Symbiotic binary
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A symbiotic star is a type of binary star.[1] They are usually a white dwarf with a companion late sequence red giant. They are indicated by the exhibition of spectral lines of the highly contrasting temperatures of the red giant (~3000 K) and the white dwarf (~20 000 K). The white dwarf gains mass from stellar wind from the red giant or gravitational streaming.
Symbiotic stars are of particular interest to astronomers as they can be used to learn about stellar evolution. They are also vital in the study of stellar wind, ionized nebulae, and accretion[2] because of the unique interstellar dynamics present within the system.
Discovery
The term 'symbiotic star' was first used in 1958 in a publication about 'stars of composite spectra'.[3] 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.[4]
Variability
Many symbiotic binaries show brightness changes and are classified as variable stars. Typical behaviour 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.
Quiescent phase
When the accretion, mass-loss and ionization processes are all in equilibrium between the stars, the system is said to be in quiescence. At this point, the system will continue to release energy at an approximately average rate.[5] This can be observed through the spectral energy distribution (SED) of the star which will be remain relatively constant.
Active phase
If the equilibrium of a quiescent symbiotic star is disturbed, it will transition into an active phase. This phase is shown through a large change in both the nature of the radiation from the star, and a brightening of the optical emission of the star by several magnitudes. The transitions between phases are poorly understood, and it is currently difficult to predict when a star will transition into an active phase from quiescence, or when it will return to quiescence. Many systems have not yet been observed to enter an active state. Others, however, such as AG Draconis enter active phases on a regular and cyclical basis.[5]
Sub-types
Symbiotic binaries are often divided into two sub-types based on the nature of the continuum in their spectra. S-type systems have a stellar continuum since the giant component is not obscured. D-type systems are surrounded by optically thick dusty nebulosity and the star itself is not directly visible. D-type systems tend to contain a Mira variable or other long period variable star.[6]
Jets
Some symbiotic stars have jets which are a collimated outflow of material. These typically are bi-polar and extend from both poles of the white dwarf. Jets are most commonly observed on stars which are currently in active phase or outburst. Once the outburst has ended, the jet fades and the ejected emissions dissipate. It has been argued that the jets present in symbiotic stars could help further the understanding of jets in other systems, such as in active galactic nuclei.[1]
References
- ^ a b Sokoloski, J. L. (June 20, 2003). "Symbiotic Stars as Laboratories for the Study of Accretion and Jets: A Call for Optical Monitoring". Journal of the American Association of Variable Star Observers. 31 (2): 89–102. arXiv:astro-ph/0403004. Bibcode:2003JAVSO..31...89S.
- ^ "Symbiotic binaries". Astrophysics Group. University of Exeter.
- ^ Tomokazu Kogure; Kam-Ching Leung (5 May 2010). The Astrophysics of Emission-Line Stars. Springer Science & Business Media. pp. 5–. ISBN 978-0-387-68995-1.
- ^ Mikołajewska, Joanna (2002). "Orbital and stellar parameters of symbiotic stars". ASP Conference Series. 303: 9. arXiv:astro-ph/0210489. Bibcode:2003ASPC..303....9M.
- ^ a b Skopal, Augustin (8 May 2008). "How to understand the light curves of symbiotic stars". Journal of the American Association of Variable Star Observers. 36 (1): 9. arXiv:0805.1222. Bibcode:2008JAVSO..36....9S.
- ^ Mikołajewska, J (2007). "Symbiotic Stars: Continually Embarrassing Binaries". Baltic Astronomy. 16: 1. Bibcode:2007BaltA..16....1M.
External links
- List of symbiotic stars discovered by IPHAS