WZ Sge (WZ Sagittae) type dwarf novae

What are WZ Sge-type dwarf novae?

[Compiled and adapted from Kato et al. (2001) PASJ 53, 1191; Kato et al. (2002) PASJ 54, 999; Kato (2002) PASJ 54, 11L etc.]

(Outburst image of the prototype WZ Sge presented by G. Masi. The image was taken on 2001 July 23)

Although presently classified as a dwarf nova, WZ Sge is peculiar in many respects. The star has exhibited three historical outbursts (1913, 1946, 1978) separated by 32.5 years, and most recently experienced its fourth outburst in 2001. The light curves of its outbursts resemble that of a fast nova in that they show:

rapid initial rise
rather rapid (about 0.3 mag/day) initial decline
subsequent slower decline lasting 60 to 100 d
large outburst amplitudes (about 8 mag)

(Light curve of WZ Sge in 2001, from Ishioka et al. 2002)

From these features, the object had been long believed to be a recurrent nova, resembling U Sco. Numerous books on variable stars (mainly historical ones) classfied WZ Sge as a fast nova (if you meet such a book, please inform the author or the publisher that the classification is wrong!). The star, however, has a large proper motion, indicating that the distance to the object is far smaller than usual (classical) novae. The small distance indicates a low instrinsic luminosity. The concept of dwarf novae was thus derived.

Spectroscopic and photometric observations during the 1978 outburst, however, confirmed that WZ Sge is a dwarf nova, and not a classical nova. (At the time of the 1978-1979 outburst, the light curve was far more fragmentary than the 2001 outburst given above).

Note: Although the early outburst spectra (IAUC 3311, 3313, 3315) were already indicative of a dwarf nova, rather than a recurrent nova, the first correct classification as a dwarf nova was suggested by Bohusz and Udalski (1979) IBVS 1583 based on photometric observations. Later authors also supported this interpretation].

Bailey (1979, MNRAS 189, 41P), extended this concept to other two large-amplitude, rarely outbursting dwarf novae (WX Cet and UZ Boo). Downes and Margon (1981, MNRAS 197, 35P) further extended this concept to a larger set of dwarf novae. The subclass of WZ Sge-type dwarf novae has been thus recognized.

The most striking discovery was the presence of superhumps by Bohusz and Udalski (1979).

Note: The emergence of a slightly longer period than the orbital period was first noted by Guinan and McCook (IAUC 3319) and Targan (IAUC 3320). Targan yielded the first estimate of this period. However, the period increase was initially considered to be a consequence of the "nova" outburst. Bohusz and Udalski (1979) first correctly described this period as superhumps, and identified WZ Sge to be an SU UMa-type dwarf nova. The first indication of the emergence of superhumps in reported photometry can be found in Heiser and Henry (1979) IBVS 1559 and Targan (IAUC 3320).

Theoretical Interpretation of WZ Sge-type outbursts

There had been several competing theories to explain the peculiar nature of WZ Sge. One is the mass-transfer burst model, which was used to explain the large-amplitude periodic humps early in its 1978 outburst (Patterson et al. 1981). The period of the humps was equal to the orbital period (Porb), and were considered to reflect a hot spot enhanced by the mass-transfer burst, although the hump maxima occurred 0.17 orbital phase prior to the orbital humps in quiescence.

The other is an extension of the thermal and tidal instability theory of SU UMa stars (Osaki 1989, PASJ 41, 1005) towards the lowest mass-transfer rate (Osaki 1995, PASJ 47, 47). Numerical simulations have shown that a thermal instability of the accretion disk occurs rarely in such conditions, and it always leads to a tidal instability to trigger a superoutburst.

Lasota and Hameury (1995, A&A 302, L29) assumed evaporation of the inner disk, and argued that rare outbursts can be caused by a small increase of the mass-transfer rate, without an assumption of a very low quiescent viscosity. This scenario may be considered as a combination of the mass-transfer burst model and the disk-instability model.

