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STScI Preprint #1204
Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-2655
We present and analyze Hubble Space Telescope Faint observations of the eclipsing nova-like cataclysmic variable UX UMa obtained with the Faint Object Spectrograph. Two eclipses each were observed with the G160L grating (covering the ultraviolet waveband) in August of 1994 and with the PRISM (covering the near-ultraviolet to near-infrared) in November of the same year. The system was ~50% brighter in November than in August, which, if due to a change in the accretion rate, indicates a fairly substantial increase in M·acc by 50%.
The eclipse light curves are qualitatively consistent with the gradual occultation of an accretion disk with a radially decreasing temperature distribution. The light curves also exhibit asymmetries about mid-eclipse that are likely due to a bright spot at the disk edge. Bright spot spectra have been constructed by differencing the mean spectra observed at pre- and post-eclipse orbital phases. These difference spectra contain ultraviolet absorption lines and show the Balmer jump in emission. This suggests that part of the bright spot may be optically thin in the continuum and vertically extended enough to veil the inner disk and/or the outflow from UX UMa in some spectral lines.
Model disk spectra constructed as ensembles of stellar atmospheres provide poor descriptions of the observed post-eclipse spectra, despite the fact that UX UMa's light should be dominated by the disk at this time. Suitably scaled single temperature model stellar atmospheres with Teff ~ 12,500-14,500 K actually provide a better match to both the ultraviolet and optical post-eclipse spectra. Evidently, great care must be taken in attempts to derive accretion rates from comparisons of disk models to observations.
One way to reconcile disk models with the observed post-eclipse spectra is to postulate the presence of a significant amount of optically thin material in the system. Such an optically thin component might be associated with the transition region ("chromosphere") between the disk photosphere and the fast wind from the system, whose presence has been suggested by Knigge & Drew (1997). In any event, the wind/chromosphere is likely to be the region in which many, if not most, of the UV lines are formed. This is clear from the plethora of emission lines that appear in the mid-eclipse spectra, some of which appear as absorption features in spectra taken at out-of-eclipse orbital phases.