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Astron. Astrophys. 324, 943-948 (1997)

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Polar spots and stellar spindown: is dynamo saturation needed?

S.K. Solanki 1, S. Motamen 1 and R. Keppens 2

1 Institute of Astronomy, ETH-Zentrum, CH-8092 Zürich, Switzerland
2 FOM institute for Plasma-physics Rijnhuizen, P.O. Box 1207, 3430 BE Nieuwegein, The Netherlands

Received 20 September 1996 / Accepted 7 February 1997


Dynamo saturation is often invoked when calculating the rotational evolution of cool stars. At rapid rotation rates a saturated dynamo reduces the angular momentum carried away by the stellar wind. This, in turn, may explain the high rotation rates present in the distribution of rotation periods in young clusters.

Here we point out that concentration of magnetic flux near the poles of rapidly rotating cool stars provides an alternative to dynamo saturation. A high-latitude concentration of field on rapid rotators saturates the angular momentum loss induced by the stellar wind, due to the reduced torque arm. We show that the inclusion of this effect in model calculations is able to reproduce the observed high rotation rates without the need for dynamo saturation. Taken together with the results of O'Dell et al. (1995) this argues against dynamo saturation at low rotation rates.

Key words: stars: late-type – stars: rotation – stars: magnetic field – starspots

Send offprint requests to: S.K. Solanki

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© European Southern Observatory (ESO) 1997

Online publication: May 5, 1998