(Early superhumps in AL Com, from Kato et al. 1996)
(Mainly based on Kato (2002) PASJ 54, 11L)
WZ Sge-type dwarf novae are a small subgroup of dwarf novae characterized by a long (~10 yr) outburst recurrence time and a large (~8 mag) outburst amplitude. They are a subclass of SU UMa-type dwarf novae in that they show superhumps during their long, bright outbursts (superoutbursts).
The most remarkable signature of WZ Sge-type outbursts is the presence of "early superhumps" (This feature is also referred to as orbital superhumps or outburst orbital hump by different authors), during the earliest stage of superoutbursts. Early superhumps have a period extremely close to the binary period (The best-established case is the 2001 superoutburst of WZ Sge). In a few other systems, such as AL Com, EG Cnc, the periods of early superhumps have been found to be in good agreement with their quiescent photometric periods, most likely representing orbital periods. The early superhumps commonly show a double-humped profile in contrast to ordinary superhumps of SU UMa-type dwarf novae.
Early superhumps are the most discriminative feature of WZ Sge-type outbursts, and have not been conspicuously detected in other dwarf novae. The origin of the early superhumps was speculated to be an enhanced hot spot (Patterson et al. 1981, who subsequently proposed a concept of outburst orbital humps), which, however, is most likely a result of mis-interpretation of observations (see e.g. Osaki and Meyer 2003, A&A in press, astro-ph/0302140).
During the early stage of the 2001 outburst of WZ Sge, several people (Steehgs et al., IAUC 7675 and Baba et al. IAUC 7678) detected two-armed arch-like structures on Doppler tomograms of He II and C III/N III emission lines. The feature resembled those of "spiral structure" seen on Doppler tomograms of the dwarf nova IP Peg during an outburst.
The spiral structure in Doppler tomograms of IP Peg has been widely believed to represent tidally induced spiral shocks or spiral waves (Steeghs et al. 1997, 1998). However, it is not clear whether the same interpretation can apply to the completely different (very low mass-ratio, q=M2/M1 < 0.1, in contrast to q~0.5 in IP Peg, and very low-mass transfer rate, several orders of magnitudes lower than that of IP Peg) binary parameters of WZ Sge.
At the epoch of the detection of two-armed arch-like structures in WZ Sge, the star showed double-wave early superhumps. Although there have been suggestions that early superhumps in photometry and two-armed structures on Doppler tomograms have the same origin, no promising idea has been proposed to explain these features in the same scheme.
Most recently, Osaki and Meyer (2002) explained early superhumps (which they call early humps by considering the manifestation of a tidal 2:1 resonance in the accretion disks of binary systems with extremely low mass ratios.
Most recently, an alternative idea has been proposed to explain arch-like structures in the outbursting IP Peg disk, by considering the irradiation of the elevated disk formed by the horizontal convergence of three-body orbits (Smak 2001). This interpretation successfully reproduced the arch-like structures, and the observed strong intensity in the region as observed, which is difficult to explain within the scheme of conventional tidally induced spiral shocks. Oglivie (2001) studied the tidal distortion of fluid disks, and succeeded to reproduce the basically same features. Oglivie (2001) showed, in realistic fluid disks, that the m=2 inner vertical resonance plays a more important role even in high-q binaries.
In low-q systems, like WZ Sge, the tidal force of the secondary is so weak on a compact disk in which a condition is usually achieved that angular momentum is more effectively transported by viscosity than the tidal force of the secondary (Lin and Papaloizou 1979). However, during the early stage of vigorous outbursts of WZ Sge-type stars, the disk can sufficiently expand. In usual SU UMa-type dwarf novae, growth of the 3:1 resonance, which is responsible for superhumps, is rapid enough to effectively truncate the disk at the resonance radius. In low-q WZ Sge-type stars, the slow growth can allow the disk to expand beyond the 3:1 resonance.
The maximum phases of early superhumps have also been problematic. Early superhumps in WZ Sge-type dwarf novae consist of two maxima of different amplitudes. In WZ Sge, the brighter maximum is around a binary phase 0.6-0.7 (Ishioka et al. 2001). Although models involving an enhanced hot spot, as in Patterson et al. (1981), would require such a maximum corresponding to the orbital phase of a stronger emission feature, the available observation shows a complete phase reversal. Such a reversal can be naturally understood if one considers that the source of the emission lines and stronger continuum is an irradiated surface of the accretion disk.
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