CH Cygni 1987-89: The inactive state as a precursor to a new outburst

In order to help answering the question generally posed for symbiotic stars: are the apparently inactive periods quiescent at all, or should they be regarded merely as a transition between adjacent outbursts, we tried to establish correlation between the subtle phenomena in the time evolution of the cool giant˘s photosphere and emission line profiles of the symbiotic star CH Cygni during the period 1987-89. Optical spectra of this star in its inactive state taken at the Haute Provence Observatory as well as the spectra of late type giants of the same spectral type have been used. The comparison of the results obtained suggests that the time evolution of the physical parameters of CH Cyg reflect the symbiotic phenomena rather than the intrinsic variability of the cool component.The time behaviour of the Balmer emission line profiles rules out the general validity of the established models. An acceptable model valid at least for the observed period has been proposed. According to it, an envelope of variable optical thickness surrounds the hot component. The development of its inner radius, of Halpha and H beta line profiles and of the lambda5007~[OIII] nebular line leads to the conclusion that the inactive state is not stationary, but is gradually evolving in time toward the new activity of the star.