Far Ultraviolet Observations of Dwarf Novae made with the Hopkins Ultraviolet Telescope

Abstract

Observations with the Hopkins Ultraviolet Telescope on the Astro-1 and Astro-2 space shuttle missions have provided the first set of moderate (3 A) resolution far ultraviolet (FUV) spectra of dwarf novae to include the wavelength range between Lyα and the Lyman limit. Important lines which are detected in the HUT spectra of dwarf novae in this wavelength range include S vi λλ933, 945, C iii λ977, O vi λλ1032, 1038, P v λλ1118, 1128 and C iii λ1176, as well as the higher order Lyman lines. The observations confirm earlier IUE observations that two dwarf novae — U Gem and VW Hyi — have quiescent FUV spectra dominated by the white dwarf, but suggest that the quiescent FUV emission from three other dwarf novae — SS Cyg, WX Hyi and YZ Cnc — are dominated by emission from a hot portion of the disk or a disk corona. The spectra obtained of the dwarf novae Z Cam and EM Cyg in outburst and also of the nova-like variable IX Vel can be modeled reasonably successfully in terms of steady stated disks constructed by adding appropriatly-weighted stellar model spectra.