Sion, Huang, Szkody (U. of WA), Cheng (U. of MD) and Hubeny (NASA
GSFC) have analyzed a far ultraviolet spectrum of the dwarf nova VW
Hydri obtained during quiescence with the Hubble Space Telescope Faint
Object Spectrograph (HST/FOS). Their HST observation occurred 10 days
after the return to optical quiescence from a superoutburst of VW Hydri.
The spectrum reveals a very strong Stark-broadened Lyman-
absorption with narrow geocoronal emission, and a very rich metallic
absorption line spectrum dominated by strong resonance absorption
features of singly and doubly ionized silicon and carbon, the first
solid identification of metallic absorption features arising in the
accreted atmosphere of the white dwarf. They confirm the reported low
resolution IUE detection of the underlying white dwarf photosphere by
Mateo & Szkody. A synthetic spectral analysis with hot, high gravity
LTE model atmospheres yielded a best fit model with the following
parameters: T
K, Log g 
, with
chemical abundances of O
, N
and all
other heavy elements 
. Based upon their absorption
line measurements in the observations at different orbital phases, they
found no conclusive evidence of equivalent width variations versus
orbital phase. In the absence of any significant reduction of the white
dwarf's core mass by past nova explosions, its lower limit cooling age
is approximately 50 million years.
They obtained a far ultraviolet spectrum of the dwarf nova VW Hydri
in quiescence, with the Hubble Space Telescope GHRS, covering the region
of the Si IV (1393, 1402) resonance doublet. The broad,
shallow Si IV doublet feature is fully resolved, has a total
equivalent width of 2.8 Å, and is the first metal absorption feature
to be clearly detected in the exposed white dwarf. Their synthetic
spectral analysis, using a model grid constructed with the code TLUSTY,
resulted in a reasonable fit to a white dwarf photosphere with T
K, log g , an approximately solar Si/H
abundance, and a rotational velocity, v sin 
km s
. 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/EUV 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 
M and rotating at 600 km s,
corresponding to VW Hydri in quiescence, is 
ergs s when
proper account is taken for 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 
K. This is essentially
identical to the photospheric luminosity and temperature determined in
far ultraviolet photospheric analyses. If the boundary layer area is
of the white dwarf surface area, then 
K.
Huang, Sion, P. Szkody, and K. Long investigated the cooling of the white dwarf in U Geminorium following heating by two different outbursts. They present the analysis of 13 orbital phase-resolved IUE (large and small aperture), SWP spectra, sampling the quiescent intervals of the prototype dwarf nova U Geminorium following two individual outbursts, the outburst of 1992, 29 August-14 September and the outburst of 1993, 19 March-5 April. During quiescence, the photospheric radiation of the exposed white dwarf dominates the far ultraviolet. The variations in absorption line strengths and continuum flux levels are analyzed as a function of both orbital phase and elapsed time since the return to optical quiescence, by fitting the IUE spectra with a grid of high gravity, LTE, model atmospheres with solar composition constructed with TLUSTY and SYNSPEC (Hubeny 1992). They present evidence from both absorption line variations and continuum variations, as a function of time since the outbursts, that the white dwarf photosphere has cooled by several thousand degrees. Within the signal to noise limitations, they find no evidence of a difference in heating and cooling between the two outbursts. The quiescent interval following the outburst of 29 August 1993 is the longest ever recorded.
Huang, Sion, Szkody, Hubeny and Cheng also investigated the HST/FOS
spectroscopy of VW Hydri in superoutburst. They present an analysis of
two HST/FOS UV spectra of the SU UMa type dwarf nova, VW Hyi, obtained
on 1993, October, 24, about 5 days after the optical rise of a
superoutburst. The absorption features in the first spectrum appear to
consist of two components: a broad-winged component (with velocity width
of about 3000km/s), and a sharp core narrow component. This is the
first time the narrow core is clearly resolved in superoutburst spectra
of a dwarf nova system. The sharp core appears absent in the second
spectrum obtained about 10 minutes later. The broader component is
mainly from the accretion disk. By comparing the spectra with a grid of
LTE model accretion disk atmospheres constructed with TLUSTY, SYNSPEC
and DISKSYN, they present two possible disk fits to the observed
spectra: a steady state disk with solar abundance and 
which can account for all the broad absorption
features except for NV(
1240), and a model with a discontinuous
distribution in which there is a contribution to the
NV(1240) absorption feature. They provide arguments supporting
the possibility that the sharp cores are due to gas streams in the
system. They also point out the far less likely possibility that the
sharp cores form in a hot, high gravity atmosphere. The synthetic
fitting results may imply that the hot matter is accreted from the inner
part of the disk onto the surface of the white dwarf through a highly
inhomogeneous gas flow. They also tied in to this model their FOS
detection of highly asymmetric inverse P Cygni profile structure in the
narrow stellar components at CIV(1550).
Huang, Sion, and Sparks (LANL) continue the development of the two dimensional, fully implicit, radiative hydrodynamic, Lagrangian code to simulate the accretion processes near the boundary layer of cataclysmic variable systems. After the code passing the free fall test last year, this year they have included the radiative physics into the code for more realistic testing cases. Another major progress in the code development is that the implementation of the Interface Reconstruction, a numerical technique originally invented for Eulerian scenario, into their Lagrangian code is near its completion. This is the first time the Interface Reconstruction technique has ever been applied to astrophysical simulations.