The paper is divided into two parts. In the first part, the common envelope phase that follows a nova outburst is discussed. It is shown that this phase leads to mass loss, preferentially in the orbital plane. It is argued, that the common envelope can explain the rapid appearance of a nebular spectrum in novae and the shaping of the nebula.

In the second part, the present status of the cyclic-evolution (“hibernation”) scenario is reviewed. It is argued that novae and dwarf novae are the same systems, transforming from one class to the other. Observational and theoretical evidence is presented, that shows that both the accretion rates and the white dwarf masses in observed nova systems should be higher than the average. Novae should therefore not be regarded as “typical” cataclysmic variables. Some critical observations that can provide observational tests are suggested.

The cyclic evolution of classical novae (CN) cannot be observed, as in the case of recurrent or dwarf novae. The duration of a typical cycle may range from a few thousand to a few 105 years. Thus we have to rely on theoretical studies for understanding the periodic outbursts — due to thermonuclear runaways (TNR) — on the surface of accreting white dwarfs (WD). One may distinguish between two kinds of studies: a) detailed investigations of a specific aspect or a particular evolutionary phase; b) studies which emphasize the cyclic behavior of novae and their long-term evolution. The latter, which form the subject of this brief review, may be divided into four groups, each adopting a different approach to the problem:

1. 1. Steady-state (semi-analytic) solutions;

2. 2. One- (or two-) zone models;

3. 3. Quasi-static (and/or steady burning) numerical calculations;

4. 4. Full-scale hydrodynamic evolutionary computations.

The nova outburst requires an energy source that is energetic enough to eject material and is able to recur. The Thermonuclear Runaway (TNR) model, coupled with the binary nature of nova systems, satisfies these conditions. The white dwarf/red dwarf binary nature of novae was first recognized as a necessary condition by Kraft (1963,1964, and these conference proceedings). The small separation characteristic of novae systems allows the cool, red secondary to overflow its Roche lobe. In the absence of strong, funneling magnetic fields, the angular momentum of this material prevents it from falling directly onto the primary, and it first forms a disk around the white dwarf. This material is eventually accreted from the disk onto the white dwarf. As the thickness of this hydrogen-rich layer increases, the degenerate matter at the base reaches a temperature that is high enough to initiate thermonuclear fusion of hydrogen. Thermonuclear energy release increases the temperature which in turn increases the energy generation rate. Because the material is degenerate, the pressure does not increase with temperature, which normally allows a star to adjust itself to a steady nuclear burning rate. Thus the temperature and nuclear energy generation increase and a TNR results. When the temperature reaches the Fermi temperature, degeneracy is lifted and the rapid pressure increase causes material expansion. The hydrogen-rich material either is ejected or consumed by nuclear burning, and the white dwarf returns to its pre-outburst state. The external source of hydrogen fuel from the secondary allows the whole process to repeat.

Abundance determinations for the ejected shells of classical novae are reviewed. Substantial enrichments (relative to hydrogen) of helium, the CNO elements, and elements in the range from neon to aluminum are found to be characteristic of nova ejecta. The source of these enrichments is believed to be the dredge-up of underlying white dwarf core matter, thus reflecting the white dwarf composition and confirming the presence of oxygen-neon-magnesium (ONeMg) as well as carbon-oxygen (CO) degenerate dwarfs in cataclysmic variable systems. The implications of these abundance enrichments, for the behavior of nova systems in outburst and for nucleosynthesis, are examined.

Using the distribution of white dwarf masses in zero-age cataclysmic variables (CVs) calculated by Politano and Webbink (these proceedings), the relative frequency of classical nova outbursts as a function of white dwarf mass is calculated. These results are compared with the results obtained by Truran and Livio (1986), who calculated the same function, but used a distribution of white dwarf masses in CVs calculated from a Salpeter initial mass function and a particular progenitor mass-white dwarf mass relationship for single stars.

