The Rotation of Arcturus and Active Longitudes on Giant Stars

Abstract

ABSTRACTFrom 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−1, based on a radius of 25.4 R⊙. From detailed Fourier analysis of the broadening and shapes of the spectral lines, we derive vsin i = 1.5 ± 0.3 km s−1 and a radial‐tangential macroturbulence dispersion of 5.2 ± 0.2 km s−1. 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.