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Observational and theoretical investigations provide evidence for non-uniformspot and magnetic flux distributions on rapidly rotating stars, which have asignificant impact on their angular momentum loss rate through magnetisedwinds. Supplementing the formalism of MacGregor & Brenner (1991) with alatitude-dependent magnetised wind model, we analyse the effect of analyticallyprescribed surface distributions of open magnetic flux with different shapesand degrees of non-uniformity on the rotational evolution of a solar-like star.The angular momentum redistribution inside the star is treated in a qualitativeway, assuming an angular momentum transfer between the rigidly-rotatingradiative and convective zones on a constant coupling timescale of 15 Myr; forthe sake of simplicity we disregard interactions with circumstellar disks. Wefind that non-uniform flux distributions entail rotational histories whichdiffer significantly from those of classical approaches, with differencescumulating up to 200% during the main sequence phase. Their impact is able tomimic deviations of the dynamo efficiency from linearity of up to 40% andnominal dynamo saturation limits at about 35 times the solar rotation rate.Concentrations of open magnetic flux at high latitudes thus assist in theformation of very rapidly rotating stars in young open clusters, and ease thenecessity for a dynamo saturation at small rotation rates. However, since ourresults show that even minor amounts of open flux at intermediate latitudes, asobserved with Zeeman-Doppler imaging techniques, are sufficient to moderatethis reduction of the AM loss rate, we suggest that non-uniform fluxdistributions are a complementary rather than an alternative explanation forvery rapid stellar rotation.

A further “degree of freedom” in the rotational evolution of stars