Rotational modulation and flares on RS CVn and BY DRA stars. IV. The spatially resolved chromosphere of AR Lacertae.

The authors observed the RS CVn system AR Lacertae systematically over an orbital period with the International Ultraviolet Explorer in October 1983. Contemporaneous radio observations were obtained at the Very Large Array. The spectra of the Mg II k emission line were analyzed using a Doppler imaging technique. In this way, the authors identified three discrete regions of emission in the outer atmosphere of the K star - two "plages" and a chromospheric brightening that was related to a radio flare. The widths of the plage profiles indicate that the two plages together cover about 2% of the visible stellar hemisphere, and their v sin i values indicate that they lie close to the equator of the K star. The Mg II k surface flux in the plages is about five times the mean Mg II k surface flux of the K star. The authors then used the far-ultraviolet spectra obtained at the eclipse phases to separate the individual contributions of the two stars and the plage and flare regions in order to estimate their line surface fluxes.