Multifrequency observations of the eclipsing symbiotic triple system CH Cyg during the 1992--94 active phase

The authors present UBV photoelectric photometry, plus optical and ultraviolet low- and high-resolution spectroscopy and MERLIN, VLA and JCMT radio/mm-wave observations of the symbiotic star CH Cyg during the period 1989.7-1995.5 which covers the 1992 outburst. On 1990 July 22 the spectrum displayed only absorption features characterized by broad hydrogen profiles, indicating that CH Cyg underwent a dramatic transient change, losing its circumstellar shell. In early 1992 the star's brightness increased by about 2.5 mag in the U band, and then gradually increased from U ≡ 8.5 at that time to U ≡ 6.7 in mid-1994. In 1992 September - November and 1994 October - December, the two deep minima caused by an eclipse of the active component by the cool component in the symbiotic pair of the triple CH Cyg system were observed. A decrease of the star's brightness to U ≡ 11 in mid-1995 indicates that this active phase is over. During the outburst the energy distribution of the hot component exhibits two maxima: the first in the far-UV (1500-1900 Å) region, and the second one around the U band. Evolution of the spectrum and variation in the hydrogen line profiles result from an irregular, high-velocity and in part bipolar outflow from the central star. Radio observations indicate a radio flux increase and suggest an expansion of the outer radius of the wind material around the active component.