gliese 729 and Gliese 873 (EV Lac). High-resolution (R = 120,000), low-noise (similar to 0.5%) spectra show clear Zeeman-shifted sigma components from which we infer field strengths of 2-4 kG, independent of uncertainties in model atmospheres. Similar observations of a sequence (M0 V-M5 V) of low-activity M dwarfs demonstrate that the wing components in the 8468.40 Angstrom line are not due to the ubiquitous TiO lines in the vicinity. This strongly suggests that discrepancies in the shape of the magnetically sensitive iron line are due to magnetic fields rather than differences in photospheric temperature. By fitting the ratio of active to inactive line profiles with simple one-component models, we estimate that 50 +/- 13% of the photosphere of EV Lac. is covered by 3.8 +/- 0.5 kG magnetic fields, while 50 +/- 13% of Gliese 729 is covered by 2.6 +/- 0.3 kG fields. This confirms earlier reports of large magnetic fields on M dwarf flare stars. We see evidence for a distribution of magnetic field strength, spatially across the surface and/or with depth. The twofold increase in field strength relative to G and K dwarfs is predicted by flux tube equilibrium arguments, but the 1 kG difference in the measured field strengths for these two stars suggests that field strengths may also grow with activity as the photospheric filling factor approaches unity.