Compared to early models the current Disc Instability Model (DIM) has become rather com- plex. With the addition of physical mechanisms not taken into account in the original model the current version is able to describe many aspects of the dwarf nova outburst cycle: irradiation, evaporation, stream impact heating and mass transfer variations.
Irradiation from the hot central body (Hameury et al., 1999; Schreiber & G¨ansicke,
2001) changes the relation between the mid-plane and surface temperature of the disc. This irradiation will stabilise the inner disc when the central disc temperature is pushed above the hy- drogen ionisation range. At larger radii irradiation will destabilise the outer disc by increasing the mid-plane temperature bringing it close to hydrogen partial ionisation. Smak (2000) plot- ted the ratio of mean irradiation to the secondary’s intrinsic flux finding that DN not showing superoutbursts had a ratio less than 10. Systems with superoutburst have a much larger ratio of greater than 20 indicating that irradiation plays a crucial role in superoutburst systems.
Evaporation of the inner disc into a corona (Meyer & Meyer-Hofmeister, 1994) provides
a means for the white dwarf to accrete during quiescence. The inner disc is evaporated by a siphon flow into a corona. The matter retains its angular momentum which supports it against the gravitational pull of the white dwarf and can then be accreted solving the problem of the intense X-rays observed from CVs during quiescence.
Stream impact heating and tidal dissipation respectively heat and truncate the outer disc
(Buat-M´enard et al., 2001a). The gas stream from the secondary hits the accretion disc, creating a bright spot that heats the outer disc. It allows outside-in heating waves to occur for lower mass transfer rates. The outer radius of the disc is truncated by tidal effects for close binaries and angular momentum transported through the disc is returned to the binary orbit. A range of mass transfer rates are obtained where the model produces alternating short and long outbursts. The outer fraction of the gas stream from the secondary is able to skim over and under the outer disc while the inner fraction of the gas stream bores into the outer disc and is stopped (Schreiber & Hessman, 1998). The overflowing stream interacts with the heating and cooling fronts. However, significant overflow is required to change the outburst.
Mass transfer variations (Smak, 1999; Schreiber et al., 2000; Buat-M´enard et al., 2001b)
can occur on almost every possible time scale leading to the disc exhibiting differing outburst profiles. Models with the mass transfer variations during outburst show that moderate enhance- ment of the mass transfer rate correspond to narrow outbursts and major enhancement of the mass transfer rate correspond to wide outbursts.
Despite these additions to the DIM there are still a number of areas that require im- provement before the predictions made by the model can be properly tested. It was realised quite early in the development of the DIM that in order to reproduce observed amplitudes and durations the parameterαmust have different values in outburst and quiescence, requiring two separate S-curves to be used (Smak, 1984a).
An artifact of using the same α value on the upper and lower branches is that the DIM produces small amplitude variations in the disc luminosity instead of dwarf nova type lightcurves. This indicates that the parameterαmust have different values in outburst and quies- cence, however, this has yet to be confirmed. The DIM is also unable to produce a sequence of narrow and wide outbursts of roughly the same amplitude, however, mass-transfer enhancements may be able to solve this (Smak, 1999).
The single largest issue confronting the DIM today is quiescence (Lasota, 2001). The DIM predicts low quiescent temperatures (. 4000 K) (Gammie & Menou, 1998) below the critical value needed for the Balbus-Hawley instability to operate. However, quiescent DN emit an impressive quantity of hard X-rays with luminosities of 1030−1032 ergs s−1(Verbunt et al., 1997). X-ray eclipsing systems clearly show that the X-rays are emitted by the accretion flow close to the white dwarf (Mukai et al., 1997). Thus for the DIM to be valid another viscosity mechanism must be in operation during quiescence. The low predicted quiescent temperatures also do not correspond with observation (Wood et al., 1986, 1989).
A further problem of the DIM is the quiescent disc, if it were to extend down to the surface of the white dwarf and the X-rays emitted by a hot boundary layer, the required accre- tion rates would be about two and a half orders of magnitude higher than those allowed by the DIM (Meyer & Meyer-Hofmeister, 1994). Reviewing the disc instability model, Lasota (2001) states that the truncation of the inner accretion disc is a necessary ingredient in the explanation of
quiescent X-ray fluxes. Holes present in the inner disc can be formed in a number of ways: mag- netic fields (Livio & Pringle, 1992); evaporation into a siphon flow (Meyer & Meyer-Hofmeister, 1994); or an Advection Dominated Accretion Flow (Menou, 2000). Truss et al. (2004) presented two-dimensional accretion disc models achieving the same effect with a small portion of the disc remaining in a high-viscosity state. The DIM also predicts that quiescent fluxes are increasing, however observations show that they are constant or decreasing (McGowan et al., 2004).
The DIM can describe many aspects of the DN outburst cycle if it is complemented by additional physical mechanisms not taken into account in the original version. However, before the DIM can be considered as providing a credible description of DN outbursts these problems described above must be solved.