This chapter is organized as follows: first, three basic types of star are studied in some detail: main-sequence stars, white dwarfs, and black holes; other transient and related types are then discussed more descriptively, classified according to the parameters mass and time.

Main-sequence stars

Main-sequence stars or normal stars are self-gravitating bodies, in hydrostatic equilibrium, consisting of hydrogen, helium, and other minor constituents, in a physical state that can reasonably be described as that of an ideal gas, releasing nuclear energy from the fusion of hydrogen to helium.

The mass loss in the fusion process 4H → He is very large; some 0.7% of the rest mass is converted to radiative and kinetic energy. If the star were pure hydrogen, the fusion of hydrogen could supply energy of 0.001 Mc2, where M is the star's mass. Other fusion processes are much less effective. Atoms heavier than iron cannot be produced by exothermic fusion reactions. A hypothetical process 14 He → Fe would have a mass loss of only 0.1%, supplying only approximately O.OO1Mc2. Hydrogen is thus the major fuel constituent and the most energetic nuclear process is 4H → He. After leaving the main sequence other heavier nuclei are produced, but the energy available is less and the post-main-sequence period is short. Before nuclear burning, a star radiates for a short time due to gravitational contraction. As the largest fraction of the energy is released during the main-sequence epoch, this will be the longest lasting stellar phase.

With less powerful magnetic fields on the primaries, synchronism cannot be achieved. This opens the way for a multitude of periodic phenomena and generates the class of magnetic CVs known as intermediate polars (IPs).

Historical Development

Charles et al. (1979) identified the X-ray source 2A0526–328 as a thirteenth magnitude star with a spectrum like AM Her but lacking detectable polarization; this was the first CV to be discovered by means of its X-ray emission. Photometry (Motch 1981) and spectroscopy (Hutchings et al. 1981a) revealed an orbital period (from RV modulation) of 5h 29.2 min and a photometric period of 5h 11.5min. The beat period between these, viz. 4.024 d, was also present in the light curve.

TV Col (as 2A0525–328 was designated) has turned out to be more complicated than even those initial observations suggested, so although Hutchings et al. proposed the correct basic model for the system, it is now applied to TV Col in a different manner (see below). For simplicity we will therefore move attention to the case history of AO Psc, which was being studied almost at the same time as TV Col.

The X-ray source H2252–035 was identified with a thirteenth magnitude previously unrecognized variable (later designated AO Psc) by Griffiths et al. (1980). Its spectrum resembled a DN at quiescence and photometry showed an orbital modulation of 3.6 h with a prominent 14.3 min periodic luminosity variation superimposed (Warner 1980; Patterson & Price 1980).

It is probable that magnetic fields play a role in all CVs – even in nominally nonmagnetic systems by driving orbital evolution through magnetic braking (Section 9.1.2.1) or in providing the source of viscosity in accretion discs (Section 2.5.2.2). However, in systems where the primary has a relatively weak magnetic field (105 G) the movement of gas from the secondary to the primary is determined predominantly by dynamical and hydrodynamical flows.

This chapter provides an overview of how the physical properties of the various components in non-magnetic CVs can be treated theoretically and deduced observationally.

Classification of CVs

The present system of assigning individual CVs to specific types is a simplification of earlier, more detailed schemes that were developed in response to the steadily revealed diversity of CV behaviour. All early classifications were based on morphology of light curve. The distinction between novae and DN was maintained until the discovery of RN and the later realization that the recurrence times and outburst ranges of RN and DN overlap. Then appeal to their different spectroscopic characteristics became necessary.

Detailed properties of the subtypes that exist among the CN, DN, RN, NL and magnetic CV classifications are given in the respective chapters. For the purpose of this chapter, which examines the underlying binary and accretion disc structure common to all non-strongly-magnetic CVs, it is necessary only to expand a little on the types already introduced in the first chapter.

