Eruptive Variables

Eruptive variables involve violent transient events (explosions). These may vary from small flares (on stars of the main sequence) to "flare stars" (where the energies may be as much as 103 times greater) to complete destruction of the star (supernovae). The GCVS (Kholopov 1985) lists the following types of variable stars in the "eruptive" category: Stars with brightness increase due to "violent processes and flares" but they include also "shell events or matter outflow" in stellar winds with possible interaction with interstellar matter. As a consequence, this categorization includes many slower phenomena as well as stars undergoing very rapid changes. The GCVS types belonging to this category include

• FU Orionis variables (typically 100-fold brightening over intervals of months, followed by constancy or slow decline over years to decades. Possibly associated with the T Tauri evolution stage "Orion variables". A reflecting nebula spectrum is always seen, and emission lines are seen at outburst);

• y Cassiopeiae variables [Rapid rotators, these objects are hot, emission line stars (Be stars), and Doppler-shifted spectral lines indicate rapid outflow. Their light variation may be as much as 1.5 magnitudes];

• Orion variables (irregular variables associated with nebulosity. Some are found to be T Tauri stars, an early stage in stellar evolution; some are rapidly varying; spectral types distinguish subcategories);

• R Coronae Borealis variables [described as both eruptive and pulsating, these objects suffer fading and gradual recovery over intervals of months; the variation is thought to involve ejections of carbon (soot, basically)];

• S Doradus variables (high-luminosity stars characterized by envelope ejections; P Cygni and n Carinae are members of this class. The latter, at least, has been identified as an Asymptotic Giant Branch star undergoing thermal pulses, a late stage in stellar evolution);

• UV Ceti variables (typically late-type dwarf stars undergoing flares with rise times of seconds and recoveries of minutes of duration; a related group is associated with "Orion variables". In clusters, these objects are simply known as "flare stars");

• Wolf-Rayet variables (very hot stars with irregular light variations).

In addition, the GCVS identifies RS Canum Venaticorum variables among the eruptive variables. These objects are typically giant or subgiant interacting stars characterized by strong magnetic field interactions and active chromospheric regions, causing quasi-cyclic variations in light curve shape outside of eclipse (cf. Hall (1975)). As these are enhanced forms of phenomena associated with all late-type, main sequence stars (such as the Sun) there is disagreement about whether or not RS CVn-like systems deserve a separate group designation. In this book, we prefer to call systems that exhibit enhanced active regions merely as "RS CVn-like."

The GCVS has another category in which the more violent members of eruptive variables are placed; the "cataclysmic" variables, described as having outbursts produced by thermonuclear processes either on the surface or in the interior. The name "cataclysmic variable" usually means a slightly different type of object to much of the variable star community; it involves a white dwarf and a cooler, less evolved star that has filled its inner Lagrangian surface (see Sect. 3.1.6) resulting in a semi-detached binary star system. The system is further characterized by a stream of material feeding either directly on to, or into a disk surrounding, the white dwarf. As the material in the disk loses angular momentum, it spirals onto the white dwarf's surface. This is, in fact, a nova or even supernova, waiting to happen, but it is (usually!) not happening (yet!). See Warner (1995) for a definitive discussion of CVs and Hellier (2001) for practical details. In any case, the GCVS category includes novae of various types, namely "fast," "slow," or "very slow," depending on the rate of decline from maximum outburst), "recurrent" (i.e., having been seen to recur), or "nova-like" (having spectra resembling novae at minimum light); and the supernovae. Supernovae types I and II are associated with stellar populations II and I, respectively. Population I stars are relatively young, and supernovae from this population are thought to include at least some single massive stars that have undergone catastrophic collapse due to the formation of iron in an endothermic reaction in their cores. The removal of energy in this process causes a deficiency in the pressure so that the weight of the overlying layers cannot be sustained, resulting in a massive implosion and catastrophic explosion as the imploding material bounces off the highly compressed core, dispersing the atmosphere in the surrounding interstellar medium.

Novae ("new" stars) have been known since ancient times, but the far more luminous phenomena, supernovae, are newly recognized. Walter Baade recognized that some novae were extraordinarily luminous and called these "Hauptnovae," (or "chief novae"; others used the terms "giant novae," or "more luminous novae," or even "super-novae" to describe them) and he and Fritz Zwicky are said to have coined the term "supernovae" (see Osterbrock (2001)). The relative energy of the outburst ranges from ~ 1044 for novae to ~ 1048 ergs for supernovae. It is now known that novae are the products of mass exchange in highly evolved binary star systems, involving a white dwarf as the recipient star. Some supernovae (type Ia in particular) are similar. However, in the case of novae, the system survives, and the nova phenomenon may recur. In the case of supernovae, there are drastic changes to the exploding star, and a recurrence is not possible. Three known outcomes of this catastrophic event are

• the core of the star collapses to form a neutron star, an object so dense that electrons and protons are forced into neutrons. Rotating neutron stars are seen as pulsars;

• the core collapses into a black hole, an object so dense that light cannot escape its overwhelmingly strong gravitational field within a certain distance of its center;

• the star dissipates into a rapidly expanding debris cloud without a core remnant.

This concludes our brief summary of variable stars. We now concentrate on eclipsing systems and their treatment.

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