Algolstern

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The term Algolsterne (short Algols ) describes both a class of eclipsing stars , the brightness of which does not or hardly changes at maximum, as well as a group of interacting binary stars . Both star classes are named after their prototype Algol in the constellation Perseus .

Eclipsing stars of the Algol type

Animation of an eclipsing double star with resulting light curve.

Algol stars ( GCVS systematic abbreviation : EA ) are binary star systems consisting of two spherical or only slightly ellipsoidally deformed single stars due to centrifugal forces . The plane of the orbit is so in space that the stars cover each other on their orbit and less radiation reaches the earth. The time of the beginning and the end of a minimum is clearly defined in the case of Algol stars, in contrast to the Beta-Lyrae stars and W-Ursae-Majoris stars , which show a continuous change of light due to the strong deformation of the stars in these binary star systems.

Between the minima, the brightness of the eclipsing stars of the Algol type remains approximately constant. A slight change in brightness can be the result of a reflection effect, elliptical deformation of the star components or intrinsic variability. The periods of Algol stars are between about 0.2 and over 10,000 days, with the longest known period of 27 years being held by the star Almaaz in the constellation Fuhrmann . The amplitude of the changes in brightness in the Algol stars can be up to a few magnitudes .

The Algol stars were named after the star Algol in the constellation Perseus , the first eclipse variable discovered (1669 by Geminiano Montanari ).

Occurrence in star catalogs

The General Catalog of Variable Stars currently lists around 5000 stars with the abbreviation EA , which means that around 10% of all stars in this catalog belong to the class of Algol stars.

Interacting binary stars of the Algol type

The second star class with the name Algolsterne describes double stars in which a star with less mass is more developed than a more massive star. This contradicts the stellar evolution of single stars, which runs faster with increasing mass and is also known as the Algol paradox. In the narrower sense, it is a binary star system consisting of a BA main sequence star and a cooler FG giant star , with the cool giant filling its Roche boundary volume . These algol stars often show that the more massive star rotates faster than the orbital period of the binary star system. With some Algol stars, a mass flow from the companion with less mass to a hot spot on the heavy star can also be detected. In the hot spot, matter meets the atmosphere and kinetic energy is converted into thermal energy . The star, which is now lower in mass, originally had the greater mass and has evolved from the main sequence . It began to expand until it exceeded the Roche limit volume. If this limit is exceeded, matter flows to the companion and within a short period of time there is a mass reversal. Therefore, in the Algol phase after the rapid mass transfer, the star with lower mass is further developed and the more massive star rotates at high speed due to the transfer of angular momentum between the two stars. In the observable phase, the mass transfer from the evolved subgiant or giant to the heavy main sequence star, an increase in orbit duration should occur. Analyzes of the behavior of algol stars, however, often show cyclical period changes with both decreasing and increasing orbital times, the cause of which may be the magnetic activity of the mass-donating component.

In the stable phase after the rapid mass transfer, the formerly more massive star is a cool subgiant with an extended atmosphere with convective energy transport. At the same time, the speed of rotation of the subgiant is quite high because of the bound rotation in the narrow binary star systems. A fast differential rotation in combination with a convective atmosphere leads to a pronounced magnetic activity in algal stars due to magnetohydrodynamics , which is noticeable in the form of flares in the range of radio and X-rays as well as emission lines of the Balmer series . Temporary accretion disks have also been found in long-period Algol stars , which do not always lie in the plane of the binary star system. These deviations cannot be explained by simple models that only consider gravitational and centrifugal forces . The gas masses outside the orbital plane are associated with the magnetic activity of the mass donating star:

  • due to coronal expectorations
  • Magnetic fields on the subgiant interact with the ionized matter flowing through the Lagrange point L1
  • Accreted gas is diverted from the plane of the orbit because a backwater forms at the point where the gas flow hits the primary star
  • A superhump- like phenomenon has been detected in radio observations. It is likely that the gas flow is deflected in a helical manner by a magnetic field near the mass dispenser. Since the early stars have no intrinsic magnetic fields, these magnetic field lines are probably generated by the plasma flow itself.

Examples

See also

literature

  • W. Krat: About the derivation of the edge darkening law of the Algol stars . In: Zeitschrift für Astrophysik , Vol. 5, 1932, pp. 60–66, bibcode : 1932ZA ...... 5 ... 60K

Web links

Individual evidence

  1. ↑ D. Gossman: Light Curves and Their Secrets. In: Sky & Telescope. October 1989, p. 410.
  2. ^ John R. Percy: Understanding Variable Stars . Cambridge University Press, Cambridge 2007, ISBN 978-0-521-23253-1 .
  3. ^ Cuno Hoffmeister , G. Richter, W. Wenzel: Veränderliche Sterne . JA Barth Verlag, Leipzig 1990, ISBN 3-335-00224-5 .
  4. ↑ Astro-Lexicon V 1 (Andreas Müller)
  5. ↑ Variability types General Catalog of Variable Stars, Sternberg Astronomical Institute, Moscow, Russia. Retrieved September 1, 2019 .
  6. ^ R. Deschamps, L. Siess, PJ Davis, A. Jorissen: Critically rotating accretors and non-conservative evolution in Algols . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1306.1348v1 .
  7. ↑ SN Shore, M. Livio, EPJ van den Heuvel: Interacting Binaries . Springer-Verlag, Berlin 1992, ISBN 3-540-57014-4 .
  8. ↑ L. Jetsu, S. Porceddu, J. Lyytinen, P. Kajatkari, J. Lehtinen, T. Markkanen, J. Toivari-Viitala: Did the ancient egyptians record the period of the eclipsing binary Algol - the Raging one? In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1204.6206v1 .
  9. ↑ F. Baron, JD Monnier, E. Pedretti, M. Zhao, G. Schaefer, R. Parks, X. Che, N. Thureau, TA ten Brummelaar, HA McAlister, ST Ridgway, C. Farrington, J. Sturmann, L. Sturmann, N. Turner: Imaging the Algol Triple System in H Band with the Chara Interferometer . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1205.0754v1 .
  10. ^ Mercedes T. Richards, Michail I. Agafonov, Olga I. Sharova: New Evidence of Magnetic Interactions between Stars from 3D Doppler Tomography of Algol Binaries: Beta Per and RS Vul . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1210.0081 .
  11. ↑ Eric Raymer: Three-Dimensional Hydrodynamic Simulations of Accretion in Short Period Algols . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1209.2167 .