Hubble Space Telescope STIS Spectroscopy and Modeling of the Long-Term Cooling of WZ Sagittae following the 2001 July Outburst

We present the latest Hubble Space Telescope Space Telescope Imaging Spectrograph (HST STIS) E140M far-ultraviolet (FUV) spectrum of the dwarf nova WZ Sge, obtained in 2004 July, 3 yr following the early superoutburst of 2001 July. With a temperature T ≈ 15,000 K, the white dwarf (WD) is still ~1500 K above its quiescent temperature, it has a FUV flux level almost twice its preoutburst value, and its spectrum does not distinctly exhibit the quasi-molecular hydrogen feature around 1400 Å, which was present in the International Ultraviolet Explorer (IUE) and HST Goddard High Resolution Spectrograph (GHRS) preoutburst data. This is a clear indication that even 3 yr after outburst, the system is still showing the effect of the outburst. We model the cooling curve of WZ Sge, over a period of 3 yr, using a stellar evolution code including accretion and the effects of compressional heating. Assuming that compressional heating alone is the source of the energy released during the cooling phase, we find that (1) the mass of the WD must be quite large (≈1.0 ± 0.2 M☉), and (2) the mass accretion rate must have a time-averaged (over 52 days of outburst) value of order 10-8 M☉ yr-1 or above. The outburst mass accretion rate derived from these compressional heating models is larger than the rates estimated from optical observations (Patterson et al.) and from a FUV spectral fit (Long et al.) by up to 1 order of magnitude. This implies that during the cooling phase the energy released by the WD is not due to compressional heating alone. We suggest ongoing accretion during quiescence as an additional source of energy.