On the angular momentum loss of late-type stars

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Abstract
The observed surface angular velocity of main-sequence stars shows a sharp decrease at about spectral type F6. We suggest that stars more massive than F6 cannot experience an appreciable angular momentum loss because their convection zones cannot sustain a magnetic dynamo: without a magnetic field the angular momentum loss is very small. The influence of rotation on the convective motions is essential for the existence of a solar type dynamo. Rotation can influence these convective motions only if the typical convective time is larger than the rotation time, i.e., if l/u c > 1/Ω, where uc and l are typical values of the convective velocity and mixing length in the lower part of the convection zone and Ω is the star's angular velocity. For main-sequence stars of different masses and chemical compositions we evaluate the dimensionless parameter uc/Ω[odot] l and show that it increases very sharply for stars whose mass, M, exceeds that defined by log(M /M [odot]) ≑ 0.1 (Ω[odot], and M[odot], are the sun's angular velocity and mass, respectively). Thus even for large angular velocities, magnetic dynamos are not feasible if log(M/M[odot]) appreciably exceeds 0.1.

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