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Title:
FUSE Observations of U Geminorum during Outburst and Decline
Authors:
Froning, Cynthia S.; Long, Knox S.; Drew, Janet E.; Knigge, Christian; Proga, Daniel
Affiliation:
AA(Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218; , ), AB(Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218; , ), AC(Imperial College of Science, Technology and Medicine, Blackett Laboratory, Prince Consort Road, London SW7 2BZ, UK; ), AD(Department of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK; ), AE(NASA Goddard Space Flight Center, Laboratory for High Energy Astrophysics, Code 662, Greenbelt, MD 20771; )
Publication:
The Astrophysical Journal, Volume 562, Issue 2, pp. 963-984. (ApJ Homepage)
Publication Date:
12/2001
Origin:
UCP
ApJ Keywords:
Accretion, Accretion Disks, Stars: Binaries: Close, Stars: Novae, Cataclysmic Variables, stars: individual (U Geminorum), Ultraviolet: Stars
DOI:
10.1086/323860
Bibliographic Code:
2001ApJ...562..963F

Abstract

We have obtained far-ultraviolet (904-1187 Å) spectra of U Gem in outburst with the Far Ultraviolet Spectroscopic Explorer. Three of the observations were acquired during the plateau phase of the outburst, while the fourth was obtained during late outburst decline. The plateau spectra have continuum shapes and fluxes that are approximated by steady state accretion disk model spectra with m~=7×10-9 Msolar yr-1. The spectra also show numerous absorption lines of H I, He II, and 2-5 times ionized transitions of C, N, O, P, S, and Si. There are no emission features in the spectra, with the possible exception of a weak feature on the red wing of the O VI doublet. The absorption lines are narrow (FWHM~500 km s-1), too narrow to arise from the disk photosphere, and at low velocities (<=700 km s-1). The S VI and O VI doublets are optically thick. The absorption lines in the plateau spectra show orbital variability: in spectra obtained at orbital phases 0.53<=Φ<=0.79, low-ionization absorption lines appear and the central depths of the preexisting lines increase. The increase in line absorption occurs at the same orbital phases as previously observed EUV and X-ray light-curve dips. If the absorbing material is in (near-) Keplerian rotation around the disk, it must be located at large disk radii. The final observation occurred when U Gem was about 2 mag from optical quiescence. The spectra are dominated by emission from an ~=43,000 K, metal-enriched white dwarf (WD). The inferred radius of the WD is 4.95×108 cm, close to that observed in quiescence. Allowing for a hot heated region on the surface of the WD improves the fit to the spectrum at short (less than 960 Å) wavelengths. Based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer (FUSE). FUSE is operated for NASA by Johns Hopkins University under NASA contract NAS 5-32985.
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