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CH Cygni: TiO molecule as a probe of temperature variations in the atmosphere of the cool component

We report our determination of Tvib from band sys- tems of TiO in the visible spectra of the symbiotic star CH Cygni during its 1987-89 quiescent phase as well as And and 30 Her, which are taken as comparison stars. The problems usually met by attempting to perform quantitative analysis of the atmo- spheres of late M-type stars have been taken into account and discussed in detail. Assuming the LTE, Born-Oppenheimer ap- proximation, constant electronic transition moment for an elec- tronic band and neglecting the vibration-rotation interaction but thanks to the sensitivity of the method used, the vibrational tem- perature of the outer atmospheric layers of the cool component of CH Cygni has been determined to be changing from about 2000K to 2200K during the observed two year period. The er- rors of the applied method are small enough to allow the in- terpretation of this temperature variation as a result of intrinsic variariability of the cool component. The possible influence of the symbiotic mechanism has been excluded. The results ob- tained for CH Cygni fit well with the temperatures of the two comparison stars determined from the spectra on one hand and taken from the literature on the other.