We wish to calculate the frequency of classical nova outbursts as a function of white dwarf mass. To do this, we require two quantities: 1) the recurrence frequency of classical nova outbursts (i.e., how many outbursts per year) and 2) the white dwarf mass spectrum in classical novae (i.e., number of novae per white dwarf mass). We discuss each of these in turn.

The influence of accretion in the evolution of CO white dwarfs is calculated, up to the thermonuclear runaway, with a previous analysis of the H-He burning shell. For this later study, a two-zone model is developed, consisting of two thin shells in plane-parallel approximation. The influence in the inner core evolution is discussed.

We have obtained the effective growth rate of white dwarf masses which are suffering mass loss during both hydrogen and helium nova outbursts. If the mass transfer rate from the companion is smaller than 10−7 M⊙/yr, the net growth rate is reduced to less than one tenth of the mass transfer rate from the companion star. It is suggested that the white dwarf mass is hard to grow to the Chandrasekhar mass unless its initial mass is very close to the Chandrasekhar limit.

A zero-age cataclysmic variable (ZACV) we define as a binary system at the onset of interaction as a cataclysmic variable. We present here the results of calculations of the distributions of white dwarf masses and of orbital periods in ZACVs, due to binaries present in a stellar population which has undergone continuous, constant star formation for 1010 years. These results differ from previous work (Politano and Webbink 1988) in that an improved criterion for stability against rapid mass transfer by the secondary to the white dwarf has been used. A brief outline of the method and key assumptions made in this calculation is given in Politano and Webbink (1988).

The white dwarf mass distribution of ZACVs (shown in Figure 1) contains two components: systems with helium white dwarfs and systems with C-0 white dwarfs. Systems with helium white dwarfs comprise slightly greater than 50% of all ZACVs. The helium white dwarfs have masses which range from 0.27 to 0.46 solar mass. The C-0 white dwarfs have masses which range from 0.54 solar mass up to the Chandrasekhar mass. (Note: systems with O-Ne-Mg white dwarfs are not distinguished from systems with C-0 white dwarfs in this calculation. Presumably, these O-Ne-Mg systems comprise the upper end of the white dwarf mass distribution.)

The main outstanding problem in our full understanding of the classical nova mechanism is the apparent discrepancy between mass transfer rates (10−9 to 10−8M⊙/yr) inferred from observations (Patterson, 1984, Ap.J.Suppl.231, 789), and those required by numerical models in order to reproduce nova characteristics (10−11 to 10−9M⊙/yr). The low accretion rates are needed in order to obtain powerful runaways and high values of Z in the ejecta by the diffusion-convection mechanism. The discrepancy seems to have sharpened by the realization that accretional heating (Shaviv and Starrfield, 1987, Ap.J.321, L51) and angular momentum transfer (Sparks and Kutter, 1987, Ap.J.321, 394 and Kutter and Sparks, 1987, Ap.J.321, 386) tend to lower the theoretical upper limit for Ṁ to about 10−10M⊙/yr. On the other hand, scenarios invoking variable mass accretion rates — hibernation (Shara et al, 1986, Ap.J.311, 163) or ‘mild’ hibernation (Livio, Shankar and Truran, 1988, Ap.J.330, 264) — have shown that the high observed rates immediately prior and following outbursts can be reconciled with lower average accretion rates over the period between outbursts.

According to modern conceptions the cause of nova outbursts is a thermal runaway (TR) taking place in the deep layers of the accreted envelope of a white dwarf (WD), component of a close binary system. The theory of TR was considered by many authors and, especially, has been developed in detail by Starrfield et al. (1974, 1984). It has been shown that when the pressure at some level in the envelope reaches a critical value Pcr, the thermonuclear reactions of the “hot” CNO cycle become very fast, and after a time interval of several tenths seconds the temperature at this level rises to (2 ÷ 3)× 108 K. If the CNO abundances are high enough, the total outburst energy may exceed 1047 ergs. As a consequence of TR, a considerable fraction of the envelope mass (20% ÷ 50%) must be torn off by shock waves. As suggested by Starrfield et al. (1974), the remaining matter forms an extended atmosphere (R ≃ 1011 cm) and the star transforms itself into a supergiant having a luminosity L* ≲ Ledd during several months.

