FK Comae Berenices star

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FK-Comae-Berenices-Stars , or FK-Comae-Stars for short , are a small group of rapidly rotating variable stars . They are giants with the spectral types  G to K and an amplitude of less than 0.5  mag . The stellar activity due to the rapid rotation leads to strong emissions in the range of X-rays .

properties

spectrum

FK Comae stars are giants of the spectral class G to K with broad emission lines of calcium (H and K). Sometimes the spectra also show H-alpha emission.

Light curve

The FK comae stars show a low amplitude variability of less than 0.5 mag, which in most cases does not exceed 0.1 mag. Neither the cycle length of a few days nor the amplitude or the shape of the light curve is constant. In addition to photometric variability, the chromispherical spectral lines of Ca II and the Balmer lines are also subject to fluctuations with the same cycle lengths.

Cause of the light change

Originally, the FK-Comae stars were defined as single stars in contrast to the RS-CVn stars. However, this assumed property could not be maintained after the discovery of periodically changing radial velocities at UZ Lib and at FK Com, which are interpreted as orbital motion in a binary star system . Two hypotheses have been developed as the cause of variability:

  • A rotating bright spot creates the cyclical variability. It arises at the place where matter from the companion meets the giant star.
  • The variability is a result of unevenly distributed star spots on the surface of the rotating giant

Today the second hypothesis is generally considered to be correct.

Star spots

Using the light curve, photometry can be used to reconstruct the size and distribution of the star spots. However, this solution is dependent on a number of assumptions such as B. a circular shape, since the solution is underdetermined even with multicolor photometry. In contrast, high-resolution Doppler tomography can also clearly determine the temperature and shape of the star spots. Due to the rotation speed of more than 100 km / s, it is possible, due to the rotation broadening in the spectra , to assign individual areas of the surface of the star to sections of an absorption line . As a result of the rotation, these areas move through the absorption line and, through the analysis of several lines, enable an indirect resolution of the star surface of a few degrees.

Properties of the star spots

The magnetic flux density in the star spots on FK Comae stars can reach maximum values ​​of a few hundred Gauss . The strength of the hydrogen emission lines fluctuates in parallel with the brightness and the magnetic field strength . The temperature in the star spots is about 1000 Kelvin lower than on the undisturbed surface, which is an average temperature, as it is not possible to differentiate between umber and penumbra . Differential rotation could be detected on the FK-Comae stars , since the speed of the migration of the star spots depends on the stellar latitude and the speed of rotation decreases towards the pole like the sun. In addition, the flip-flop effect could also be observed on FK comae stars . This effect, also described for the sun and BY-Draconis stars , is based on the observation that after a large star spot has disappeared, a new star spot often appears offset by 180 ° a short time later, i.e. on the back, on the star's surface. In addition, there is sometimes a dislocation in the phase of the dominant star spots. The difference between the flip-flop effect and the phase jump is that the shift does not happen by 180 °. An explanation of these phenomena in the context of the theory of stellar magnetic fields by the magnetohydrodynamic dynamo is still pending .

Occurrence in star catalogs

The General Catalog of Variable Stars currently lists just under 10 stars with the abbreviation FKCOM , which means that only 0.02% of all stars in this catalog belong to the class of FK-Comae-Berenices stars.

development

The stage of a star at which it is observed as an FK comae star is quite short. Due to the stellar activity, a stellar wind is created in the hot corona , which is also responsible for intense X-ray emissions. The stellar wind follows the open magnetic field lines extending into space and must therefore be dragged along by the star. This leads to a loss of torque and the speed of rotation of the star decreases. Dynamically, FK Comae stars are not young stars that were only formed a few million years ago. Therefore, the high speed of rotation was achieved either by the transfer of matter from a companion to the giant, with both matter and torque being exchanged. Or it is a merger of a binary star system that resulted in a rapidly rotating single star. When the rotation speed of the FK-Comae stars has decreased, they are perceived as blue stragglers because they are too massive for their age. The W-Ursae-Majoris stars and the Luminous Red Novae are considered to be potential forerunners of the FK Comae Berenices stars .

However, the discovery of a lithium line in the spectrum of some of the FK Comae stars speaks against an emergence from a merging binary star system. The element lithium is destroyed inside the star at temperatures below the ignition temperature for hydrogen burning and a merger should be accompanied by a strong mixing of the star's interior. An alternative scenario assumes that the zone with convective energy transport reaches the high-speed rotating core of FK Comae stars and therefore freshly synthesized lithium is washed to the surface.

Examples

Individual evidence

  1. ^ Cuno Hoffmeister , G. Richter, W. Wenzel: Veränderliche Sterne . JA Barth Verlag, Leipzig 1990, ISBN 3-335-00224-5 .
  2. ^ H. Korhonen, S. Hubrig, SV Berdyugina, Th. Granzer, T. Hackman, M. Scholler, KG Strassmeier and M. Weber: First measurement of the magnetic field on FK Com and its relation to the contemporaneous starspot locations . In: Astrophysics. Solar and Stellar Astrophysics . 2008, arxiv : 0812.0603v1 .
  3. ^ O. Cohen, JJ Drake, VL Kashyap, H. Korhonen, D. Elstner, TI Gombosi: Magnetic Structure of Rapidly Rotating FK Comae-Type Coronae . In: Astrophysics. Solar and Stellar Astrophysics . 2010, arxiv : 1006.3738v1 .
  4. ^ Thomas Hackman et al .: Flip-flops of FK Comae Berenices . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1211.0914 .
  5. ↑ Variability types General Catalog of Variable Stars, Sternberg Astronomical Institute, Moscow, Russia. Retrieved August 4, 2019 .
  6. ↑ Gaitee AJ Hussain: Magnetic braking in convective stars . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1202.5075v1 .
  7. ↑ DH Bradstreet, EF Guinan: Stellar Mergers and Acquisitions: The Formation and Evolution of W Ursae Majoris Binaries . In: Astronomical Society of the Pacific . tape 56 , 1994, pp. 228-243 .
  8. ↑ R. Tylenda, M. Hajduk, T. Kamiński, A. Udalski, I. Soszyński, M. K Szymański, M. Kubiak, G. Pietrzyński, R. Poleski, Ł Wyrzykowski, K. Ulaczyk: V1309 Scorpii: merger of a contact binary . In: Astrophysics. Solar and Stellar Astrophysics . November 1, 2010, arxiv : 1012.0163 .
  9. ↑ Fekel, FC, Balachandran, S .: Lithium and rapid rotation in chromospherically active single giants . In: The Astrophysical Journal . tape 403 , 1993, pp. 708-721 .
  10. ^ John R. Percy: Understanding Variable Stars . Cambridge University Press, Cambridge 2007, ISBN 978-0-521-23253-1 .