W Ursae Majoris Star

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W-Ursae Majoris star , also W-Ursae Majoris variable ( GCVS -Systematikkürzel: EW ) are eclipsing star whose binary pair is in surface contact and shows a continuous change of light. They are surrounded by a common shell that has formed between the inner and outer Roche boundary . They are named after the prototype W Ursae Majoris .

properties

spectrum

W-Ursae-Majoris stars mostly have a spectral class from F to K, with their components being approximately equally bright with a different mass.

Light curve

The orbit period is usually less than a day, with almost all periods between 0.22 and 0.8 days. The amplitude in visual light is usually less than 0.8 magnitudes , with the two minima only differing slightly.

The light curve differs from the discrete darkening of classic cover changers through a continuous change in brightness. This is a consequence of the elliptical shape of the stars due to their proximity, which is caused by gravitational distortion and centrifugal force . The light curve often does not repeat itself strictly, because stellar activity occurs due to the short orbital period and the convective energy transport in the shell . Star spots and flares are therefore often observed. Another characteristic of W Ursae Majoris stars is the constant color index over the entire change in light, which is also used to differentiate between pulsating variables such as Delta Scuti stars and RR Lyrae stars . The constant color index results in an almost identical surface temperature for two stars with different masses. This is a violation of the Vogt-Russell theorem , according to which the mass and chemical composition clearly determine both the radius and the luminosity of the star. Today it is assumed that a W Ursae Majoris star is embedded in a common shell and this leads to the identical surface temperature.

O'Connell Effect

With many contact systems and especially with W-Ursae-Majoris variables, the O'Connell effect can be observed, in which the maxima in the light curve show a different height of up to 0.1 magnitude . The asymmetry in the change of light increases the more the stars are elliptically distorted and the greater the ratio of the radii of the stars. The O'Connell effect is optionally explained as the result of a hot spot between the two stars due to a mass exchange, star spots on the components of the binary star system and circumstellar matter in a ring around the eclipse variable. This is accompanied by the so-called W-phenomenon. After that, the deeper minimum of the eclipsing light change occurs for most W-UMa stars when the secondary star is covered by the more massive primary star. This has been linked to an accumulation of star spots on the primary star, making the average temperature of its photosphere lower than that of its companion.

Subgroups

The W-Ursae Majoris stars are divided into the following subclasses:

  • Type A: The more massive star of the two has the larger radius and a higher effective temperature , with both stars having a higher surface temperature than the sun with a spectral type A or F with an orbital period of 0.4 to 0.8 days
  • Type W: The more massive star has a larger radius and a lower effective temperature than its partner. Both stars have a spectral class G or K with an orbital period of 0.22 to 0.4 days
  • Type H: These W-Ursae-Majoris stars have a mass ratio q = M 1 / M 2 of more than 0.72. With these double stars, the energy transfer between the components is very inefficient.

Occurrence in star catalogs

The General Catalog of Variable Stars currently lists around 3500 stars with the abbreviation EW , which means that 7% of all stars in this catalog belong to the class of W-Ursae-Majoris stars.

development

The total mass of a W-Ursae-Majoris binary star system does not exceed 2.5 solar masses. The primary star is developing on the main sequence , while the companion with a lower mass has a radius up to seven times larger than a single star with an identical mass and chemical composition. The increased diameter could be the result of convective energy transfer from the primary star to the companion.

W-Ursae-Majoris variables and other contact systems do not occur in star formation regions or in young open star clusters . In contrast, they are often found in older open star clusters that are more than a billion years old and in globular clusters that are around 12 billion years old . Contact systems arise in a time-based process known as magnetic torque loss. Since the rotation of the stars is bound in close, initially still separated binary star systems, the period of rotation of these stars can only be identical to the period of rotation in the binary star system of a few days. Because convection dominates the transport of energy on the surface of the late stars , global magnetic fields develop. The matter given off in the stellar wind is ionized , therefore frozen in the magnetic field and must follow the rotation of the star. This dragging along reduces the angular momentum present in the binary star system and as a result the distance between the two components is reduced until they form a common shell. In the case of more massive W-Ursae-Majoris variables, nuclear development dominates. After the hydrogen reserves have been exhausted by thermonuclear processes , the star expands in order to remain in hydrostatic equilibrium and thus comes into contact with its companion. This evolutionary path is characteristic of the W-UMa stars of type A. In both evolutionary paths, the binary star system is in contact and exchanges matter for only 10 percent of its characteristic lifespan of up to about 8 billion years. The mass ratio is not more extreme than a tenth.

Due to the constant exchange of matter and energy between the two stars in a shared shell, the total angular momentum of the binary star system is further reduced. Hence, the distance between the two components decreases until the two stars merge. In the process of merging a close binary star system, a large amount of energy is released and this is observed as a luminous red nova . In the case of V1309 Sco , the variability of the coverage was even documented before the outbreak. As a result of the luminous red nova, a rapidly rotating single star is formed consisting of the mass of the two components of the binary star system. The successors of this merger are the FK-Comae-Berenices stars and the blue stragglers .

