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Novae outbursts have been observed for over 2000 years. However, it was not until the 1920s that it was realised that classical novae represented a different phenomena

Figure 1.7: Light curves of the fast nova V1500 Cyg in red and slow nova HR Del in blue. The data was taken from AAVSO [1].

to supernovae [24]. Like dwarf novae, novae occur in cataclysmic variable systems and are a consequence of the accretion onto the white dwarf. Material striped from the secondary star forms an accretion disc which gradually deposits the material onto the primary (see above). Over time this layer of accreted hydrogen builds up putting the lower layers under increasing pressure. Eventually this become sufficiently great to initiate thermonuclear burning [150] which leads to a rapid runaway reaction causing the entire accreted layer to burn or be ejected as an expanding nova shell. This greatly increases the luminosity of the system, typically by 8-15 magnitudes [63]. The exact properties of the nova are determined by the white dwarf mass, the white dwarf temperature and the mass accretion rate [176]. Once the nova eruption is complete the system returns to its accreting state indicting that no long term change has occurred to the system as a whole. This process and the physics behind it is again well reviewed in Brian Warner’s bookCataclysmic Variable Stars [167].

and spectral properties [24]. Systems that are only known to have had one nova eruption are known as classical novae. The duration of these eruptions can vary dramatically. V1500 Cyg only took 3.6 days to decrease 3 magnitudes from peak outburst while HR Del took 230 days to do the same. A plot showing the light curves of a slow nova (in blue) and a fast nova (in red) is shown in Figure 1.7. As can be seen from this plot, fast novae can resemble WZ Sge systems around peak outburst which is an important consideration when classifying transient candidates. Novae that are seen to erupt multiple times are classified as recurrent novae. Typically recurrent novae have recurrence times of decades and often have large or- bital periods with giant secondaries [152]. These short recurrence times also require a high mass primary (often close to the Chandrasekhar limit) and comparatively high accretion rates [149]. Currently only 10 recurrent novae are known in our Galaxy with several others identified in neighbouring galaxies [24].

Observations of novae are scientifically useful for several reasons. Firstly, using detections of the peak apparent magnitude and the shape of the decline it is possible to use novae as distance indicators [38]. There are also several significant unanswered questions in our understanding of novae themselves [24]. For example, the role of novae in CV evolution is still poorly understood. It is thought that there may be a link with recurrent novae and supernovae of type Ia. In classical novae heavy elements are often detected in the nova shell suggesting that the nova eruption blasted off more than just the accreted material causing a net reduction in the white dwarf mass. However, in recurrent nova it is thought that the white dwarf mass (already near the Chandrasekhar limit) may be increasing [148]. The natural conclusion for this would be a type Ia supernova as described in Section 1.5.4. Secondly it is not understood whether classical nova and recurrent nova really represent the same population (with all novae being recurrent and our current recurrent nova population being those that repeat on the shortest time scales).

The potential for a survey using the WASP observatories to address these questions is somewhat limited. The current discovery rate for galactic novae is ∼8yr−1[120]. The overall estimated rate in the Galaxy is∼34yr−1[38] with∼12yr−1

predicted to be brighter than 11th magnitude in V [90]. This low rate of galactic novae means that they are normally discovered in the galactic plane. Fast novae in particular are confined to within 100 pc of the plane (although slow novae can reach up to 1000 pc) [24]. The WASP observatories do not observe in the galactic plane because of the problem of stars blending together (see Section 2.1). While it is possible for WASP to detect nova in other galaxies (such as M31), only the brightest examples could be found and only in ideal observing conditions.

An alternative source of detectable novae could be tramp novae which are thought to be stripped out of galaxies during galactic collisions [146]. Measurements of the positions and rates of these could provide a way to estimate the quantity and distribution of stars in intergalactic regions. From this it may also be possible to establish the proportion of mass in the universe that is found as these intergalactic stars. Novae are one of the best ways to measure intergalactic populations because they can be used as standard candles to accurately estimate their distance.

Overall, while WASP is not well suited to identifying novae in the galactic plane, it may be possible to detect intergalactic or halo novae. These could be used to estimate the distribution and quantity of stars in these regions. Novae also form an important consideration when classifying WZ Sge candidates because of the similarities in their light curves.