We present a synthetic spectral analysis of nearly the entire far ultraviolet IUE archive of spectra of dwarf novae in or near their quiescence. We have examined all of the systems for which the signal to noise ratio permitted an analysis. The study includes 53 systems of all dwarf nova subtypes both above and below the period gap. The spectra were uniformly analyzed using synthetic spectral codes for optically thick accretion disks and stellar photospheres along with the bestavailable distance measurements or estimates. We present newly determined approximate white dwarf temperatures or upper limits and estimated accretion rates. The implications of our study for disk accretion physics and CV evolution are discussed. The average temperature of white dwarfs in dwarf novae below the period gap is ∼ 18, 000K. For white dwarfs in dwarf novae above the period gap, the average white dwarf temperature is ∼26,000K. There is a flux component, in addition to a white dwarf photosphere, which contributes > 60% of the flux in the FUV in 53% of the quiescent dwarf novae in this study. We find that for 41% of the dwarf novae in our sample, a white dwarf photosphere provides > 60% of the FUV flux. Accretion rates estimated from the FUV alone for the sample of DNe during quiescence ranged from 10 −12 M ⊙ /yr to 10 −10 M ⊙ /yr. The additional flux component is almost certainly not an optically thick accretion disk since, according to the disk instability model, the disk should be optically thin and too cool during dwarf nova quiescence to be a significant FUV continuum emitter. Among the candidates for the second component of FUV light are the quiescent inner disk, a hot accretion belt at low white dwarf latitudes centered on the equator, and hot rotating ring where the outer part of the boundary layer (the UV boundary layer) meets the inner disk and possibly heats it. The implications of our findings are discussed.
We explore the origin of FUSE and HST STIS far UV spectra of the dwarf nova, EY Cyg, during its quiescence using combined high gravity photosphere and accretion disk models as well as model accretion belts. The best-fitting single temperature white dwarf model to the FUSE plus HST STIS spectrum of EY Cygni has T ef f = 24, 000K, log g = 9.0, with an Si abundance of 0.1 x solar and C abundance of 0.2 x solar but the distance is only 301 pc. The best-fitting composite model consists of white dwarf with T ef f = 22, 000K, log g = 9, plus an accretion belt with T belt = 36, 000K covering 27% of the white dwarf surface with V belt sini = 2000 km/s. The accretion belt contributes 63% of the FUV light and the cooler white dwarf latitudes contribute 37%. This fit yields a distance of 351 pc which is within 100 pc of our adopted distance of 450 pc. EY Cyg has very weak C iv emission and very strong N v emission, which is atypical of the majority of dwarf novae in quiescence. We also conducted a morphological study of the surroundings of EY Cyg using direct imaging in narrow nebular filters from ground-based telescopes. We report the possible detection of nebular material associated with EY Cygni. Possible origins of the apparently large N v/C iv emission ratio are discussed in the context of nova explosions, contamination of the secondary star and accretion of nova abundance-enriched matter back to the white dwarf via the accretion disk or as a descendant of a precursor binary that survived thermal timescale mass transfer. The scenario involving pollution of the secondary by past novae may be supported by the possible presence of a nova remnant-like nebula around EY Cyg.
We present the results of the first multicomponent synthetic spectral analysis of International Ultraviolet Explorer (IUE) archival spectra of the long-period dwarf nova RU Peg during quiescence. The best-fit, highgravity, solar composition photosphere models yield T eff ¼ 50; 000 53; 000 K with scale factor distances of 250 pc. Optically thick accretion disk models imply accretion rates between 1 Â 10 À9 and 1 Â 10 À10 M yr À1 in order to match the steeply sloping far-UV continuum. However, the best-fit accretion disk models yield distances from 600 to 1300 pc, well beyond the estimated distance range of 130-300 pc. Using rough theoretical flux arguments and the distance estimates, we find better agreement between the observed far-UV luminosity and the predicted far-UV luminosity of a hot, massive, white dwarf model than with an accretion disk model. RU Peg appears to contain the hottest white dwarf yet found in a dwarf nova. We cannot rule out that the far-UV energy distribution is due to a multitemperature white dwarf with cooler, more slowly rotating higher latitudes and a rapidly spinning, hotter equatorial belt. We discuss implications of our analysis for theoretical predictions of the disk instability theory of dwarf nova outbursts. We discuss a comparison between RU Peg's white dwarf and the observed properties of other analyzed white dwarfs in dwarf novae.
We have analyzed the Far Ultraviolet Spectrocopic Explorer (FUSE) spectra of two U Gem-Type dwarf novae, SS Aur and RU Peg, observed 28 days and 60 days (respectively) after their last outburst. In both systems the FUSE spectra (905 − 1182Å) reveal evidence of the underlying accreting white dwarf exposed in the far UV. Our grid of theoretical models yielded a best-fitting photosphere to the FUSE spectra with T ef f =31,000K for SS Aur and T ef f =49,000K for RU Peg. This work provides two more dwarf nova systems with known white dwarf temperatures above the period gap where few are known. The absence of C iii (1175Å) absorption in SS Aur and the elevation of N above solar suggests the possibility that SS Aur represents an additional accreting white dwarf where the surface C/N ratio derives from CNO processing. For RU Peg, the modeling uncertainties prevent any reliable conclusions about the surface abundances and rotational velocity.
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