HST/FOS Eclipse Observations of the Nova-like Cataclysmic Variable UX Ursae Majoris

摘要

[abridged abstract] We present and analyze Hubble Space Telescope observations of the eclipsingnova-like cataclysmic variable UX UMa obtained with the Faint ObjectSpectrograph. Two eclipses each were observed with the G160L grating (coveringthe ultraviolet waveband) in August of 1994 and with the PRISM (covering thenear-ultraviolet to near-infrared) in November of the same year. The system was50% brighter in November than in August, which, if due to a change in theaccretion rate, indicates a fairly substantial increase in Mdot_acc by >~ 50%. Model disk spectra constructed as ensembles of stellar atmospheres providepoor descriptions of the observed post-eclipse spectra, despite the fact thatUX UMa's light should be dominated by the disk at this time. Suitably scaledsingle temperature model stellar atmospheres with T_eff = 12,500-14,500 Kactually provide a better match to both the ultraviolet and opticalpost-eclipse spectra. Evidently, great care must be taken in attempts to deriveaccretion rates from comparisons of disk models to observations. One way to reconcile disk models with the observed post-eclipse spectra is topostulate the presence of a significant amount of optically thin material inthe system. Such an optically thin component might be associated with thetransition region (``chromosphere'') between the disk photosphere and the fastwind from the system, whose presence has been suggested by Knigge & Drew(1997).