Hubble Space TelescopeSTIS Spectroscopy of the White Dwarfs in the Ultrashort‐Period Dwarf Novae VY Aquarii and WX Ceti

Abstract

We present the results of a multicomponent synthetic spectral analysis of Hubble Space Telescope (HST) STIS spectra of the ultrashort-period dwarf novae VY Aqr and WX Ceti during their deep quiescence following their last superoutburst. The white dwarf in these extremely low accretion rate systems dominates the far-UV light. We find that the accreting white dwarfs in VY Aqr and WX Ceti are remarkably similar. Both systems contain white dwarfs with Teff = 13,000-13,500 K, a rotation velocity below 800-1200 km s-1, and subsolar metallicity. Both white dwarfs are better fitted with a two-temperature white dwarf plus accretion belt model in which part of the white dwarf is cooler and slowly rotating and part is hotter, smaller, and spinning at the Keplerian speed. We discuss the implications of the surface temperatures we have derived for the white dwarfs in VY Aqr, WX Ceti, and the nine other WZ Sge-like dwarf novae below the period gap which have been observed with HST. Their surface temperatures cluster closely around 15,000 K and their orbital periods are between 1.3 and 1.5 hr. We show that long-term compressional heating due to time-averaged accretion is the mechanism responsible for the clustering around 15,000 K. The time-averaged accretion rate corresponding to this cluster of observed temperatures is almost precisely what is predicted if gravitational wave emission is driving mass transfer below the period gap.