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 TK, 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.