The topic of nova explosions introduces areas of physics not touched upon in previous chapters: white dwarf structure, nuclear reactions, hydrodynamics of explosive mass loss, common envelope structures, non-LTE conditions in low density ejecta, dust formation. At the same time these are areas that have been comprehensively reviewed and referenced in recent books and articles. This chapter therefore concentrates on the basics of eruption physics, expanding only in those parts that relate strongly to the properties and evolution of the classes of CVs discussed in other chapters. Novae between eruptions are discussed in Chapter 4.

Two books provide detailed reviews of observation and theory of nova eruptions: The Galactic Novae (Payne-Gaposchkin 1957) and Classical Novae (Bode & Evans 1989). The latter contains a comprehensive list of references to observational papers on novae published to the beginning of 1987. Modern review articles, and a few early ones still of value, are Stratton (1928), McLaughlin (1960a), Gallagher & Starrfield (1978), Truran (1982), Bode & Evans (1983), Starrfield (1986, 1988, 1990, 1992), Starrfield & Snijders (1987), Gehrz (1988), Shara (1989), Seitter (1990). Specialized conference proceedings appear in Friedjung (1977) and Cassatella & Viotti (1990).

Lists of novae, finding charts and references are given in Duerbeck (1987), Bode & Evans (1989) and Downes & Shara (1993).

Nova Discovery

Prior to the introduction of wide-field sky photography in the late nineteenth century novae were found generally as naked eye objects.

A variety of stars was early recognised to have eruptions that bear some resemblance to those of novae. Consequently, a very heterogeneous class of nova-like variables (NLs) was introduced (see, e.g., Campbell & Jacchia 1941; Kukarkin et al. 1958; Petit 1987), which, with hindsight, is seen to include many types of object that are totally unrelated structurally to the true novae. For example, η Car, γ Cas and P Cyg stars do not have the duplicity of CVs. Symbiotic stars, on the other hand, may all be close binaries and some may contain degenerate components as in the CVs (Kenyon 1986).

With the removal of these eruptive objects to their own classes, paradoxically only the non-eruptive residue remained as NLs (many of which, however, may have ‘low states’). It is from their short time scale spectroscopic and photometric behaviour (including the evidence of binary structure) that such stars are recognized as resembling novae between eruptions (they could more appropriately have been termed ‘NR-like’).

Classifications

From the incomplete discovery of novae in earlier centuries (Section 1.1) it is clear that among the NLs there should be many unrecognized NRs. Similarly, the fact that many novae discovered this century can be found as blue objects on archival sky survey plates shows that among the currently known NLs must be a number of pre-novae.

This first chapter is designed to give the reader an historical perspective on the subject of cataclysmic variable (CV) stars. Ground-based photometric and spectroscopic observational developments up to 1975 are treated in detail. Since that date instrumental methods in the optical region have been to some extent fixed, and to continue the historical approach would be repetitive of much of what appears in later chapters. The introduction of observational techniques in other wavelength regions is, however, followed beyond 1975

Pre-1900 Observations of Novae

If the ancient philosophers had been correct in their assertion that the distant stars are immutable, incorruptible and eternal, astronomy would be the dullest of disciplines. Fortunately, they were wrong on all counts. The stars possess variability on all time scales and amplitudes, sufficient to satisfy all interests, from the exotic to the commonplace, from the plodding to the impatient.

Among these, the most prominent celestial discordants are the novae Stella: new stars, challenging the ancients in their own times, but, such was the power of Aristotelian philosophy, passing almost entirely unacknowledged in European and Middle Eastern societies until the post-Copernican era (Clark & Stephenson 1977). In China, however, records of celestial events (kept mostly for astrological purposes) have been maintained since c. 1500 BC, and there are supporting and supplementary records in Japan from the seventh century AD and in Korea from c. 1000 AD (Clark & Stephenson 1976, 1977). Among these are numerous accounts of temporary objects, from which may be sifted comets, meteors, novae and supernovae.

