ULTRAVIOLET EMISSION-LINE RATIOS OF CATACLYSMIC VARIABLES

We present a statistical analysis of the ultraviolet emission lines of cataclysmic variables (CVs) based on approximate to 430 ultraviolet s pectra of 20 sources extracted from the International Ultraviolet Expl orer Uniform Low Dispersion Archive. These spectra are used to measure the emission-line fluxes of N V, Si IV, C IV, and He II and to constr uct diagnostic flux ratio diagrams. We investigate the flux ratio para meter space populated by individual CVs and by various CV subclasses ( e.g., AM Her stars, DQ Her stars, dwarf novae, nova-like variables). F or most systems, these ratios are clustered within a range of similar to 1 decade for log Si IV/C IV approximate to 0.5 and log He II/C IV N approximate to-1.0 and similar to 1.5 decades for log N V/C IV approx imate to-0.25. These ratios are compared to photoionization and collis ional ionization models to constrain the excitation mechanism and the physical conditions of the line-emitting gas. We find that the collisi onal models do the poorest job of reproducing the data. The photoioniz ation models reproduce the Si IV/C IV line ratios for some shapes of t he ionizing spectrum, but the predicted N V/C IV line ratios are simul taneously too low by typically similar to 0.5 decades. Worse, for no p arameters are any of the models able to reproduce the observed He II/C IV line ratios; this ratio is far too small in the collisional and sc attering models and too large by typically similar to 0.5 decades in t he photoionization models.