HUBBLE-SPACE-TELESCOPE HIGH-RESOLUTION SPECTROSCOPY OF THE EXPOSED WHITE-DWARF IN THE DWARF NOVA VW HYDRI IN QUIESCENCE - A RAPIDLY ROTATING WHITE-DWARF

We obtained a far-ultraviolet spectrum of the dwarf nova VW Hyi in quiescence, with the Hubble Space Telescope Goddard High Resolution Spectrograph covering the region of the Si Iv lambda lambda 1393, 1402 resonance doublet. The broad, shallow Si rv doublet feature is fully resolved, has a total equivalent width of 2.8 Angstrom, and is the first metal absorption feature to be clearly detected in the exposed white dwarf. Our synthetic spectral analysis, using a model grid constructed with the code TLUSTY, resulted in a reasonable fit to a white dwarf photosphere with T-eff = 22,000 +/- 2000 K, log g = 8.0 +/- 0.3, an approximately solar Si/H abundance, and a rotational velocity, v sin i similar or equal to 600 km s(-1). This rotation rate, while not definitive because it is based upon just one line transition, is 20% of the Keplerian (breakup) velocity of the white dwarf and hence does not account for the unexpectedly low boundary-layer luminosity inferred from the soft-X-ray/extreme ultraviolet bands where most of the boundary-layer luminosity should be radiated. The predicted boundary-layer luminosity for a 0.6 M. white dwarf accreting at the rate 10(-10) M. yr(-1) and rotating at 600 km s(-1), corresponding to VW Hyi in quiescence, is 2 x 10(32) ergs s(-1) when proper account is taken of the rotational kinetic energy going into spinning up the white dwarf. If the boundary-layer area is equal to that of the white dwarf, then T-bl = 24,000 K. This is essentially identical to the photospheric luminosity and temperature determined in far-ultraviolet photospheric analyses. If the boundary-layer area is 10(-3) of the white dwarf surface area, then T-bl = 136,000 K.