POLAR SPOTS AND STELLAR SPINDOWN - IS DYNAMO SATURATION NEEDED

Dynamo saturation is often invoked when calculating the rotational evo lution of cool stars. At rapid rotation rates a saturated dynamo reduc es the angular momentum carried away by the stellar wind. This, in tur n, may explain the high rotation rates present in the distribution of rotation periods in young clusters. Here we point out that concentrati on of magnetic flux near the poles of rapidly rotating cool stars prov ides an alternative to dynamo saturation. A high-latitude concentratio n of field on rapid rotators saturates the angular momentum loss induc ed by the stellar wind, due to the reduced torque arm. We show that th e inclusion of this effect in model calculations is able to reproduce the observed high rotation rates without the need for dynamo saturatio n. Taken together with the results of O'Dell et al. (1995) this argues against dynamo saturation at low rotation rates.