A STUDY OF EXTREME-ULTRAVIOLET EMISSION FROM CATACLYSMIC VARIABLES

Voyager far- and extreme-ultraviolet (500-1200 A) spectrophotometric observations of five cataclysmic variables (the dwarf novae SS Cyg and VW Hyi and the novalike variables V3885 Sgr, RW Sex, and IX Vel) are combined with neutral hydrogen column densities derived from the curve-of-growth analysis of interstellar absorption lines in high-resolution IUE spectra to place upper limits on the emitted flux in the 600-700 A EUV band. The Voyager observations of SS Cyg were obtained during outburst, and those of VW Hyi were obtained during both normal and superoutbursts. No EUV emission was observed in any of the five cataclysmic variables. Detailed accretion disk model calculations show that most of the 600-7000 Å flux in these systems should originate in the inner accretion disk rather than in the boundary layer. The relatively high neutral hydrogen column densities (>1019 cm-2) derived for SS Cyg, V3885 Sgr, RW Sex, and IX Vel when combined with the model calculations produce estimates for the observed 600-700 Å EUV flux that are substantially smaller than the Voyager upper limits. Thus, for these four systems, no scientifically useful information regarding the intrinsic 600-700 Å flux is available. However, for VW Hyi, the low neutral hydrogen column (≈6 × 1017 cm-2) and excellent Voyager superoutburst data place the observed upper limit to the 600-700 Å flux well below the expected EUV flux from the model calculations. Possible explanations for this EUV "deficiency" in VW Hyi are presented.