The active phase of the hot component of Z Andromedae

Abstract

Z And is considered the prototype of the symbolic class. Quiescence data show that its hot component lies close to the central stars of planetary nebulae in the H-R diagram. This system underwent an outburst at the beginning of 1984. The activity phase lasted for approximately 900 days with two maxima in 1984 April and 1985 December. In this work we present UV and radio data which cover the entire activity phase, as well as part of the pre- and post-outburst period. All the evidence indicated that the two maxima are different unrelated outbursts in which an optically thick shell was ejected at moderately high velocities of the order of 200-300 km/s. The two outbursts were characterized by a general increase at all the wavelength ranges (except in the IR). In the UV there is a broadening of all the emission lines, and a change in the shape of the continuum, which became cooler. The behavior of lines of different ionization levels shows that the shell ejected in the second outburst was more massivce, leading to a substantial blocking of the UV radiation and the subsequent effects on the high-ionization lines.