The Effect of a Superoutburst on the White Dwarf and Disk of VW Hydri as observed with FUSE

Abstract

We have used FUSE to obtain thirteen observations of the dwarf nova VW Hyi covering the period from the end of a superoutburst through the following normal outburst of the system. Here, we present the quiescent spectra. They contain at least three components. The dominant component is the white dwarf (WD), which cools following the superoutburst. The amount of cooling depends somewhat on the WD models used. For log g of 8.0, the temperature drops from 24,000K just after the outburst to 20,000 K just before the normal outburst. For this model, and a distance of 65 pc, the WD radius is ~8 x 10**8 cm and v sin(i) is ~420 km/s. The fact that the derived radius is smaller than expected for a WD with log g=8 suggests a higher gravity WD or that VW Hyi is somewhat further than 65 pc. Either is possible given the current distance uncertainty of +-20 pc. Earlier suggestions that the WD photosphere shows evidence of CNO processed material are confirmed, but our analysis also shows that issues remain in terms of analyzing the spectra of WDs in such unusual physical situations. The second component is relatively featureless and shows modulation on the orbital (and just after outburst, the superhump) period. It is likely associated with the hot spot where material from the secondary encounters the disk, rather than the boundary layer region between the inner disk and WD. The second component fades ~10 days after the superoutburst. There is also a third component, clearly visible in broad emission lines of C III 977, N III 991, Lyman Beta and O VI 1032,1038, which appears to be accompanied by a flat continuum. The strength of the emission lines, which are almost surely associated with the accretion disk, appear relatively constant throughout the observations.