Ultraviolet Emission‐line Ratios of Cataclysmic Variables

Abstract

We present a statistical analysis of the ultraviolet emission lines of cataclysmic variables (CVs) based on B430 ultraviolet spectra of 20 sources extracted from the International Ultraviolet Explorer Uniform Low Dispersion Archive. These spectra are used to measure the emission-line Ñuxes of N V, Si IV, C IV, and He II and to construct diagnostic Ñux ratio diagrams. We investigate the Ñux ratio parameter space populated by individual CVs and by various CV subclasses (e.g., AM Her stars, DQ Her stars, dwarf novae, nova-like variables). For most systems, these ratios are clustered within a range of D1 decade for log Si IV/C IV B [0.5 and log He II/C IV B [1.0 and D1.5 decades for log N V/C IV B [0.25. These ratios are compared to photoionization and collisional ionization models to constrain the excitation mechanism and the physical conditions of the line-emitting gas. We Ðnd that the collisional models do the poorest job of reproducing the data. The photoionization models reproduce the Si IV/C IV line ratios for some shapes of the ionizing spectrum, but the predicted N V/C IV line ratios are simultaneously too low by typically D0.5 decades. Worse, for no parameters 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 scattering models and too large by typically D0.5 decades in the photoionization models.