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Hubble Space Telescope Eclipse Observations of the Nova Like Cataclysmic Variable UX Ursae MajorisWe present and analyze Hubble Space Telescope 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 about 50% brighter in November than in August, which, if due to a change in the accretion rate, indicates a fairly substantial increase in Mass accretion by about 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 mideclipse 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 posteclipse 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 posteclipse 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 T(sub eff) approximately equals 12,500-14,500 K actually 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 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 and 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.
Document ID
19990042036
Acquisition Source
Goddard Space Flight Center
Document Type
Reprint (Version printed in journal)
Authors
Knigge, Christian
(Space Telescope Science Inst. Baltimore, MD United States)
Long, Knox S.
(Space Telescope Science Inst. Baltimore, MD United States)
Wade, Richard A.
(Pennsylvania State Univ. University Park, PA United States)
Baptista, Raymundo
(Santa Catarina Univ. Florianopolis, Brazil)
Horne, Keith
(Saint Andrew's Univ. United Kingdom)
Hubeny, Ivan
(NASA Goddard Space Flight Center Greenbelt, MD United States)
Rutten, Rene G. M.
(Isaac Newton Group Santa Cruz de La Palma, Spain)
Date Acquired
August 19, 2013
Publication Date
May 30, 1998
Publication Information
Publication: Astrophysical Journal
Publisher: American Astronomical Society
Volume: 499
Subject Category
Astronomy
Funding Number(s)
CONTRACT_GRANT: NAS5-2655
CONTRACT_GRANT: GO-5448
CONTRACT_GRANT: GO-3683.03-91A
CONTRACT_GRANT: NAGw-3171
Distribution Limits
Public
Copyright
Other

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