The temperature dependence of rotation and turbulence in giant stars

Rotation and turbulence velocities of 23 luminosity class III giants are obtained from their line profiles by Fourier analysis. The rotational discontinuity at G5 III is clearly delineated. It is shown that a dynamo brake is the most likely explanation, that the brake turns on and off before the star evolves more than one- or two-tenths in spectral class, and that all giants leave the braking stage with the same rotation rate of 5 km/s. The macroturbulence dispersion is double valued for most stars in the G8 III to K2 III interval. The larger of the two values, derived from the weaker lines, shows a linear change with spectral class from 7.0 km/s at G5 III to 4.6 km/s at K2 III. Expressed as a function of effective temperature, the macroturbulence dispersion varies as Teff to the 2.6 power.