Post-Main-Sequence Changes in Rotational Velocities

For 1377 dwarfs and 381 giants of types B and A, we present mean observed projected rotational velocities for normal plus abnormal spectral types combined. For the dwarfs, V sin i is statistically constant at 127 ± 15 km s^-1 from B0 V to A5 V. Using the interior models of Bertelli et al., we predict the rotational velocities of the giants. We assume an age of 30 × 10^6 yr for the observed dwarfs. The predicted giant rotational velocities agree well with the observed values if angular momentum is conserved in rigid-body rotation; the observed minus the computed is -7 ± 4 km s^-1. Significant loss of angular momentum by mass loss (stellar winds) cannot occur because the observed rotational velocities of the giants are already at the maximum level allowed by either conservation mechanism. Comparison with three other studies indicates that angular momentum is conserved by rigid-body rotation if the expansion is less than a factor of 4, but conserved in shells if the expansion is more.