A time-series IUE archival study of the white dwarf in the ultrashort-period dwarf nova WZ Sagittae

The IUE archive offers a series of far-UV spectra of the ultrashort-period dwarf nova WZ Sge over a long time baseline and hence the opportunity to monitor the ongoing accretion, the cooling of the white dwarf, and temporal variations in the line spectra. We have carried out a quantitative analysis of these spectra including model atmosphere simulations. We find an indicated cooling of the white dwarf by 5140 K from 20,510 to 15,370 K with a thermal e-folding time of 690 days. This cooling rate is well represented by heating of the white dwarf by accretion of matter with angular momentum during the 1978 December outburst followed by subsequent cooling. We find that the abundance of carbon in the white dwarf photo-sphere is elevated above solar. This finding is consistent with the results of recent Hubble Space Telescope observations. We find marginal evidence that the C abundance was 6 times solar close to the outburst and 2 times solar in the most recent spectra obtained 13.7 years later. However, uncertainties in the abundances do not allow any definitive conclusion regarding temporal abundance variations. The origin of the elevated carbon abundance is discussed in terms of accretion and diffusion while arguments are presented which may rule out ordinary convective dredge-up and dredge up due to forced convection associated with shear mixing.