Ultraviolet Emission‐line Ratios of Cataclysmic Variables

We present a statistical analysis of the ultraviolet emission lines ofcataclysmic variables (CVs) based on $\approx 430$ ultraviolet spectra of 20sources extracted from the International Ultraviolet Explorer Uniform LowDispersion Archive. These spectra are used to measure the emission line fluxesof N V, Si IV, C IV, and He II and to construct diagnostic flux ratio diagrams.We investigate the flux ratio parameter space populated by individual CVs andby various CV subclasses (e.g., AM Her stars, DQ Her stars, dwarf novae,nova-like variables). For most systems, these ratios are clustered within arange of $\sim 1$ decade for log Si IV/C IV $\approx -0.5$ and log He II/C IV$\approx -1.0$ and $\sim 1.5$ decades for log N V/C IV $\approx -0.25$. Theseratios are compared to photoionization and collisional ionization models toconstrain the excitation mechanism and the physical conditions of theline-emitting gas. We find that the collisional models do the poorest job ofreproducing the data. The photoionization models reproduce the Si IV/C IV lineratios for some shapes of the ionizing spectrum, but the predicted N V/C IVline ratios are simultaneously too low by typically $\sim 0.5$ decades. Worse,for no parameters are any of the models able to reproduce the observed He II/CIV line ratios; this ratio is far too small in the collisional and scatteringmodels and too large by typically $\sim 0.5$ decades in the photoionizationmodels.Comment: LaTeX format, uses aaspp4.sty, 28 pages, 11 Postscript figures, accepted for publication in The Astrophysical Journal 10/16/9