BY-Draconis star

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BY-Draconis stars are stars of the spectral classes G to M. Many BY-Draconis stars are red dwarfs , but some also have a higher mass than red dwarfs. They show low amplitude variability due to the rotation of star spots in their photosphere . The prototype is BY Draconis .

properties

spectrum

BY-Draconis stars are stars of the spectral classes G to M. Compared to calm red dwarfs, the magnetically active stars show a rotational broadening of the spectral lines . The rotation broadening is caused by the Doppler effect due to the high rotation speed. In addition, emission lines of calcium (H and K) and the Balmer series occur. As with the sun, the lines are created by a chromosphere and corona heated from below . BY Draconis stars, like the sun, can also be detected in the X-ray range, as their corons reach temperatures of several million Kelvin.

Light curve

The variability has a low amplitude of up to 0.5 mag and usually does not exceed 0.1 mag. The period ranges from fractions of a day to 120 days. The cause of the changes in brightness are star spots (similar to sunspots ), which become periodically visible due to the rotation of the star, and chromospheric activity , which manifests itself in the form of flares , flares (hotter areas on the star's surface) and changes in the half-width of the emission lines. If a flare is detected in a BY Draconis star, the star also belongs to the class of UV Ceti stars . The star spots of the BY Draconis stars and the flares of the UV Ceti stars are two properties of magnetically active stars that do not differ in their physical properties.

Star spots

The position and extent of the star spots can be derived by modeling their light curves . They can cover up to a fifth of the star's surface, while sunspots only cover up to one percent of the sun's surface. In contrast to the sun, many star spots are located at the rotation poles of the stars, while sun spots are observed in the range of ± 30 degrees along the equator. In both types of spots, however, the cause is the break-through of the surface by bundled magnetic field lines and the resulting cooling due to the hindrance of the energy transport from the star's interior. There seems to be no difference in the physical parameters of stars that show a modulation of brightness by star spots and stars with similar rotation speeds, temperatures and radii in the same star cluster whose brightness is constant. While some of these inactive stars can go through a Maund Minimum , this cannot apply to a third of the potential BY-Dra stars. It is possible that these stars have an approximately rotationally symmetrical distribution of small star spots, which means that fluctuations in brightness would be below the detection limit.

Magnetic fields

The cause of all stellar activity lies in magnetic fields in the photosphere of the late dwarfs. The high magnetic fields are created by a high rotation speed in combination with convection currents of the electrically charged plasma. These movements of electrical currents lead to the creation of magnetic fields via a dynamo effect . The rotation is sustained in double star systems by tidal forces for a long time due to a bound rotation . Therefore, individual BY-Draconis stars are often young, while active stars occurring in binary star systems can be several billion years old. According to high-resolution spectrographic measurements of the Zeeman effect, the flux density of the magnetic fields reaches up to 500 Gauss .

Physical Properties

The radii of stars can be measured with great accuracy in eclipsing stars . Apparently, the radii of BY-Draconis stars are between 3 and 12% larger than theoretically expected. In addition, the temperatures in the photosphere seem to be 3% below the expected values. These discrepancies are associated with the magnetic activity in the binary stars. First, the cooler star spots on the surface lead to reduced radiation, and the star responds by expanding to restore hydrodynamic equilibrium . Second, the bound rotation should lead to an increase in the convection in the photosphere and thus also contribute to an expansion of the radius. The BY-Draconis stars in close binary star systems are therefore only partially representative of the physical properties of red dwarfs .

Occurrence in star catalogs

The General Catalog of Variable Stars currently lists almost 1000 stars with the abbreviation BY , which means that almost 2% of all stars in this catalog belong to the BY Draconis class.

Examples

It turns out that some BY Draconis stars are also accompanied by exoplanets .

See also

Individual evidence

  1. ^ Cuno Hoffmeister , G. Richter, W. Wenzel: Veränderliche Sterne . JA Barth Verlag, Leipzig 1990, ISBN 3-335-00224-5 .
  2. ^ KG Helminiak, M. Konacki, MW Muterspaugh, SE Browne, AW Howard and SR Kulkarni: New high precision orbital and physical parameters of the double-lined low-mass spectroscopic binary BY Draconis . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1109.5059v1 .
  3. ↑ J. Lehtinen, L. Jetsu, T. Hackman, P. Kajatkari, and GW Henry: Spot activity of LQ Hya from photometry between 1988 and 2011 . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1203.1555v1 .
  4. ^ John R. Percy: Understanding Variable Stars . Cambridge University Press, Cambridge 2007, ISBN 978-0-521-23253-1 .
  5. ^ J. MacDonald and DJ Mullan: Precision modeling of M dwarf stars: the magnetic components of CM Draconis . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1106.1452v1 .
  6. ↑ RJ Jackson and RD Jeffries: Why do some young cool stars show spot modulation while others do not? In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1204.4066v1 .
  7. ↑ Ribeiro, T. et al .: Activity on the M star of QS Vir . In: Astrophysics. Solar and Stellar Astrophysics . 2009, arxiv : 0912.0912v1 .
  8. ↑ Jayne Birkby et al .: Discovery and characterization of detached M-dwarf eclipsing binaries in the WFCAM Transit Survey . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1206.2773v1 .
  9. ↑ Variability types General Catalog of Variable Stars, Sternberg Astronomical Institute, Moscow, Russia. Retrieved May 19, 2019 .