Eruptive variable star
The light change of eruptive changers differs from that of other changeable stars in that the apparent brightness changes abruptly and not periodically .
Breakout Mechanisms
The change in light of the eruptive variable stars is mainly based on the following physical mechanisms:
- Suddenly onset of thermonuclear processes such as in novae and supernovae
- Increased flow of matter through accretion disks on a compact star , whereby the accretion disk lights up, for example in dwarf novae and FU Orionis stars
- Flares as a result of a magnetic short circuit in the atmosphere of the stars, for example in the case of UV Ceti stars
- Absorption and re-emission at other wavelengths as a result of ejected matter, for example in Gamma Cassiopeiae stars
Subgroups
The eruptive variables as a subgroup of variable stars are divided into the following classes:
- Novae are close binary star systems with orbital periods between 0.05 and 150 days. A thermonuclear reaction ignites on the surface of one partner in the binary star system, a white dwarf. This leads to an increase in brightness from 7 to 19 mag within days. The brightness slowly falls back to the brightness before the outbreak, which happens over a period of months to decades. The novae are divided into subclasses based on the rate at which the brightness decreases.
- More than one eruption has been recorded in the Recurrent Novae during the historical period .
- Supernovae increase their brightness by 20 mag or more during an outbreak and then slowly decay. The structure of the breaking out object changes completely. The result can be a neutron star , black hole or the complete annihilation of the previous star . Radial speeds of several 1000 km / s are observedduring the eruptions. The ejected shell can bedetectedas a supernova remnant a few decades to centuries after the eruption.
- Dwarf novae are close binary star systems consisting of a late dwarf or subgiant and a white dwarf . Around the white dwarf there is an accretion disk made of material that was transferred from the late companion. The orbital period in the binary star systems is between 0.05 and 0.5 days. The light change consists of various components and is dominated by outbreaks that are localized in the accretion disc.
- Nova-like variables are mostly little-studied objects that belong to one of the other groups of cataclysmic variables .
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Symbiotic variables consist of a hot star (usually a white dwarf), a red giant and an extensive shell that is excited to shine by the hot star. The light change consists of the following components:
- Eruptions on or near the white dwarf
- long period light change of the red giant
- possibly change of covering light and / or change of reflection light with the period of the orbital period of the binary star system
- The light change of the FU Orionis stars consists of an increase of 6 mag in several months, a stay at the maximum brightness for years and a slow decrease in brightness over several decades. These are young stars that are classified as T-Tauri stars before and after the eruption . The cause of the outbreak is a lighting up of the accretion disk around the protostar .
- Gamma Cassiopeiae stars are rapidly rotating giants with a type B spectral with emission lines ; it is a special subgroup of the Be stars , which also belong to the group of "shell stars". The eruptions are caused by the formation of a pseudo photosphere in a ring around the equator of the B star.
- R-Coronae Borealis stars are hydrogen-poor, but helium- and carbon-rich giants with great luminosity. Their light change is characterized by deep minima up to 9 mag. They are also called antinovae. The change in light is a sequence of soot clouds that are ejected from the star at irregular intervals.
- UV Ceti stars are red dwarf stars with radiation bursts ( flares ) in the way that the sun shows at times of high activity. Due to the low luminosity of the dwarf star, a flare can increase its brightness by orders of magnitude.
- The rapidly irregular variables are an inhomogeneous group that cannot be classified based on the change in light. This subgroup contains:
- T-Tauri stars
- Young stars with a non-periodic light changes, the on Algol variable recalls
- Young stars with flares similar to solar flares
- The YY Orionis stars similar to the T Tauri stars. They also show signs of accretion of matter on the star's surface
- Little studied stars with rapid light changes
- The AM Herculis stars are built up close binary star systems like the dwarf novae. The difference lies in the strong magnetic field of the white dwarf , which suppresses the formation of an accretion disk and the matter is transferred from the companion directly to the poles of the white dwarf. Polars show strong linear and circular polarization .
- The magnetic field of the closely related DQ Herculis stars is not strong enough to completely suppress the formation of an accretion disk. However, it is sufficient to force accretion to the magnetic poles of the white dwarf and therefore the DQ Herculis stars also show a strong polarization.
- In the Wolf-Rayet stars , only a slight change in light is observed, which is the result of a change in pseudo- photosphere caused by a variable stellar wind .
- The RS Canum Venaticorum stars are close binary star systems with chromospheric activity in the form of star spots , flares and calcium lines in emission. They are also X-ray sources due to stellar activity.
- S-Doradus stars are also called Luminous Blue Variables (LBV). They are very massive stars at the limit of stability. A variable stellar wind caused by the radiation pressure forms envelopes around the star, in which ultraviolet radiation is absorbed, which isre- emitted in the optical . The outbreaks can be triggered by the narrow passage of a companion .
See also
literature
- Cuno Hoffmeister , G. Richter, W. Wenzel: Variable stars . 3. Edition. JA Barth Verlag, Leipzig 1990, ISBN 3-335-00224-5 .
- John R. Percy: Understanding Variable Stars . Cambridge University Press, Cambridge 2007, ISBN 978-0-521-23253-1 .