Light curves during eruption

Modern studies of nova light curves still depend largely on the descriptions introduced 50 or more years ago. Almost all novae rise rapidly (1-3 d) and are not then sufficiently well observed to warrant division into types. To quantify the very different rates of decline from maximum light, Payne-Gaposchkin (1957) introduced the 'speed classes' listed in Table 2.2 (where it will be noted that there are no 'normal' novae, they are either fast or slow; cf. dwarf and giant stars). The notation tn is used to designate the number of days, t, that the nova took to fall n magnitudes from maximum. The implied rate of decline, d V/dt, is also listed. From a correlation of measured values, Warner (1995) finds that t3 « 2.75 t^88 (see also Chapters 1 and 14).

In many compilations, including the Downes and Shara (1993) catalogue and its later versions, the speed classes are contracted into the notation NA, NB and NC, meaning fast, slow and very slow. Recurrent novae are designated NRA if they have giant secondaries, and NRB if they have non-giant secondaries.

By compression of their time-scales McLaughlin (1939, 1960) found that all nova optical light curves can be made to resemble each other, and that the spectral evolutions are at the same time brought into better accord. The idealized light curve is shown in Figure 2.2.

2.3.1 Initial rise, pre-maximum halt and final rise

Few novae have been caught on the initial rise - though it is sometimes possible to construct at least part of the early rise from patrol images of the sky taken by amateurs (V1500 Cyg 1975 is a good example: Liller et al, 1975). The brightening to within about 2 magnitudes from maximum takes at most 3 days, even for the slowest novae. In many novae there is a pause, ranging from a few hours in fast novae to a few days in slow novae, about 2 magnitudes below maximum. The nova then brightens to maximum, taking 1-2 days for fast novae and several weeks for the slowest novae. The maximum phase itself is relatively short-lived, being only hours for very fast novae and only a few days for slow novae.

2.3.2 Early decline and transition

The initial fall from maximum is usually smooth, except for slow novae, which have brightness variations on time-scales of 1-20 days with amplitudes up to 2 mag. At 3 to 4 magnitudes below maximum three distinct behaviours are possible: (i) some novae fall into a minimum 7 to 10 mag deep and lasting for months or even years, after which the star recovers and follows an extrapolated decline; (ii) other novae start large amplitude oscillations with quasi-periods of ~5-15 days and amplitudes of up to 1.5 mag; (iii) a few novae, comprising

Example Novae Light Curves
Fig. 2.2. Morphology of a nova light curve. From Classical Novae, first edition (Bode & Evans, 1989).

about one-third of the fast and very fast systems plus a few of the slower ones, pass through this transition region without any noticeable peculiarities.

The deep minimum that occurs on the decline of some novae is due to the formation of dust in the gas ejected by the eruption. This was first made clear by infrared photometry of FH Ser, which developed an infrared excess when the optical light curve began to fall into its 5 mag minimum (Hyland & Neugebauer, 1970; Geisel, Kleinmann & Low, 1970; see also Chapters 1 and 13). The sum of the optical and infrared energies remained approximately constant at ~ 1.5 x 104 L©. A second indication that a nova eruption is roughly a constant bolometric luminosity event came soon after - the realization that in the early stages of decline the fall in optical flux is counterbalanced by an increase in ultraviolet flux was a result of OAO-2 satellite measurements (Gallagher & Code, 1974). Here the reason is that as the ejected shell becomes optically thinner we see radiation coming from deeper in towards the central binary with its heated primary (see Chapters 4 and 5).

The modern understanding of nova eruptions is that they have bolometric luminosities that are all at or somewhat above the Eddington luminosity, and the total radiant output lies in the range 1045-1046 erg. The parameter determining the speed class is the initial rate of generation of energy, which itself is largely a function of the mass of the primary.

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