Line formation regions of the UV spectrum of CI Cygni

Abstract

<i>Aims.</i> They are the interpretation of the emission line formation regions in CI Cygni. <i>Methods.</i> They involve the examination of radial velocities and fluxes of ultraviolet emission lines at different epochs, deduced from archival IUE and GRHS/HST spectra. <i>Results.</i> The line fluxes give electron densities and were in addition used to calculate emission measures, suggesting line formation in regions rather smaller than the binary separation. Examination of the radial velocities led to us to find a systematic redshift of the high ionization resonance lines with respect to the intercombination, and He ii lines. Possible explanations of the redshift and the high resolution GHRS C iv profile are discussed. We favour that involving resonance line absorption by a circum-binary region most probably in an asymmetric wind interaction shell or in a wind from the accretion disk.