Introduction

The composite nature of a typical symbiotic star is shown effectively in Figure 1.1, which exhibits the spectroscopic features commonly associated with these interesting variables, namely (i) a bright red continuum, (ii) strong TiO absorption bands, (iii) prominent high ionization emission lines, and (iv) a weak blue continuum. Berman (1932) and Hogg (1934) were the first to suggest that these phenomena are most naturally explained by a binary star. Both authors suggested that the behavior of an “average” star with combination spectrum could be understood if an M-type giant had a faint O- or B-type companion star similar to that known to exist in o Ceti. This faint star would give rise to the weak blue continuum, and surrounding nebulosity would provide an obvious emission-line region. The small-scale fluctuations observed in these lines and in the continuum are a result of binary motion, while the larger flares are caused by instabilities in the hot source itself.

A few years later, Kuiper (1940) proposed an alternative binary model, suggesting that a system composed of a normal main sequence star and a Roche lobe-filling companion might explain such peculiar stars as Z And and (3 Lyr. Matter lost by the giant falls onto its fainter companion, giving rise to a hot emission region. In this case, flares and other types of random variability are a result of instabilities in the mass-losing star, rather than in the hot, compact star. This explanation gained some support with the detection of gas streams from the giant components in AX Mon and 17 Lep (Cowley 1964 and references therein).

Introduction

In the preceding Chapters, we have seen that symbiotic stars are neither an entirely homogeneous group of variables, nor a random collection of stellar misfits. Although they possess an exciting diversity of hot white dwarfs, hot subdwarfs, and accreting main sequence stars, the unifying factor among symbiotic systems is the evolved red giant star losing mass via stellar wind or tidal overflow. This giant, whether it be a Mira, a semi-regular, or a non-variable red star, severely limits the length of the symbiotic relationship, as the evolutionary timescale for this star is < 106 yr (Becker 1979; Iben 1967). This is a very small fraction of the lifetime of any binary system, and it is therefore important to understand (i) how a given binary manages to become symbiotic, and, once it finds itself in this unusual state of affairs, (ii) how the binary extricates itself from a period of symbiosis.

The formation of binary stars is a process that is not understood in detail, although considerable progress has been made in recent years. The currently most popular scenario for the formation of binary stars begins with the collapse of a rotating interstellar cloud threaded by a magnetic field. The loss of angular momentum by magnetic braking allows the cloud to contract, and instabilities encountered during the early phases of the collapse result in the formation of dense fragments (cf. Mouschovias 1978). Although detailed calculations for the subsequent evolution of the cloud are not yet possible, it is believed that the rapidly rotating fragments undergo fission into smaller components which then contract into normal stars.

Introduction

As we shall see in Chapter 5, the outbursts of symbiotic stars are very exciting events which last from a few weeks to many decades. The appearance of P Cygni-like emission line profiles in the early stages of these outbursts is naturally explained by mass ejection at a velocity of ≈100 km s−1 (e.g., Beals 1951), suggesting that some type of explosive event causes the 2–7 mag increases in brightness (cf. Figures 1.1 and 5.1–5.3). Spectroscopic observations generally show A or F-type supergiant absorption features at maximum visual light (e.g., Belyakina 1979), which diminish in intensity shortly thereafter. In this respect, symbiotics resemble classical novae, which also have A or Ftype spectra near visual maximum (Payne-Gaposchkin 1957). A few systems tend to resemble planetary nebulae at maximum, with fairly prominent Wolf-Rayet emission features pointing to rapid mass ejection via a stellar wind (Thackeray 1977).

With the development of the binary model by Berman and Hogg in the 1930's, the outburst was presumed to be associated with the hot component, although the nature of the instability was left to the imagination. Once single-star models were developed for quiescent symbiotic stars, mechanisms were proposed in which such an object could undergo multiple outbursts. Bruce (1955, 1956) likened symbiotics to tremendous thunderstorms, with outbursts resulting from electrical discharges in the extended atmosphere of an evolved red giant. Gauzit (1955), a champion of the coronal model, postulated that scaled-up versions of solar flares would explain the behavior of eruptive symbiotics, an interpretation supported by Aller and Menzel.

The past two decades have seen a renaissance in the study of symbiotic stars, and modern results have established these objects as belonging to an independent class of binary system. The next decade will see a new generation of satellite observatories, and many cherished theories may crumble under the onslaught of information from Space Telescope, the Hopkins Ultraviolet Telescope, the Extreme Ultraviolet Explorer, and the Advanced X-ray Astronomy Facility (among others). However, the basic properties of symbiotic stars are now well-established, and it is important to understand these characteristics when planning future observational and theoretical projects.

1. Symbiotic stars are binaries with periods ranging from ∼200 days to > 10 years. They are members of the old disk population, and therefore have masses of 1-3 M⊙. Spectroscopic orbits are available for a few systems, and these generally confirm kinematic mass estimates.

