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

摘要

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 λλ11393, 1402 resonance doublet. The broad, shallow Si Ⅳ doublet feature is fully resolved, has a total equivalent width of 2.8 A, 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 approx= 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 solar mass white dwarf accreting at the rate 10~(-10) solar mass 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.