Accretion occurs from a disc or possibly from polar caps in the case of magnetic novae: it is doubtful whether a spherically symmetric and homogeneous envelope configuration is eventually reached. A number of works (see in particular Kippenhahn & Thomas, 1978, King & Shaviv, 1978, Livio & Truran, 1988) adressed to this problem. Even if the matter can be almost homogeneously distributed - at least for high accretion rates - it is very likely that the chemical composition would not be uniform because of the shear mixing in the equatorial belt. Moreover, there can be temperature gradients. These and several non spherically symmetric factors (see the discussion of Tutokov & Yungelson, 1974, on the effects of rotation and magnetic fields) could lead to runaway only in a portion of the envelope. Instead of having a spherically symmetric explosion there would be a local thermonuclear runaway (LTNR). Estimates of Shara (1982) indicate that, with certain sets of parameters, the time for thermalization of the envelope would be much longer than the time to reach the conditions for the runaway and the outburst in such a case would occur only in a certain portion of the envelope. A non-spherically symmetric outburst could be the cause for the peculiar equatorial belts - polar caps geometry of the shells of many novae. There are other explanations (for instance interaction of the secondary with the expanding shell, like suggested by Livio and by Shankar et al. in this colloquium), but bidimensional spectroscopy seems to favour also difference in chemical composition in the envelope and in the polar caps (Duerbeck 1987) and high resolution Hα spectroscopy of Nova Cen 1986 hints that the blobs in the shell have formed at the time of the outburst (Bandiera & Focardi 1989) and cannot be explained by a spherically symmetric TNR model.

Thermonuclear models of recurrent novae demand white dwarf accretors near the Chandrasekhar mass. In this case, the known recurrent novae should possess classical counterparts bearing the same structural parameters and space distribution, save for having only marginally less massive white dwarfs. Furthermore, recurrent novae should occur exclusively on ONeMg white dwarfs, and display in their ejecta either neon-group overabundances (if the white dwarfs are eroded through an outburst cycle) or no heavy element enhancements whatever (if the white dwarfs increase in mass).

The known recurrent novae are reviewed in the light of these and other characteristics of thermonuclear runaway models, and also in terms of accretion-powered events, with special attention to the difficulties encountered by both models. Pivotal tests to distinguish between between thermonuclear and accretion models rely on the fact that the latter require far more mass transferred than the former to produce the same outburst energetics. Thus, photospheric opacities in thermonuclear recurrent novae are dominated by scattering; those in recurrent accretion events by true absorption. Orbital period changes through outburst are 103 times greater in accretion models than in thermonuclear models.

Symbiotic Novae are composite spectrum variables, whose light history is characterized by a single nova-like outburst. Their behaviour and time evolution is rather similar to that of classical novae, except for the much longer time scale, and the associated physics could be the same. We give an overview of the basic characteristics of the symbiotic novae, and their time behaviour in different frequencies. We find that the observed amplitude of outburst is mostly determined by the red star visual luminosity. The actual luminosity increase of the compact object might be much larger than that observed, and closer to that of classical novae. In some systems, the red star luminosity is possibly weakened by circumstellar dust. In symbiotic novae accretion should occur from the red star wind. Although the actual value of the accretion rate is still uncertain, thermonuclear runaway seems to better describe the symbiotic novae phenomenon. But new observational and theoretical work is needed.

The recurrent nova RS Ophiuchi has had five outbursts in 1898, 1933, 1958, 1967 and 1985. We present here in brief the spectroscopic results of the 1985 outburst and subsequent quiescence, based on data obtained at the Vainu Bappu Observatory (VBO), Kavalur. Detailed results of the 1985 outburst appear in Anupama & Prabhu (1989).

