DISKS, WINDS, AND VEILING CURTAINS - DISSECTING THE ULTRAVIOLET-SPECTRUM OF THE DWARF NOVA Z-CAMELOPARDALIS IN OUTBURST

We present a far-ultraviolet spectrum of the dwarf nova Z Cam near the peak of a normal outburst as observed with the Hopkins Ultraviolet Te lescope (HUT) on the Astro-2 mission. The continuum shape and luminosi ty are almost identical to an Astro-1 HUT spectrum of the same object in a similar state obtained about 4 years or 50 outburst episodes earl ier. This suggests that, following the onset of an outburst, the syste m quickly reaches a (quasi-) steady state that is insensitive to the i nteroutburst history. A variance analysis of the Astro-2 data reveals no evidence for spectral variability on a timescale of minutes. The rm s amplitude of any intrinsic fluctuations is <5% of the flux in both c ontinuum and lines. Z Cam's continuum can be described moderately well in terms of an optically thick, steady state accretion disk with M(ac c) similar or equal to 3 x 10(17) g s(-1) if the disk is assumed to ra diate as an ensemble of stellar atmospheres. This type of model reprod uces the turnover in the data at about 1050 Angstrom, but the predicte d spectrum is somewhat too blue at longer wavelengths, causing it to u nderpredict the flux longward of about 1500 Angstrom. This discrepancy appears to be resistant to all potential remedies we have tested, whi ch include differential limb-darkening, reddening, and white dwarf, bo undary layer, or hot spot spectral components. This suggests either th at our modeling of the standard accretion disk picture is too simplist ic - the effects of radial interactions and disk irradiation, for exam ple, have been ignored - or that the standard picture itself may requi re modification. Blackbody disk models actually match the data better at longer HUT wavelengths, but the redder color of these models is a d irect consequence of the neglect of all radiative transfer effects. Th e same neglect prevents blackbody models from reproducing the turnover in the spectrum and causes them to overpredict the accretion rate. We use a Monte Carlo line profile synthesis code to model five of the hi gh-ionization lines in Z Cam's spectrum in terms of a simple, kinemati c description of a rotating, biconical accretion disk wind. Adopting t he picture of such an outflow that has recently been proposed for anot her cataclysmic variable, UX UMa, we find that acceptable fits to the data can be obtained. The relative mean ionization fractions we derive for the ionic species included in our wind modeling appear to be cons istent with photoionization by a radiation field with T similar to 1.2 x 10(5) K. This temperature is within the range that has recently bee n inferred for Z Cam's soft X-ray component from ROSAT data and simila r to the boundary layer temperature that has been derived on the basis of Extreme-Ultraviolet Explorer (EUVE) observations for the dwarf nov a U Gem in outburst. An important feature of our adopted outflow model is the existence of a vertically extended, dense, slow-moving ''trans ition region'' between the disk photosphere and the fast-moving wind. Using a static LTE slab to crudely model this region, we find many of the absorption features in Z Cam's line spectrum that we have not mode led with our Monte Carlo code. The physical conditions expected in the extended disk atmosphere - n(e) similar to 10(12) cm(-3), N-H similar to 10(22) cm(-2), and T similar or equal to few x 10(4) K - are simil ar to those in the ''Fe II curtain'' that has been found to veil the w hite dwarf in the dwarf nova OY Car in quiescence.