The history of cataclysmic variable star research mirrors the objects themselves: periods of relative inactivity punctuated by heightened or even explosive advances. Until about 1970 each resurgence of interest was a result of a distinct technological advance. In the past two decades the technological improvements have been almost continuous and the interest in cataclysmic variables has burgeoned from the realization that they have so much to offer. Not only are they of interest per se, exhibiting a challenging range of exotic phenomena covering the electromagnetic spectrum from radio waves to TeV gamma rays, and time scales from fractions of a second to millions of years, they are important for their relevance to other exciting areas of astrophysics.

For example, it has become evident that accretion discs are one of the most commonly occurring structures – probably all stars form from disc-like configurations, with material left over to provide planetary systems. A large fraction of binary stars form accretion discs at some stage of their evolution. Accretion discs are important in X-ray binaries – matter accreting onto neutron stars or black holes. Entire galaxies are initially gaseous discs, and most may develop central discs intermittently that fuel their active nuclei.

But it is in cataclysmic variables (CVs) that accretion discs are observed to best advantage – quasi-stable discs, unstable discs and transformations between them. In dwarf novae during outburst, or in nova-like variables in their high state, the light is dominated by emission from discs – and being almost two-dimensional their observed properties are strongly affected by the viewing angle. All are close double stars, and those with eclipses present unrivalled opportunities for determining spatially resolved physical structures.

From systems that are weakly or covertly magnetic we turn to ones in which the magnetic field of the primary is strong enough to control the accretion flow, preventing the formation of an accretion disc and generating the signatures of magnetic accretion: large linear and circular optical polarization and strong X-ray emission.

Historical Development

The discovery of the polars provides a lesson that even relatively familiar objects may reveal exotic phenomena if interrogated in the correct way. The star AM Her had been discovered as a variable in 1924 and listed as a NL on the basis of slow variations in brightness over a range of 3 mag and an emission-line spectrum. In 1976 Berg & Duthie (1977) suggested that AM Her could be the optical counterpart of the Uhuru X-ray source 3U 1809+50 and Hearn, Richardson & Clark (1976) using the SAS-3 satellite found a variable soft X-ray source near the same position. The similarity of this source to the low mass X-ray binaries Sco X-l and Cyg X-2 stimulated Cowley & Crampton (1977) to obtain spectra, which revealed a 3.09 h orbital period.

The main surprise came, however, when Tapia discovered in August 1976 that AM Her is linearly and circularly polarized at optical wavelengths (Tapia 1977a). Its linear polarization varies from zero up to 7% and its circular polarization from −9% to +3%, both changing smoothly over the period of 3.09 h (Figure 1.12). The high degree of circular polarization, previously only seen in magnetic white dwarfs (Angel 1978), suggested the presence of a strong magnetic field.

The DN, already introduced in Sections 1.2 and 1.3 with their classification scheme described in Section 2.1, are arguably the most valuable of objects for the study of accretion discs. Among them examples may be found of optically thin discs and optically thick discs, of face-on discs and edge-on discs, of non-steady discs and of nearly steady state discs and of transitions between them. Furthermore, the brightest DN at maxima reach apparent magnitudes of 8–10, at which time the entire flux is conveniently of almost pure accretion origin.

Well-Observed DN

It is inevitable that a few relatively bright DN, especially the eclipsing systems, have been preferentially observed. Although over 200 DN have been classified by their light curves, only a small fraction have been studied sufficiently to establish their orbital periods. It will be seen in this chapter that POrb plays an important rôle in the systematics of DN. Among the DN in general, 12 Z Cam stars, 29 definite U Gem stars (including, slightly unconventionally, the three systems BV Cen, GK Per and V1017 Sgr with large POrb), 34 SU UMa stars and 22 objects suspected of belonging in the DN class have known orbital periods. The SU UMa stars may be overrepresented because their orbital periods are easy to estimate, independent of inclination, from photometric observations made during super outbursts. Orbital periods for the U Gem and Z Cam class have come predominantly from spectroscopic observations, with the addition of a few found from photometric orbital variations (eclipses, bright spot modulation, IR ellipsoidal modulation).

