The Dwarf Novae during Quiescence

We present a synthetic spectral analysis of nearly the entire far-ultraviolet (FUV) IUE archive of spectra of dwarf novae (DNe) 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 DN 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 best-available distance measurements or estimates. We present newly determined approximate white dwarf (WD) 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 WDs in DNe below the period gap is ~18,000 K. For WDs in DNe above the period gap, the average WD temperature is ~26,000 K. There is a flux component, in addition to a WD photosphere, which contributes >60% of the flux in the FUV in 53% of the quiescent DNe in this study. We find that for 41% of the DNe in our sample, a WD 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 to 10^-10 M_☉ yr^-1. 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 DN 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 WD 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.