Far Ultraviolet Observations of Disks, Winds, and White Dwarfs in Cataclysmic Variables

Abstract

Cataclysmic variables are the nearest and most easily studied laboratories for understanding accretion onto compact objects. FUSE has provided the first significant sample of high resolution, high S/N spectra of such objects in the wavelength range 900–1183 Å. As expected, the FUSE spectra of most nova‐like variables and dwarf novae in outburst show broad features associated with emission from the inner accretion disk. Detailed modeling has proven difficult, however, because the flux near the Lyman limit is higher than expected, indicating that the vertical structure of the disk differs significantly from that of normal stellar atmospheres of similar temperature and gravity. Wind features, due to OVI and CIII, are also evident in many systems, and in most cases are accompanied by fairly narrow lower ionization‐state lines that are blue shifted by no more than a few 100 km s−1. These narrower features and the small number of systems with P‐Cygni‐like profiles suggest that most of the wind features observed with FUSE arise in material fairly near the disk surface, a picture quite different from that inferred from analyses of longer wavelength HST data. Dwarf nova systems in optical quiescence show FUV emission from the WD, which is heated by and cools after outbursts, and from the disk, and, surprisingly, at least in some cases, the hot spot. Here I will briefly illustrate the contributions FUSE has made to our understanding of cataclysmic variables, describe the quantitative state of the modeling, and suggest where we might go from here.