Swifter than the Sun

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Science  24 Feb 2012:
Vol. 335, Issue 6071, pp. 893
DOI: 10.1126/science.335.6071.893-c

Stars form from the gravitational collapse of clouds of gas and dust. Before they attain a stable radius, they spin faster and faster as they contract, much like a spinning ice skater does as she folds her arms to her body. After gravitational collapse stops, stars start slowing down because of magnetic braking—the loss of stellar angular momentum as material gets removed from the star because of a magnetized stellar wind. Stars thus start as fast rotators and then slow down as they age. Current theories of angular momentum evolution reproduce the rotation data for Sun-like stars, but fail to account for those with masses lower than half that of the Sun. These very low-mass stars don't seem to slow down with age as much as expected, and the lower their masses, the faster they rotate at a given age. Reiners and Mohanty reexamined the theories of angular momentum evolution for low-mass stars, and show that angular momentum evolution must depend on stellar radius if the rotation of a star is related to its magnetic field strength. Stars with lower masses have smaller radii and lower magnetic braking efficiencies, meaning that they will take longer to slow down.

Astrophys. J. 746, 43 (2012).

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