Hubble Space Telescope Eclipse Observations of the Nova‐like Cataclysmic Variable UX Ursae Majoris

Abstract

We present and analyze Hubble Space T elescope 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 1994 August and with the PRISM (covering the near-ultraviolet to near-infrared) in November of the same year. The system was D50% brighter in November than in August, which, if due to a change in the accretion rate, indicates a fairly substantial increase in by The eclipse light curves are qualitatively consistent with the gradual M 0 acc Z50%. occultation of an accretion disk with a radially decreasing temperature distribution. The light curves also exhibit asymmetries about mideclipse that are likely due to a bright spot at the disk edge. Bright-spot spectra have been constructed by di †erencing the mean spectra observed at pre-and posteclipse orbital phases. These di †erence 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 outÑow from UX UMa in some spectral lines. Model disk spectra constructed as ensembles of stellar atmospheres provide poor descriptions of the observed posteclipse spectra, despite the fact that UX UMaÏs light should be dominated by the disk at this time. Suitably scaled single temperature model stellar atmospheres with K actually T eff ^ 12,500È14,500 provide a better match to both the ultraviolet and optical posteclipse 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 posteclipse spectra is to postulate the presence of a sig-niÐcant amount of optically thin material in the system. Such an optically thin component might be associated with the transition region ("" chromosphere ÏÏ) between the disk photosphere and the fast wind from the system, whose presence has been suggested by Knigge & Drew. 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 mideclipse spectra, some of which appear as absorption features in spectra taken at out-of-eclipse orbital phases.