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Magnetic fields affect the local structure of the photosphere of stars. Theycan considerably influence the radiative properties near the optical surface,flow velocities, and the temperature and pressure profiles. We aim atunderstanding qualitatively the influence of small magnetic flux concentrationsin unipolar plage regions on the centre-to-limb variation of the intensity andits contrast and on the shape of spectral line profiles in cool main-sequencestars. We analyse the bolometric and continuum intensity and its angulardependence of 24 radiative magnetohydrodynamic simulations of the near-surfacelayers of main-sequence stars with six different sets of stellar parameters(spectral types F to early M) and four different average magnetic fieldstrengths (including the non-magnetic case). We also calculated disc-integratedprofiles of three spectral lines. The small magnetic flux concentrations formedin the magnetic runs of simulations have a considerable impact on the intensityand its centre-to-limb variation. Spectral lines are not only broadened owingto the Zeeman effect, but are also strongly affected by the modifiedthermodynamical structure and flow patterns. This indirect magnetic impact onthe line profiles is often bigger than that of the Zeeman effect. The effectsof the magnetic field on the radiation leaving the star can be considerable andis not restricted to spectral line broadening and polarisation by the Zeemaneffect. The inhomogeneous structure of the magnetic field on small lengthscales and its impact on (and spatial correlation with) the localthermodynamical structure and the flow field near the surface influence themeasurement of the global field properties and stellar parameters. Theseeffects need to be taken into account in the interpretation of observations.

astronomy and astrophysics

Three-dimensional simulations of near-surface convection in  main-sequence stars. IV. Effect of small-scale magnetic flux concentrations  on centre-to-limb variation and spectral lines