The Dwarf Novae during Quiescence

Abstract

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.

A new absolute magnitude calibration with 2MASS for cataclysmic variables

“…Therefore, only a lower limit to the distance can be obtained by using this method due to the effects of the disc and the irradiated area of the secondary star (Berriman, Szkody & Capps 1985). Almost all distance estimation methods attempts to use properties of a component of the system such as surface brightness of the secondary star (Bailey 1981), spectra of the white dwarf (Sion et al 1995;Urban & Sion 2006) and M V − P orb relationship of dwarf novae at outburst (Warner 1995;Harrison et al 2004). However, none of them can give a distance as precise as trigonometric parallax due to the contaminations from the other components and the lack of the information about the individual contributions of the components to the observed light.…”

Orbital periods of cataclysmic variables identified by the SDSS

“…Urban & Sion 2006;Townsley & Gänsicke 2009) -by comparison the well-studied dwarf nova U Gem has overall properties which are very similar to SDSS J1006 but harbours a WD with T WD ≈ 30 000 K (Sion et al 1998;Long et al 2006). Given that SDSS J1006 is a high-inclination system, it might be possible that the WD is partially veiled by extended structures above the accretion disc, similar to those seen in OY Car (Horne et al 1994).…”

FUSEObservations of the Dwarf Novae UU Aql, BV Cen, and CH UMa in Quiescence

“…Perhaps the least understood topic in cataclysmic variable (CV )/ DN research (along with what drives the wind outflow in outburst) is the state and structure of the boundary layer and accretion disk during quiescence and the physics of how the long-term accretion of mass, angular momentum, and energy affects the WD. Our studies with archival IUE data and Hubble Space Telescope (HST ) STIS have found that $50% of the DNe in quiescence are dominated (i.e., >60% of UV flux) by a component of FUV flux other than the WD called the ''accretion disk''; $25% are dominated by the WD, and $25% have nearly equal contributions of WD and accretion disk (40%Y60% each) ( Urban & Sion 2006).…”

“…A number of studies (Araujo-Betancor et al 2003;Sion 1991;Sion 1999;Urban & Sion 2006) have shown that CV WDs above the gap are typically on-average 10,000 K hotter than CV WDs below the period gap (almost certainly a consequence of higher time-averaged accretion rates of systems above the gap but possibly with system total age also being a factor). Since the WD surface temperature is crucial for understanding CV evolution and whether CVs evolve across the period gap, the use of cooling ages and long-term evolutionary model sequences with accretion (including the effects of nova explosions; Townsley & Bildsten 2003) must rely on the empirical WD temperature of the photosphere in equilibrium with long-term compressional heating from accretion.…”

“…For CH UMa, we adopted values from Friend et al (1990). For all three systems, the distances were the same as those in Urban & Sion (2006), where the Warner (1995) and Harrison et al (2004) M v(max) versus P orb relations, calibrated with trigonometric parallaxes, were used. The reddening values were the same as those quoted in Urban & Sion (2006), which were from Verbunt (1987), LaDous (1991, and Bruch & Engel (1994).…”