Rotation, magnetic braking, and dynamos in cool giants and subgiants

Details Rotation, magnetic braking, and dynamos in cool giants and subgiants Rotation, magnetic braking, and dynamos in cool giants and subgiants

Oct 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...278...51s&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 278, no. 1, p. 51-67

Astronomy and Astrophysics

Astrophysics

33

Giant Stars, Stellar Evolution, Stellar Magnetic Fields, Stellar Rotation, Subgiant Stars, Angular Momentum, Dynamo Theory, Stellar Convection, Stellar Interiors

Scientific paper

We analyze the rotation rates of giants and subgiants evolving from the main sequence to the red giant branch. A comparison of observational data with rotational velocities expected from evolutionary models incorporating stellar magnetic braking results in the following conclusions: (a) subgiants have lost most of their total angular momentum prior to reaching the red giant branch, while luminosity class III giants have lost typically 50% of their total angular momentum by that time; (b) stellar magnetic braking, and hence probably the stellar dynamo, does not switch on at its full strength at the onset of envelope convection, but rapidly increases in strength over an interval of at least Delta(B - V) approximately equal to 0.2-0.3; (c) the radiative-equilibrium interior and the convective envelope appear to exchange angular momentum on a time scale short compared to the time scale associated with the loss of angular momentum through a magnetized wind so that the rotational shear between envelope and interior is limited to at most about a factor of four and probably less; (d) magnetic braking is strongly enhanced in giants as compared to dwarf stars: the parametrization of the angular momentum loss used in the present simulations supports a proportionality with the sixth power of the radius.

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