Magnetic braking of Sun-like stars in the alpha Persei open cluster
P. Petit, J.-F Donati, J. Barnes, A. Collier Cameron, M. Jardine, S.K. Solanki, B. Carter, C. Brown, S. Marsden

Abstract: Many young solar analogues in open clusters possess short rotation periods, a puzzling observation indicating that the magnetic braking supposed to act during pre-main sequence evolution is less efficient than expected on these objects. Several scenarii have been proposed to explain why such stars have conserved their angular momentum, all of them making predictions on the geometry of the stellar magnetic field. ESPaDOnS is the first instrument able to provide a direct test of such models, with the possibility to simultaneously estimate the respective efficiency of magnetic braking associated with the magnetized wind and with coronal mass ejections. Here we present preliminary results of this program, with a time-series of ESPaDOnS observations of the fast-rotating Sun-like star AP 149 in the alpha Persei open cluster. The spectropolarimetric data set is used to reconstruct a photospheric magnetogram and measure its evolution under the effect of differential rotation. We also map the spatial structure and short-term evolution of coronal condensations, using the rotational modulation of Halpha emission.