Ultraviolet observations of AR Lacertae.

Abstract

Short-wavelength (1150-1950 Å) low-resolution and long-wavelength (2000-3000 Å) high-resolution ultraviolet spectra of the RS CVn type binary system AR Lac obtained by IUE are examined. The surface fluxes of short-wavelength chromospheric and transition region lines show average enhancements of 20-200 times to that of the quiet Sun, respectively. The emission line spectrum taken during the distortion wave minimum is further enhanced 34% in the transition region in comparison to the distortion wave maximum. The long-wavelength spectra exhibit two separate peaks at Mg II h and k lines corresponding to the Doppler shifted positions of the two stars which is interpreted as due to the individual chromospheric activity of each star. The average flux in the Mg II lines indicates that 70% of the emission is due to the K0 IV secondary and 30% is due to the G2 IV-V primary. It is suggested that both stars of AR Lac have equal activity per unit surface area.