A series of stellar models of spectral type G is computed to study the rotation laws resulting from mean-field equations. The rotation laws of the slowly rotating Sun, the rapidly rotating MOST stars Eri and κ 1 Cet, and the rapid rotators R58 and LQ Lup can be easily reproduced. We also find that differences in the depth of the convection zone cause large differences in the surface rotation law and that the extreme surface shear of HD 171488 can only be explained with an artificially shallow convection layer. We verify the thermal wind equilibrium in rapidly rotating G dwarfs and find that the polar subrotation (dΩ/dz < 0) is due to the baroclinic effect and the equatorial superrotation (dΩ/dr > 0) is caused by the Reynolds stresses. In the bulk of the convection zones where the meridional flow is slow and smooth, the thermal wind equilibrium holds between the centrifugal and the pressure forces. It does not hold, however, in the bounding shear layers including the equatorial region where the Reynolds stresses dominate.

We continue the study of the effects of a strong magnetic field on the temperature distribution in the crust of a magnetized neutron star (NS) and its impact on the observable surface temperature. Extending the approach initiated in Geppert et al. (2004), we consider more complex and, hence, more realistic, magnetic field structures but still restrict ourselves to axisymmetric configurations. We put special emphasis on the heat blanketing effect of a toroidal field component. We show that asymmetric temperature distributions can occur and a crustal field consisting of dipolar poloidal and toroidal components will cause one polar spot to be larger than the opposing one. These two warm regions can be separated by an extended cold equatorial belt. As an example we present an internal magnetic field structure which can explain, assuming local blackbody emission, both the X-ray and optical spectra of the isolated NS RXJ 1856-3754, the hot polar regions dominating the X-ray flux and the equatorial belt contributing predominantly to the optical emission. We investigate the effects of the resulting surface temperature profiles on the observable lightcurve which an isolated thermally emitting NS would produce for different field geometries. The lightcurves will be both qualitatively (deviations from sinusoidal shape) and quantitatively (larger pulsed fraction for the same observational geometry) different from those of a NS with an isothermal crust. This opens the possibility to determine the internal magnetic field strengths and structures in NSs by modeling their X-ray lightcurves and spectra. The striking similarities of our model calculations with the observed spectra and pulse profiles of isolated thermally emitting NSs is an indication for the existence of strong magnetic field components maintained by crustal currents.

Abstract. We present shell dynamo models for the solar convection zone with positive α-effect in the northern hemisphere and a meridional circulation which is directed equatorward at the bottom and poleward at the top of the convection zone. Two different rotation patterns are used: a simple variation of the rotation rate with depth and the rotation law as derived by helioseismology. Depending on the Reynolds number associated with the meridional flow, the dynamo shows a whole "zoo" of solutions. For sufficiently small values of the eddy magnetic diffusivity (10 11 cm 2 /s), field advection by the meridional flow becomes dominant and even changes the character of the butterfly diagram. Flow amplitudes of a few m/s are then sufficient to turn the originally "wrong" butterfly diagram into a "solar-type" butterfly diagram, i.e. with activity belts drifting equatorward. This effect can easily be demonstrated with a super-rotation law (∂Ω/∂r > 0) with Ω independent of the latitude. The situation is much more complicated for the "real" rotation law with the observed strong negative shear at high latitudes. With zero meridional flow, oscillating solutions are found without any latitudinal migration of the toroidal field belts, neither poleward nor equatorward. Small but finite flow amplitudes cause the magnetic field belts to drift poleward while in case of fast flow they move equatorward.

Abstract. We present a model for the differential rotation of the present Sun as well as a solar-type star during its pre-main-sequence evolution. The model is based on the mixing-length theory of convective heat transport and a standard solar model. The resulting rotation law is in good agreement with observations and only weakly dependent on the mixing-length parameter. For the present Sun, the normalized horizontal shear decreases with increasing rotation rate, but the total shear is roughly constant. We then follow the Sun's evolutionary track from the beginning of the contraction to the arrival on the ZAMS. While at an age of 30 Myr the total shear is very similar to that of the present Sun, it is much smaller on the Hayashi track.

The spectroscopic variability of Arcturus hints at cyclic activity cycle and differential rotation. This could provide a test of current theoretical models of solar and stellar dynamos. To examine the applicability of current models of the flux transport dynamo to Arcturus, we compute a mean-field model for its internal rotation, meridional flow, and convective heat transport in the convective envelope. We then compare the conditions for dynamo action with those on the Sun. We find solar-type surface rotation with about 1/10th of the shear found on the solar surface. The rotation rate increases monotonically with depth at all latitudes throughout the whole convection zone. In the lower part of the convection zone the horizontal shear vanishes and there is a strong radial gradient. The surface meridional flow has maximum speed of 110 m/s and is directed towards the equator at high and towards the poles at low latitudes. Turbulent magnetic diffusivity is of the order $10^{15}$--$10^{16} {\rm cm^2/s}$. The conditions on Arcturus are not favorable for a circulation-dominated dynamo