2. One component of a symbiotic binary is a red giant. In longperiod D-type systems (P>5-10 yr), the giant is a Mira-like variable losing mass in a powerful stellar wind. Most of the giants in the shorter period S-type systems (P<5-10 yr) tend not to exhibit Mira-like behavior, and lose mass at a considerably lower rate. A few short-period systems contain lobefilling giants which transfer material tidally into an accretion disk surrounding a low mass companion.

3. […]

It has been over fifty years since P. Merrill and M. Humason of Mt. Wilson Observatory reported the discovery of three M-type giant stars with an unusually strong He II λ4686 emission line. Astronomers at Harvard College Observatory found each of these “stars with combination spectra” was a long-period variable, and two of them, CI Cyg and AX Per, had undergone a 3 mag nova-like eruption in the previous thirty years. New objects were added to this group of peculiar variables over the next decade: some of these exhibited the regular radial velocity variations expected of a binary system, while other systems appeared to fluctuate randomly and were thought to be single stars. P. Merrill eventually coined the term “symbiotic stars” to describe objects whose spectra simultaneously display features associated with red giant stars and planetary nebulae.

Symbiotic stars are now commonly accepted as binary systems, in which a red giant star transfers material to its hotter companion. Interactions between the components of a binary system are of special interest in astrophysics today, and symbiotic stars present an exciting laboratory in which to examine such basic physical processes as (i) mass loss from red giants and the formation of planetary nebulae, (ii) accretion onto compact stars and the evolution of nova-like eruptions, and (iii) photoionization and radiative transfer within gaseous nebulae. The physical conditions found in these systems are usually very extreme, and they therefore present activity not easily observed in other binaries.

Introduction

Decisions have to be made regarding the nature of the universe before observed properties of quasars can be transformed into intrinsic properties. We must adopt a cosmology. My objective is not to derive formally the equations of cosmology, as that has been done thoroughly many times. The ultimate purpose of this chapter is to justify these equations qualitatively while putting most of the quantitative effort into describing how to use them to study quasars. The cosmologies to be adopted were derived long before quasars were discovered, but their use became a much more serious affair once quasars had to be considered. This is because the redshifts of quasars are often sufficiently high that differences among different cosmologies become quite large. For most galaxies, by contrast, even the difference between newtonian and relativistic cosmologies can be ignored. We can certainly not yet guarantee the equations of favored cosmologies as applying to the real universe. Once a single cosmology is adopted for it, the universe is forced to become a simple place as regards the structure of spacetime. As long as such a simple universe fits what we see, it is appropriate to retain it. There is certainly little motivation for arbitrarily postulating increased complications; nevertheless, observers must forever be on the alert for those anomalies which would show the simple models to be valid no longer. There is a vested interest in having a simple universe, as that is the only kind we can understand.

Introduction

Astronomy in the early part of this century demonstrated that galaxies were systems made of tremendous numbers of stars. Spectroscopy of galaxies revealed the absorption lines that would be expected in the composite light from stars of different spectral classes. Galaxies showing dominant emission lines in their spectra were recognized as highly unusual. The first of these to be studied, NGC 1068, was commented upon even before the real size and nature of galaxies were understood (Slipher 1918). For several decades, because of their rarity, such galaxies were sufficiently outside mainstream research as to be given little attention. The subject of emission line spectroscopy for extragalactic objects suddenly became extremely important with the discovery of quasars, whose visible spectra are characterized by strong emission lines. Emission lines can provide diagnostics of velocities, temperatures and densities unavailable from any other technique. The lines which can be seen represent a wide range of ionization, so line fluxes also provide indirect measurements of unobserved portions of the continuum. Not least is the fact that emission lines are spectroscopically conspicuous, calling attention to locations where unusual events are occurring. The general similarities among the emission line spectra of quasars, and the scaling of these lines with the continuum source, means that the emission line spectrum is a characteristic quasar feature. To understand the origin of these lines, it is necessary to review the general physical concepts of spectroscopy.

Basic test for evolution

Determining whether the properties of the universe have changed as a function of its age is a major concern of observational cosmology. Not without logic, a universe maintaining the same characteristics through all of time has a satisfying nature. If we could understand it now, we would by definition understand it always. Even those who do not adhere to such a steady state universe have been loath to invoke changing characteristics to the observable galaxies in the universe. Another one of the ironies of the history of astronomy is that the cosmological tests utilized to prove that we inhabit an evolving Friedmann universe, tests applied using the bright elliptical galaxies as distance probes for cosmological purposes, could not allow evolution of those same galaxies (Sandage 1961). Constancy of the galaxy properties was a necessary prerequisite to using them for cosmological purposes. It is presumed now that such galaxies do change, even over observable time scales, and our ignorance about the proper evolutionary corrections to apply has removed much of the stimulus for drawing cosmological conclusions (Tinsley 1977).

Yet, astronomers who two decades ago accepted little evolution for galaxies were never hesitant to accept a lot of evolution for quasars. Even now, it is necessary to invoke far more evolution in quasars than in galaxies to explain the data seen. Few are troubled by this inconsistency, but it is not too surprising that some are.