Spectra were recorded in the range 6200-8900 A at dispersion of 194 A mm−1 at four epochs: 32, 59 & 60; 89 & 91 and 108 days since the 1985 outburst maximum on January 27.5. In addition, spectra were recorded in the range 4300-5600 and 5000-7600 A at dispersion of 132 A mm−1 on day 60, and in the range 5700-8000 A, at dispersion of 194 A mm−1, on days 89, 91 and 108.

From 1986 to 1988 optical spectroscopy near minimum of all then established recurrent novae (excluding recurrent X-ray novae such as V616 Mon and V404 Cyg) was carried out with the 3.6 m and 1.5 m telescopes of ESO, using EFOSC with CCD and the Cassegrain spectrograph with CCD or IDS, respectively. Flux-calibrated spectra of the six objects are shown in Figs, 1a – f. The most recent recurrent nova, V745 Sco, is included in the present discussion, because 1 month after outburst it had already reached a very late stage of outburst evolution. A spectrum of V745 Sco is shown in Schwarz et al. (1989).

The dynamical evolution and nonequilibrium X-ray emission of recurrent nova remnants have been investigated by using a spherically symmetric hydrodynamic code. We assume that the nova ejecta expand into a wind from a red-giant companion. The wind material is blast-shocked, and emits copious X-rays. The blast shock soon breaks out of the wind region and the X-ray emission declines drastically. The blast shock eventually catches up with the relatively slow ejecta of the previous outbursts. The X-ray emission may then be rejuvenated in both luminosity and spectral shape.

On May 22, 1989 the Japanese Ginga Team discovered a new X-ray source that was cataloged as GS 2023+338. This source was subsequently identified as coincident in position with a previously known nova cataloged as V404 Cygni. Its last recorded outburst was in 1938 when it rose to about 12th mag. Spectroscopic data were obtained and confirmed the nature of the outburst. Additional ground based data were obtained by us at CTIO and the VLA. The X-ray behavior of this object has been reported to be very unusual and it reached a peak of about 17 crab about one week after discovery. Since then it has varied widely in magnitude at all wavelengths at which it has been studied. We present a brief summary of the observations that have been obtained up to the time of the meeting and shortly thereafter.

Symbiotic novae are composite spectrum stars whose light history is characterized by one single major outburst. At high resolution their optical and UV spectra are very rich of emission lines belonging to a wide ionization range. Generally one can identify both narrow and broad line systems. The latter includes emission lines with multiple structure, broad emission wings, WR features, and P Cygnl lines. Frequently the broad line profiles are largely variable with time. They should be associated with the major stellar outburst. An overview of the problem with extensive references can be found in Viotti (1989).

In their study on C/N/O abundances in red giants, planetary nebulae, novae and symbiotic stars Nussbaumer, Schild, Schmid and Vogel (1988: Astron.Astrophys.198, 179) pointed out that among symbiotic stars HM Sge is quite exceptional, in that its C/N/O abundance ratios rather resemble those of novae and not of symbiotic stars.

In 1975 HM Sge brightened within a few months by at least 4m. The object has been observed in radio, visual, X-ray, and since 1978 with IUE. Here we show the changing level of activity in the UV. Figure 1 gives the FES (IUE fine error sensor) counts, representing an integrated flux at visual wavelengths. This flux shows a general decline with a secondary maximum in 1982. – Our mFES is at variance with the visual magnitude estimates given by Munari and Whitelock (1989: Mon.Not.Roy.astr.Soc. 237, 45p). They report high brightness up to at least the end of 1984.

The infrared photometry of the symbiotic nova HM Sge, with a Mira variable as a cool component, shows that since 1983 its mean IR brightness began to decrease from the previous post-outburst level (Figure 1). In 1988 the mean J magnitude was already about 1 mag fainter than it had been until 1983.

An analysis of the photometric observations shows that the decrease in the mean IR light level of HM Sge may be understood in the framework of the commonly accepted model of this binary stellar system (Taranova and Yudin 1983), if we suggest that the Mira variable and the inner parts of its circumstellar envelope (RMira envolope~ 3×l014 cm; τdust ≲ 1 at 1.25 µm until 1983) were gradually obscured by the dust originated between them and the observer.