UV spectroscopy of Z Chamaeleontis - II. The 1988 January normal outburst

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Abstract
We present IUE observations of the 1988 January normal outburst of Z Cha and compare it with the 1987 April superoutburst (Paper I). The C IVλ1549, Si IVλ1397 and N Vλ1240 lines are ∼ 40 per cent weaker than in superoutburst and are all partially eclipsed, by 10–30 per cent, which is similar to the situation during superoutburst. The spectra are bluer than in superoutburst, indicating a less extensive vertical-disc structure during normal outburst. The most dramatic difference from superoutburst is that the outburst continuum flux shows < 10 per cent orbital variation away from eclipse, implying that there is no ‘cool’ bulge on the disc to occult the brighter inner disc periodically. During outburst, the total observed luminosity of Z Cha is two orders of magnitude less than in VW Hyi. We discuss the implications for the outburst mechanism in the two types of outburst. Simultaneous IR observations during the normal outburst reveal the secondary star. Assuming that the secondary is not heated, the disc has brightened by a factor of seven from its quiescent level, and now contributes 75 per cent of the total IR flux, compared to ≲ 30 per cent during quiescence.

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