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Cool component of CH Cygni in the 1998-2000 active phase

The cool components of symbiotic binaries play an important role in the structure and evolution of these objects. Determination of the physical conditions in their photospheres is an unavoidable step in understanding the physical mechanisms of symbiotic stars and the nature of the outbursts.We study the symbiotic binary CH Cygni, in particular the physical conditions in the photosphere of its cool component which has been classfied as an M late type giant due to its spectral appearance in the optical and near IR during quiescent and active phases. Several consecutive spectra obtained with different exposure times covering the region ~4800-9000 AA were obtained in November 1999. We have concentrated our investigations on the TiO absorption bands which are the most outstanding feature in these spectra. The bands, some of which give evidence of a resolved rotational structure, have been identified as members of the a, g, g', b, d, f, e band systems. Measurement of the band head relative intensities and intensities of some rotational lines have led to determination of excitation temperatures of the coolest layers in the red giant's photosphere. These results have been compared to the excitation temperatures during the 1987-1989 quiescent phase.

CH Cyg ; excitation temperature ; TiO molecule ; optical spectrum ; vibration ; rotation

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