Magnetic Fields on M Dwarfs Measured with FeH

Measurements of magnetic fields on cool M dwarfs provide important information oil stellar dynamo models, however direct measurements are very difficult to obtain. Most of our knowledge about magnetic fields on cool stars is based on Zeeman broadened measurements of atomic and molecular lilies observed in their spectra. The FeH molecule is an excellent diagnostic for M dwarfs with many Zeeman sensitive lines persisting to very cool temperatures. Previous usage of FeH transitions for measuring magnetic fields was based on a comparison of Zeeman sensitive and insensitive lilies observed in active and inactive stars, thus, introducing an unknown bias to uncertainties in stellar temperatures, rotational and thermal broadening, etc. Thanks to the recent progress in our theoretical understanding of FeH we compared synthetic Stokes profiles with observations and determined mean magnetic field for a number of M dwarfs, independently of the thermal uncertainties. Also, we investigated the limits of determining magnetic fields by using intensity spectra only. This provides an opportunity to apply the method for cooler objects, including brown dwarfs.