2006PASP..118.1112G

From two decades of high-resolution spectroscopic measurements of Arcturus (HR 5340, K2 III), we identify a 2 yr modulation in the velocity span of the bisector of the Fe I λ6252.57 line. It seems likely that this is the rotation period, and we deduce the equatorial rotation rate to be 1.8±0.3 km/s, based on a radius of 25.4 R_☉. From detailed Fourier analysis of the broadening and shapes of the spectral lines, we derive vsini = 1.5±0.3 km/s and a radial-tangential macroturbulence dispersion of 5.2±0.2 km/s. The inclination of the rotation axis to the line of sight is then 58°±25°. Considering the periods found by H.-J. Choi et al. from variations in the Ca II H and K line emission, we suggest that giant stars have two or three active longitudes and, therefore, that periods found from the modulation of magnetic features will be one-half or one-third of the rotation period.