DETECTION OF STRONG MAGNETIC-FIELDS ON M-DWARFS

We have detected Zeeman splitting of the Fe I line (g(eff) = 2.5) at 8 468.40 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 i nfer field strengths of 2-4 kG, independent of uncertainties in model atmospheres. Similar observations of a sequence (M0 V-M5 V) of low-act ivity M dwarfs demonstrate that the wing components in the 8468.40 Ang strom line are not due to the ubiquitous TiO lines in the vicinity. Th is strongly suggests that discrepancies in the shape of the magnetical ly sensitive iron line are due to magnetic fields rather than differen ces in photospheric temperature. By fitting the ratio of active to ina ctive 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 magneti c field strength, spatially across the surface and/or with depth. The twofold increase in field strength relative to G and K dwarfs is predi cted by flux tube equilibrium arguments, but the 1 kG difference in th e measured field strengths for these two stars suggests that field str engths may also grow with activity as the photospheric filling factor approaches unity.