Decaying stellar magnetic fields, magnetic braking - Evidence from magnetic observations in Orion OB1

The magnetic fields of 13 Ap and helium-weak stars in the Orion OB 1 association are observed and compared to those of older magnetic stars. It is found that the magnetic stars in Orion have magnetic fields stronger than those of the older stars by a factor of three, in agreement with a fossil origin of the fields. The short e-folding time of decay, which is estimated to be of the order of 100 million years, is seen as evidence for the view that internal mass motions are present in upper-main-sequence stars, with potentially important effects on their evolution. The periods of variation, although uncertain, suggest that young magnetic stars rotate faster than their older counterparts. This difference implies magnetic braking on the main sequence.