Disks, Winds, and Veiling Curtains: Dissecting the Ultraviolet Spectrum of the Dwarf Nova Z Camelopardalis in Outburst

Abstract

We present a far-ultraviolet spectrum of the dwarf nova Z Cam near thepeak of a normal outburst as observed with the Hopkins UltravioletTelescope (HUT) on the Astro-2 mission. The continuum shape andluminosity are almost identical to an Astro-1 HUT spectrum of the sameobject in a similar state obtained about 4 years or 50 outburst episodesearlier. This suggests that, following the onset of an outburst, thesystem quickly reaches a (quasi-) steady state that is insensitive tothe interoutburst history. A variance analysis of the Astro-2 datareveals no evidence for spectral variability on a timescale of minutes.The rms amplitude of any intrinsic fluctuations is <5\% of the flux inboth continuum and lines.Z Cam's continuum can be described moderately well in terms of anoptically thick, steady state accretion disk with M(acc) similar orequal to 3 x 10(17) g s(-1) if the disk is assumed to radiate as anensemble of stellar atmospheres. This type of model reproduces theturnover in the data at about 1050 Angstrom, but the predicted spectrumis somewhat too blue at longer wavelengths, causing it to underpredictthe flux longward of about 1500 Angstrom. This discrepancy appears to beresistant to all potential remedies we have tested, which includedifferential limb-darkening, reddening, and white dwarf, boundary layer,or hot spot spectral components. This suggests either that our modelingof the standard accretion disk picture is too simplistic - the effectsof radial interactions and disk irradiation, for example, have beenignored - or that the standard picture itself may require modification.Blackbody disk models actually match the data better at longer HUTwavelengths, but the redder color of these models is a directconsequence of the neglect of all radiative transfer effects. The sameneglect prevents blackbody models from reproducing the turnover in thespectrum and causes them to overpredict the accretion rate.We use a Monte Carlo line profile synthesis code to model five of thehigh-ionization lines in Z Cam's spectrum in terms of a simple,kinematic description of a rotating, biconical accretion disk wind.Adopting the picture of such an outflow that has recently been proposedfor another cataclysmic variable, UX UMa, we find that acceptable fitsto the data can be obtained. The relative mean ionization fractions wederive for the ionic species included in our wind modeling appear to beconsistent with photoionization by a radiation field with T similar to1.2 x 10(5) K. This temperature is within the range that has recentlybeen inferred for Z Cam's soft X-ray component from ROSAT data andsimilar to the boundary layer temperature that has been derived on thebasis of Extreme-Ultraviolet Explorer (EUVE) observations for the dwarfnova U Gem in outburst.An important feature of our adopted outflow model is the existence of avertically extended, dense, slow-moving `'transition region'' betweenthe disk photosphere and the fast-moving wind. Using a static LTE slabto crudely model this region, we find many of the absorption features inZ Cam's line spectrum that we have not modeled with our Monte Carlocode. 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 Tsimilar or equal to few x 10(4) K - are similar to those in the `'Fe IIcurtain'' that has been found to veil the white dwarf in the dwarf novaOY Car in quiescence.