Period distribution

The distribution of the period of circulation of these contact systems has a maximum of 0.37 days. The frequency drops rapidly towards smaller periods and no W-Ursae-Majoris star is known below 0.21 days. This distribution is explained as a consequence of an unstable mass transfer. The primary star in such a close contact system has the property that its radius grows faster than the Roche limit in the binary star system with a loss of mass . The result is an exponential increase in the mass transfer rate when this lower period limit is reached. This leads to a rapid merging of the binary star system and the result is a single star rotating at high speed.

A search for contact systems at the lower end of the period distribution using the data from the SuperWASP experiment has shown that only 3 out of 53 systems show a strong reduction in the period of rotation. These period changes can neither be caused by magnetic interaction nor by radiation of gravitational waves . However, the small number for binary star systems with possibly unstable mass transfer is a problem for the current hypotheses and also not statistically significant, since there is a comparable number of contact systems with strong period extensions.

A period lower limit of 0.21 days does not seem to exist for main sequence stars consisting of two M-dwarfs . Separate systems have even been found below the period limit and the shortest known period of revolution in a contact system made up of two M dwarfs is 0.112 days. It had previously been assumed that two M-dwarfs could not have lost enough torque in the Hubble time to achieve such short periods. Whether M-dwarfs are able to convert torque faster through coupled stellar activity , or whether they already emerge from star formation as a very close binary star system , is the subject of current research.

The periods of rotation of contact systems, measured as the distance between two minima, vary with an amplitude of up to 0.01 days with a quasi-period of a few hundred days. This is caused by star spots on the surface of the stars. Star spots are areas with a lower surface temperature, which, due to their position in the hemisphere, can shift the time of minimum brightness. The quasi-periods are in turn the result of a differential rotation of the stars.

Examples

Web links

Individual evidence

  1. ^ John R. Percy: Understanding Variable Stars . Cambridge University Press, Cambridge 2007, ISBN 978-0-521-23253-1 .
  2. ^ Cuno Hoffmeister , Gerold Richter, Wolfgang Wenzel: Veränderliche Sterne . JA Barth Verlag, Leipzig 1990, ISBN 3-335-00224-5 .
  3. ↑ Quing-Yao Liu, Yu-Lan Yang: A Possible Explanation of the O'Connell Effect in Close Binary Stars . In: Chinese Journal of Astronomy & Astrophysics . tape 3 , 2003, p. 142-150 .
  4. ↑ Szilárd Csizmadia Péter Klagyivik: On the properties of contact binary stars . In: Astronomy & Astrophysics . tape 426 , 2004, p. 1001-1005 , doi : 10.1051 / 0004-6361: 20040430 .
  5. ^ Leendert Binnendijk: The W Ursae Majoris Systems . In: Small publications of the Remeis observatory in Bamberg . tape 40 , 1965, pp. 36-51 .
  6. ↑ Variability types General Catalog of Variable Stars, Sternberg Astronomical Institute, Moscow, Russia. Retrieved September 1, 2019 .
  7. ↑ Kazimierz Stepién and K. Gazeas: Evolution of Low Mass Contact Binaries . In: Astrophysics. Solar and Stellar strophysics . 2012, arxiv : 1207.3929v1 .
  8. ↑ Kaziemierz Stepién: Evolution of Cool Close Binaries - Approach to Contact . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1105.2645 .
  9. ↑ M. Yıldız and T. Dogan: On the origin of W UMa type Contact binaries - a new method for computation of initial masses . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1301.6035 .
  10. ↑ Bogumil Pilecki, Kazimierz Stepién: Light curve modeling of short-period W UMa-type stars . In: Information Bulletin on Variable Stars . tape 6012 , 2012, ISSN  1587-2440 .
  11. ↑ Romuald 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 .
  12. ↑ David H. Bradstreet, Edward 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 .
  13. ^ Slavek M. Rucinski: The short period end of the contact binary period distribution based on the All Sky Automated Survey (ASAS) . In: Monthly Notice of the Royal Astronomical Society . tape 382 , 2007, p. 393 .
  14. ↑ Dengkai Jiang, Zhanwen Han, Hongwei Ge, Liheng Yang and Lifang Li: The short-period limit of contact binaries . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1112.0466v1 .
  15. ↑ Marcus E. Lohr, Andrew J. Norton, Ulrich C. Kolb, David R. Anderson, Francesca Faedi, Richard G. West: Period decrease in three SuperWASP eclipsing binary candidates near the short-period limit . In: Astrophysics. Solar and Stellar strophysics . 2012, arxiv : 1205.1678v1 .
  16. ↑ SV Nefs, JL Birkby, IAG Snellen, ST Hodgkin, DJ Pinfield, B. Sipocz, G. Kovacs, D. Mislis, RP Saglia, J. Koppenhoefer, P. Cruz, D. Barrado, EL Martin, N. Goulding, H. Stoev, J. Zendejas, C. del Burgo, M. Cappetta, YVPavlenko: Four ultra-short period eclipsing M-dwarf binaries in the WFCAM Transit Survey. In: Astrophysics. Solar and Stellar strophysics . 2012, arxiv : 1206.1200 .
  17. ↑ K. Tran, A. Levine, S. Rappaport, T. Borkovits, Sz. Csizmadia, B. Kalomeni: The Anticorrelated Nature of the OC Curves for the Kepler Contact Binaries . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1305.4639v1 .