IUE high-resolution observations of the symbiotic star CH Cygni: confirmation of the triple-star model

We present and analyse ultraviolet high-resolution spectra of CH Cyg taken during the whole period of the International Ultraviolet Exploler (IUE) mission. Our analysis of the observed variation in the line spectrum confirms a triple-star model recently suggested for CH Cyg. Radial velocities of emission lines, which are subject to eclipse in the symbiotic pair are placed at antiphase to those of the neutral metal absorptions from the infrared/optical spectrum. This is direct evidence for the orbital motion of the hot component in the inner binary of the triple system. Variation in the line profiles of highly ionized elements and Ly alpha also supports the triple-star nature of CH Cyg. Fluxes in the SIIV, SIII and AlIII lines decreased considerably during the eclipse. Circumstellar emission in Ly alpha is produced by the radiation of the hot star. The central part of its profile (similar to 1216-1220 Angstrom) remains unchanged during the eclipse, it arises at larger distances from the star. This implies that the hot component radiation was, in fact, practically constant during this time, and thus its observed considerable decline had to be caused only by the eclipse. The outer parts of the Ly alpha profile extend up to +/-2 200 km s(-1), they are subject to eclipse, and thus reflect a high velocity outflow originating from the central star.