RS Canum Venaticorum Star

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RS Canum Venaticorum stars are a class of discrete binary stars consisting of more massive primary giants or subgiants with a spectral type G to K and a companion that is a subgiant or main sequence star of the spectral type G to M. RS-CVn stars show, outside of a possible cover light change, a modulation of the light curve with an amplitude of up to 0.6 mag in V, which is interpreted as a rotation of star spots on the surface of the stars. The pronounced magnetic activity of these stars is shown by a hot corona in the range of the X-rays and the observations of flares .

definition

There are several definitions of the RS Canum Venaticorum stars used side by side. Hall's first definition included the criteria

In addition to this definition, RS-CVn stars are not classified as close binary stars with stars with almost the same mass, in which the component with higher temperature shows the spectral class FK and the orbital period is between one day and 2 weeks. In the broadest sense, RS-CVns are described as interacting close binary stars with at least one cool star and, at best, very low mass transfer.

Occurrence in star catalogs

The General Catalog of Variable Stars currently lists around 600 stars with the abbreviation RS , which means that a little over 1% of all stars in this catalog belong to the class of RS-Canum-Venaticorum stars. Some of these stars can also be eclipsed , including the namesake RS Canum Venaticorum .

Related star classes

The stars with magnetic activity include the RS Canum Venaticorum stars

Cause of magnetic activity

All the related star classes as well as the RS-Canum-Venaticorum stars have a convective energy transport in the photosphere in combination with a high rotational speed. The rotation speed of the RS-CVns is the result of a bound rotation in the binary star system. The magnetic field is likely to arise in the RS Canum Venaticorum stars as in the sun in the Tachocline region . The MHD describes the formation of a magnetic field when electrical current in the form of a plasma due to the differential rotation flowing in the interior of a star. Because of the coupling of the period of rotation with the orbital period of this class of stars, the magnetic field strength exceeds that of the sun by orders of magnitude. But the signs of magnetic activity are the same:

  • Star spots that can occupy up to 50% of the visible surface
  • a corona heated to several million K, which can be observed in the X-ray range
  • the detection of flares in the range of X-ray, radio and ultraviolet radiation
  • Emission lines from a chromosphere like the calcium and magnesium lines

RS Canum Venaticorum stars as single stars

A K subgiant with magnetic stellar activity is counted among the RS Canum Venaticorum stars even if no signs of a companion are found. In most cases the companion is likely to be too faint to be detected. But there is also a development path for rapidly rotating single stars to transform into an active K subgiant. These stars with the spectral type early F on the main sequence form only a convection zone with shallow depth in the photosphere in the phase of central hydrogen burning. The Tachocline region only generates a weak magnetic field and the rotation speed remains high. When these stars develop away from the main sequence after the hydrogen supply in their core has been exhausted, they cross the Hertzsprung gap as an active K subgiant with a mass 1.25 to 1.5 times the solar mass. Since the Hertzsprung gap is passed within less than 10,000 years, the stars are very rare. Stars with more mass pass through the Hertzsprung gap too fast to be observed and stars with little mass develop such a strong magnetic field during their main sequence phase that they later rotate too slowly to show signs of stellar activity as K subgianti.

Examples

Individual evidence

  1. ^ John R. Percy: Understanding Variable Stars . Cambridge University Press, Cambridge 2007, ISBN 978-0-521-23253-1 .
  2. ^ Hall, DS: The RS CVn binaries and binaries with similar properties . In: Proceedings of the International Astronomical Union colloquium . tape 29 , 1976, p. 287 .
  3. ^ CJ Schrijver, C. Zwaan: Solar and Stellar Magnetic Activity . Cambridge University Press, Cambridge 2000, ISBN 978-0-521-58286-5 .
  4. ↑ Variability types General Catalog of Variable Stars, Sternberg Astronomical Institute, Moscow, Russia. Retrieved February 28, 2019 .
  5. ^ Cuno Hoffmeister , G. Richter, W. Wenzel: Veränderliche Sterne . JA Barth Verlag, Leipzig 1990, ISBN 3-335-00224-5 .
  6. ↑ Klaus G. Strassmeier: Active stars: laboratories of solar astrophysics . Springer Vienna, Vienna 1997, ISBN 978-3-211-83005-5 .
  7. ^ A. Gould et al .: MOA-2010-BLG-523: "Failed Planet" = RS CVn Star . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1210.6045 .