More recently, Osaki and Meyer (2003, A&A 401 3250) further succeeded in explaining the overall outburst features in WZ Sge basically following this scheme, by considering the effect of the 1:2 resonance. They stated that "the overall development of the 2001 outburst of WZ Sge based on the thermal-tidal instability model does not require the assumption of an unproved enhanced mass transfer". This conclusion is perfectly in line with our earlier works e.g. Kato et al. (1996, PASJ 48, L21) stating that "The double-humped profile of this variation (early superhumps) seems to preclude an explantion based on an enhanced hot spot by a mass-transfer burst from the secondary, which has been believed to be the best observational evidence of a mass-transfer burst in WZ Sge obtained during its 1978 outburst" and Ishioka et al. (2002, A&A 381, 41L) stating that "our observations reject the mass transfer instability for the trigger of the superoutburst of WZ Sge stars".

Some WZ Sge-type dwarf novae are known to show complex post-superoutburst rebrightening(s), which are rarely seen in other SU UMa-type dwarf novae. The most remarkable objects are EG Cnc, and WZ Sge, itself (the 2001 superoutburst). In order to explain this unique feature, Osaki and Shimizu (1997, PASJ 49, 19) proposed a working model, in which the disk viscosity in post-superoutburst WZ Sge-type stars are somehow maintained higher than that at the pre-superoutburst level. Although the underlying physical mechanism was not clear at the time of the initial proposition, Osaki et al. (2001, A&A 370, 488) further succeeded to explain, by using a combination of the schemes of Osaki (1995), and the decay of magnetic turbulence in the quiescent dwarf nova disk (Gammie and Menou 1998, ApJ 492, L75), the variety of phenomena in WZ Sge-type dwarf novae.

From the observational side, discrimination of the theories has been difficult owing to the rarity of outbursts of WZ Sge. Searches for more "WZ Sge-type" objects among dwarf novae have been a natural consequence of the motivation to confirm the universality of the features found in WZ Sge, itself. In spite of the efforts, a number of candidates have turned out to be rather normal SU UMa-stars exhibiting normal outbursts [e.g. WX Cet, VY Aqr], and the attempt remained rather unsuccessful. The WZ Sge-type dwarf novae and candidates have been the one of the central objects of importance in VSNET Collaboration.

Brown Dwarf Secondary?

In recent years, the secondary stars (mass-donor stars) of WZ Sge-type dwarf novae, or dwarf novae with extremely large outbursts amplitudes (Tremendous Outburst Amplitude Dwarf novae: TOADs), have been regarded as promising candidates for brown dwarfs. The existence of a brown-dwarf secondary star has also been considered to play an important role in realizing an extremely low quiescent viscosity of WZ Sge-type stars required from disk-instability theory (e.g. Meyer-Hofmeister et al. 1998, A&A 339, 507; Mineshige et al. 1998, PASJ 50, L5).

Arguments, however, exist against the extremely low quiescent viscosity. Lasota and Hameury (1995) and Warner et al. (1996) MNRAS 282, 735, assuming evaporation/truncation of the inner disk, are the best examples. The discovery of a WZ Sge-type phenomenon in a long-period system RZ Leo (Ishioka et al. 2001 PASJ 53, 905) suggests that the existence of a brown-dwarf secondary is not a necessary condition for manifestation of the WZ Sge-type phenomenon.

Observational confirmation of cataclysmic variables (CVs) with brown dwarf-secondaries is also important in that it can provide an independent estimate of the upper limit of the age of the Universe. In particular, Howell et al. (2001) ApJ 550, L57 claimed the direct spectroscopic detection of a brown dwarf in LL And, but inconsistency in this interpretation was found later.

Early Superhumps

One of the most remarkable signatures of WZ Sge-type outbursts is the presence of "early superhumps". (This feature is also referred to as orbital superhumps or outburst orbital humps). during the earliest stage of superoutbursts. Early superhumps have a period extremely close to the binary period and 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 detected in other dwarf novae.
(Time-resolved light curve obtained at the earliest stage of the 1995 superoutburst of AL Com. Low-amplitude, doubly-peaked periodic modulation is evident.)