An overview

The classical spectroscopic signature of mass loss, first reviewed in the literature by Beals (1950), is the so-called P Cygni line profile. This label has come to be attached to the profile shape in which blueshifted absorption sits alongside redshifted emission. In truth, the practice of describing just this configuration as ‘P Cygni’ does little justice to the rich variety of profile forms that are to be found in this famous star's optical spectrum—Beals himself put the case for 4 different profile types characteristic of ‘P Cygni stars.’ Interestingly, from the perspective of this collection of papers on line emission, these other forgotten types include forms that emphasise emission rather than absorption. Indeed, those of us who have taken spectra of P Cygni itself are painfully aware of just how strong the strongest emission features (in Hα, He i λ5876) really are!

Introduction

The history of ultraviolet (UV) spectroscopy in astronomy spans over three decades now and such observations have led to many discoveries regarding the physical nature of the entire gambit of astronomical objects. Hot astrophysical plasmas have line and continuum emission and absorption processes for which UV spectroscopy can probe the more energetic physical processes that cannot be studied adequately in the optical or infrared. In addition, studies of the UV spectral properties of cooler bodies, such as planetary atmospheres, comets, and interstellar dust provide important information on their physical state and composition.

This article concentrates on reviewing some of the techniques and results from the study of emission lines in astronomical UV spectroscopy. Given that the range of astronomical objects from the Earth's geocorona to quasars show UV emission lines and that during the past three decades over two thousand papers have appeared in the literature, including numerous conferences and books, a comprehensive review is unpractical.

Apart from stars and those objects which radiate reflected starlight, most of the objects in the Universe radiate an emission spectrum. It was the astronomers interest in analyzing the spectrum of the sun and other stars in the last century that motivated the development of radiative transfer, and with the newly formulated macroscopic relations of LTE early in this century, that led to our understanding of absorption spectra. The original observational stimulus for this activity had been Fraunhofer's study of the solar spectrum almost a century before.

Interest in emission-line spectra came later, when spectrographs coupled to telescopes enabled the spectra of fainter gaseous emission regions to be observed. They revealed a totally different type of spectrum than that which had been observed from stars. The fact that local thermodynamic equilibrium does not hold for emission regions has complicated the interpretation of their spectra. Huggins' initial discovery of ‘nebulium’ in gaseous nebulae and its subsequent identification with ionized oxygen by Bowen had demonstrated that rarefied conditions must pertain in nebulae. Stromgren's subsequent 1939 paper in the Astrophysical Journal was a landmark in demonstrating how far-UV continuum radiation from a hot star was absorbed by surrounding gas and converted into visible Balmer line radiation.

In the decades that followed, the realization that many interesting objects such as supernova remnants, active galactic nuclei, and quasars radiated an emission-line spectrum, motivated the analysis of emission regions.

Introduction

Gamma ray lines are the signatures of nuclear and other high energy processes occurring in a wide variety of astrophysical sites, ranging from solar flares and the interstellar medium to accreting black holes and supernova explosions. Their measurement and study provide direct, and often unique, information on many important problems in astrophysics, including particle acceleration, star formation, nucleosynthesis and the physics of compact objects.

The physical processes that produce astrophysical gamma ray emission lines are nuclear deexcitation, positron annihilation and neutron capture. Excited nuclear levels can be populated by the decay of long-lived radioactive nuclei as well as directly in interactions of accelerated particles with ambient gas. Nuclear deexcitation lines following radioactive decay have been seen from supernova 1987A (Matz et al. 1988; Tueller et al. 1990; Kurfess et al. 1992), from the supernova remnants Cas A (Iyudin et al. 1994) and Vela (Diehl et al. 1995), and the interstellar medium (Mahoney et al. 1984; Share et al. 1985; Diehl et al. 1994; 1995).