The many faces of Capella : a search for rotational modulations and astudy of systematic velocities of emission lines in the ultraviolet.

Abstract

The ultraviolet emission of the spectroscopic binary Capella has been monitored with IUE over the course of half an orbital cycle (2 months). The first objective was to detect changes in the fluxes or shapes of ultraviolet emission lines in response to the evolution of large-scale active regions or their rotation onto and off the visible hemisphere of the active F-type secondary star. Capella proved to be surprisingly steady in its ultraviolet emissions. The second objective was to investigate systematic velocities of the low-excitation and high-excitation emission lines. It is found that the latter are significantly redshifted with respect to the former. The author concludes that the redshifts indicate true downflows of 10⁵K plasma in the stellar "transition zone", rather than the operation of a "P Cygni" effect in an optically thick outflow.