NEW OBSERVATIONAL LIMITS ON DYNAMO SATURATION IN YOUNG SOLAR-TYPE STARS

We present statistically robust observational evidence which imposes n ew limits on dynamo saturation in young solar-type stars. These are in ferred from the increasing amplitude of the V-band optical flux with r otation, caused by the filling of the disc with surface spots in a non -axisymmetric pattern. Assuming spot coverage acts as a tracer of the total magnetic surface flux, we find a minimum saturation level betwee n 6 - 10 times greater than the inferred from chromospheric and transi tion line indicators, assuming that saturation occurs. We suggest that these new limits imply a minimum rotation for saturation of the dynam o and that for high rotation rates starspot coverage acts as an altern ative diagnostic for the stellar dynamo to the chromospheric and trans ition region line emission fluxes. The fact that the dynamo does not a ppear to saturate at the lower rotation rates indicated by chromospher ic indicators should assist evolutionary braking models that have to e xplain the sudden spin-down of young fast rotating G-dwarfs.