A Hubble Space Telescope STIS Observation of VW Hydri at the Exact Far-Ultraviolet Onset of an Outburst

We present an analysis of HST STIS data of VW Hyi acquired ~ 14 days after asuperoutburst. At the time of the observation the system appears to be goinginto outburst with the longest wavelengths increasing in flux by a factor of 5,while the shortest wavelengths increase by only a factor of 2. Using thedistance of 65 pc, a system inclination angle of 60 degrees and a white dwarfmass of 0.86 solar mass, we carried out model fits involving a white dwarf byitself, an optically thisck accretion disk by itself, a composite model usingan optically thick accretion disk and a white dwarf, a two-temperature whitedwarf model with a cooler more slowly rotating photosphere and a hotter,rapidly rotating accretion belt and a composite model involving a white dwarfand a rapidly rotating cooler disk ring heated up to a ``low'' temperature of\~13-14,000K. This component of temperature stays fairly constant throughoutthe HST observations while the area of the disk ring increases by a factor of12. We see evidence of a delay in the UV emission consistent with the outburstbeginning outside of a disk truncation radius.Comment: to appear in the ApJ October 15 2004 issu