Analyses of three-dimensional magnetohydrodynamic instability of antisolar latitudinal differential rotation in F, G, and K stars

Motivated by observations that only a very few stars have been found to have antisolar differential rotation, much weaker in amplitude than that of the Sun, we analyze the stability of antisolar and solar-type latitudinal differential rotations in the tachoclines of typical F, G, and K stars. We employ two three-dimensional thin-shell models, one for a Boussinesq but nonhydrostatic system and the other for a hydrostatic but non-Boussinesq system. We find that, in general, the combination of toroidal field band and differential rotation is more unstable, and unstable for lower toroidal fields, for antisolar than for solar-type differential rotation. In the antisolar case, the instability is always found to weaken the differential rotation, even if the primary energy source for the instability is the magnetic field. This favors surface antisolar differential rotations in stars being weaker than solar types, if the instability in the tachocline is felt at the surface of the star. This is most likely to happen in F stars, whose convection zones are much thinner than they are in G and K stars. This effect could help explain why the antisolar differential rotations that have been found are very weak compared with the rotation of the Sun.