Large-scale magnetic topologies of cool stars

Spectropolarimetry is the optimal tool to investigate large-scale magnetic topologies of cool low-mass stars. From phase resolved spectropolarimetric data sets, tomographic imaging can be used to obtain a spherical harmonics decomposition of magnetic fields at the surfaces of stars, and thus reveal, e.g., how strong and complex such fields are, to what degree they are axisymmetric and how they split between their poloidal and toroidal components. In this paper, we review the current knowledge on large-scale magnetic topologies of cool stars, with particular emphasis on the latest results obtained with the new spectropolarimeters ESPaDOnS@CFHT and NARVAL@TBL. Observations show that stars with masses larger than 0.5 M_⊙ host significant toroidal fields whenever their Rossby number Ro is smaller than about 1, with their poloidal field getting stronger and more non-axisymmetric as Ro decreases. On the opposite, stars with masses lower than 0.5 M _⊙ host strong poloidal fields in a mostly axisymmetric dipolar configuration. Spectropolarimetric studies can also reveal how differential rotation and magnetic cycles vary with stellar parameters.