Symbiotic binary
Binary star system with a white dwarf and red giant.
A symbiotic binary, also known simply as a symbiotic star, is a binary star system typically composed of a white dwarf and a red giant companion. The red giant sheds material through either Roche lobe overflow or its stellar wind, and this matter flows onto the hot, compact white dwarf, often forming an accretion disk. These systems are valuable to astronomers for studying stellar evolution, as well as stellar winds, ionized nebulae, and accretion processes, due to the distinctive interstellar dynamics they create.
Many symbiotic binaries exhibit brightness fluctuations and are classified as variable stars. The star Z Andromedae is often considered the prototype for this class, though it more specifically represents a subset with irregular variations of up to about 4 magnitudes. Even these Z Andromedae variables are thought to be a diverse group. A closely related type is the symbiotic nova, formally called a type NC nova, which resembles a classical nova but has extremely slow outbursts that can stay near peak brightness for years. The typical behavior of symbiotic binaries can be divided into two phases based on the accretion rate onto the compact star, with each phase having very different luminosities and often internal variability.
In the quiescent phase, the processes of accretion, mass loss, and ionization are in equilibrium between the stars. The system releases energy at a roughly average rate, and its spectral energy distribution (SED) remains relatively constant. If this equilibrium is disturbed, the system enters an active phase, marked by a large change in the nature of its radiation and a brightening of several magnitudes in optical emission. The transitions between phases are poorly understood, making it difficult to predict when a star will become active or return to quiescence. Some systems have never been observed in an active state, while others, like AG Draconis, enter active phases regularly and cyclically.
The term "symbiotic star" was first used in 1958 in a publication on stars of composite spectra, though the category had been recognized earlier. Annie Cannon identified them as a distinct class with unique spectroscopic features near the start of the 20th century. Their binary nature became clear from the simultaneous presence of spectral lines indicating a red giant and a white dwarf or neutron star.
- Prototype
- Z Andromedae
- Amplitude range
- up to about 4 magnitudes
- Sub types
- S-type and D-type
- First use of term
- 1958
- First recognized as class by
- Annie Cannon
Lore & Background
The term 'symbiotic star' was first used in 1958 in a publication about 'stars of composite spectra'. However, the distinct category of symbiotic stars had been previously known. They were first recognized as a class of stars with unique spectroscopic qualities by Annie Cannon near the beginning of the 20th century. Their binary nature was made clear by the simultaneous existence of the spectral lines indicative of a red giant and of a white dwarf or neutron star. Symbiotic stars are all binaries and so the term symbiotic binary is synonymous. Many are variable and the term symbiotic variable or symbiotic variable star is sometimes also used synonymously, but more commonly is used only for Z Andromedae variable stars.
Many symbiotic binaries show brightness changes and are classified as variable stars. The star Z Andromedae is often considered the prototype of the symbiotic binary class of stars. More commonly it is considered as the prototype of only a subset of symbiotic stars with irregular variations up to about 4 magnitudes in amplitude. Even the Z Andromedae variable stars are thought to be an inhomogeneous group. The so-called symbiotic novae are a closely related class of symbiotic binaries, more formally known as type NC novae. They appear similar to classical novae but have extremely slow outbursts that can remain near maximum brightness for years.
The typical behavior of symbiotic binaries can be divided into two phases, based on the rate of accretion to the compact component. The two phases have very different luminosities, but the systems are often also variable in each phase. When the accretion, mass-loss and ionization processes are all in equilibrium between the stars, the system is said to be in quiescence. If the equilibrium is disturbed, it will transition into an active phase, shown through a large change in both the nature of the radiation and a brightening of the optical emission by several magnitudes. The transitions between phases are poorly understood, and it is currently difficult to predict when a star will transition. Many systems have not yet been observed to enter an active state. Others, such as AG Draconis, enter active phases on a regular and cyclical basis.
Reader's Guide
Symbiotic binaries are significant because they provide a natural laboratory for studying stellar evolution, stellar wind, ionized nebulae, and accretion processes. Their unique interstellar dynamics allow astronomers to investigate mass transfer between stars and the behavior of accretion disks. The variability of these systems, particularly the distinction between quiescent and active phases, offers insight into the equilibrium and disruption of accretion and ionization processes. The poorly understood transitions between phases highlight the complexity of these systems and the need for further observation. Additionally, the presence of jets in some symbiotic stars, most commonly observed during active phases or outbursts, may help further the understanding of jets in other astrophysical contexts, such as in active galactic nuclei. The classification into S-type and D-type sub-types, based on the visibility of the giant component and the presence of dusty nebulosity, provides a framework for understanding the diversity within the class. The prototype Z Andromedae and the related symbiotic novae (type NC novae) underscore the range of outburst behaviors, from irregular variations to extremely slow novae-like events that can remain bright for years.
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