The Cooling White Dwarf in VW Hydri after Normal Outburst and Superoutburst: [ITAL]HST[/ITAL] Evidence of A Sustained Accretion Belt

Abstract

Hubble Faint Object Spectrograph (FOS) observations of VW Hyi one day after the end of a normal dwarf nova outburst reveal a heated white dwarf with deep, broad Lyα, narrow metallic absorption features, and evidence of a hotter Keplerian-broadened component manifested in quiescence as a broad continuum hump. Our best reduced χ2 fit to the data reveals (1) a DAZQ white dwarf with Teff = 22,500 ± 500 K, log g = 8, and photospheric abundances C = 0.5 solar, N = 5.0 solar, O = 2.0 solar, Si = 0.2 solar, Fe = 0.5 solar, with all other metals being 0.3 solar, and (2) a rapidly spinning accretion belt with Vrot = 3350 km s-1, Tbelt = 26,000 ± 1000 K, log g = 6.0, and a fractional belt area of approximately 11%. Our earlier FOS spectrum obtained 10 days after superoutburst reveals a cooler DAZQ white dwarf (20,500 ± 1000 K), relatively lower metal abundances, and a smaller fractional area (3%) for the accretion belt. Thus, 1 day after a normal outburst, the white dwarf is ≈ 2000 K hotter, the accretion belt fractional area is a factor of 3 greater, the accretion belt temperature appears to be cooler than at 10 days post-superoutburst, and the accreted atmosphere has relatively higher metal abundances. Finally, the accretion belt maintained during quiescence may provide a natural explanation for the 14 s soft X-ray oscillations, requires a deeper source of heating (compression and shear mixing), and implies a lower limit to the viscous spin-down timescale of 10 days.