The evolution of rotation and activity in young open clusters: IC 2391

We have undertaken a program of ground- and space-based observations to measure photometric rotation periods and X-ray luminosities for late-type stars in the young open cluster IC 2391. At an age of similar to 30 Myr, IC 2391 is an ideal cluster in which to observe conditions at the zero-age main sequence (ZAMS) since the solar-type stars within this cluster have not been on the main sequence long enough to undergo significant magnetic braking. For late-type stars, the ZAMS represents the endpoint of pre-main-sequence evolution, wherein gravitational contraction and interactions with circumstellar accretion disks dominate the angular momentum evolution of the star. The ZAMS also represents the beginning of the main-sequence life of the star, wherein its subsequent evolution is dominated by magnetic braking. Knowledge of ZAMS stellar parameters, such as rotation rates and activity levels, is therefore crucial for accurate theoretical model predictions. Our survey of IC 2391 X-ray sources has revealed a number of new candidates for cluster membership, most of these being late-type stars. Among the solar-type stars in IC 2391, we find a factor of similar to 20 spread in the distribution of rotation periods and also in the distribution of X-ray luminosities. We therefore conclude that stars arrive on the ZAMS with a wide range of rotation rates and activity levels. When we compare IC 2391 with older clusters, such as the Pleiades and the Hyades, we find there is an overall decline in the median rotation rate and X-ray luminosity with age, but the dispersion in X-ray luminosity, as a fraction of the median value, actually grows larger. This is because the young, rapidly rotating stars lie along a plateau of magnetic saturation, where L(X) has a weak dependence on rotation period, while for the older, more slowly rotating stars, the weaker L, has a strong dependence on rotation period. We find an abrupt turnover in the Rossby diagram near N-R = 0.3, which suggests there is a fundamental change in the nature of dynamo activity for rapidly rotating stars.