The MG II emission in W UMa-type binaries.

Abstract

Low-resolution IUE spectra of the Mg II emission line in eight contact binaries are presented. They are discussed together with the available observations in both resolution modes for other binaries, 15 systems altogether. The relative flux R(Mg II) = f(Mg II)/f(bol), which is known to be independent of the orbital phase, is used as a measure of chromospheric activity. The results on R(Mg II) are discussed together with the existing data on R(TR) and R(X) where TR and X are the transition-region and coronal X-ray emissions. The mutual relations between these quantities strongly depend on how R(Mg II) is estimated for the F-type contact binaries; it is possible that the scaling laws existing for non-contact stars are applicable even to contact binaries.