MHD simulations of near-surface convection in cool main-sequence stars
  The solar photospheric magnetic field is highly structured owing to its interaction with the convective flows. Its local structure has a strong influence on the profiles of spectral lines not only by virtue of the Zeeman effect, but also through the modification of the thermodynamical structure (e.g. line weakening in hot small-scale magnetic structures). Many stars harbor surface magnetic fields comparable to or larger than the Sun at solar maximum. Therefore, a strong influence of the field on the surface convection and on spectral line profiles can be expected. We carried out 3D local-box MHD simulations of unipolar magnetized regions (average fields of 20, 100, and 500G) with parameters corresponding to six main-sequence stars (spectral types F3V to M2V). The influence of the magnetic field on the convection and the local thermodynamical structure were analyzed in detail. For three spectral lines, we determined the impact of the magnetic field on the disc-integrated Stokes-I profiles. Line weakening has in many cases a stronger impact on the spectral line profiles than the Zeeman effect. Moreover, for some stars, the correlation between the magnetic field and the vertical velocity strongly influences the line shapes. These effects can impair determinations of stellar magnetic fields since currently used methods neglect the local structure of the magnetic field and its interaction with the convective flows. The MHD simulations presented can be used to quantify these effects and thus help to improve magnetic field measurements of cool main-sequence stars. 
Benjamin Beeck, Manfred Sch\"ussler, Ansgar Reiners
astro-ph.SR