HW Virginis star

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The HW Virginis stars are eclipsed stars consisting of a red dwarf or brown dwarf and a blue subdwarf . They are the result of a common envelope phase and in the course of their further development they become cataclysmic binary stars . They are also known as post-common-envelope-eclipsing variables. The HW Virginis stars have been studied intensively in recent years due to a suspected discovery of circumbinary planets around these binary stars.

properties

HW Virginis stars consist of a blue sub-dwarf with the spectral type B and a late main sequence companion with the spectral class K or M. The orbital periods of these narrow, but separate double stars are predominantly between two and six hours. The temperatures of the blue dwarfs are in the range of over 20,000 K and dominate the blue spectral range as well as the ultraviolet radiation , while the red companion with temperatures around 3,000 K emits exclusively in red and infrared radiation. The masses of the blue sub- dwarfs reach values ​​of 0.4 to 0.5 solar masses , while the red dwarfs only have a fifth to a tenth of the mass.

development

Blue subdwarfs or extreme horizontal branch stars burn helium in their core and only have an extremely thin hydrogen-rich atmosphere . They arise from a strong loss of mass on the asymptotic giant branch or, in most cases, in interacting binary star systems during a common envelope. The more massive partner develops in a close binary star system beyond its Roche border , whereby the companion is immersed in its atmosphere. The kinetic energy from the companion's orbit is transferred to the common atmosphere, whereby it is thrown off and a binary star system from the former core of the more massive star, the blue subdwarf, and its companion, the red dwarf, remain. Due to the loss of angular momentum through magnetic processes or through the radiation of gravitational radiation , the red dwarf fills its Roche boundary volume after a period of billions of years and matter flows from the main sequence star onto the cooled white dwarf . The HW Virginis stars develop into cataclysmic variables and therefore also belong to the group of pre-cataclysmic binary stars.

Circumbinary planetary systems

The light travel time effect leads to periodic shifts of an event when a body shifts the center of gravity of a binary star system via its gravitational forces. In the case of HW Virginis stars, the time of minimum brightness can be determined very precisely due to the short period of rotation and the small proportion of the red dwarf in the total light. The long-term course of the minima shows fluctuations that were interpreted as circumbinary planets.

All of these interpretations have not been confirmed by an independent method such as the transit method , direct imaging of the extrasolar planet, interferometric measurements, or by means of an infrared excess . Longer measurements of the course of the times of the minimum light at the HW Virginis stars are also more of a cyclical nature than strictly periodic, as would be expected with a circumbinary planet. Furthermore, the orbital elements of the hypothetical exoplanets were partially not dynamically stable, as the planets would have been ejected from the binary star system due to orbital resonances .

Alternative hypotheses interpret the data as the result of a dynamo effect , which leads to a change in the shape of the M dwarf in the course of a stellar cycle and thus also to a redistribution of the angular momentum in the binary star system.

See also

Examples

  • HW Virginis
  • NN Serpentis
  • DP Leonis
  • HU Aqr
  • NY Virginis
  • QS Virginis
  • RR Cae
  • UZ For

Individual evidence

  1. ^ S. Geier et al .: Substellar companions and the formation of hot subdwarf stars . In: Astrophysics. Solar and Stellar Astrophysics . 2010, arxiv : 1012.3839v2 .
  2. ^ V. Schaffenroth et al .: A new bright eclipsing hot subdwarf binary from the ASAS and SuperWASP surveys . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1303.0177v1 .
  3. ↑ V. Schaffenroth et al .: Binaries discovered by the MUCHFUSS project SDSSJ08205 + 0008 - An eclipsing subdwarf B binary with brown dwarf companion . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1103.1989v1 .
  4. ↑ S.-B. Qian et al .: MAGNETIC BRAKING AND THE EVOLUTION OF THE HW VIR – LIKE BINARY STARS . In: The Astrophysical Journal . tape 689 , 2008, p. L49-L52 .
  5. ↑ K. Beuermann et al: The Planets around the Post-Common Envelope Binary NN Serpentis . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1102.0508 .
  6. ↑ Stephen B. Potter et al: The giant planet orbiting the cataclysmic binary DP Leonis . In: Astrophysics. Solar and Stellar Astrophysics . 2010, arxiv : 1011.3905 .
  7. ↑ K. Beuermann et al .: Possible detection of two giant extrasolar planets orbiting the eclipsing polar UZ Fornacis . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1106.1404v1 .
  8. ↑ SB Qian et al.: A giant planet in orbit around a magnetic-braking hibernating cataclysmic variable . In: Monthly Notice of the Royal Astronomical Society . tape 401 , no. 1 , 2010, p. L34-L38 , doi : 10.1111 / j.1745-3933.2009.00780.x .
  9. ↑ SB Qian et al.: Detection of a planetary system orbiting the eclipsing polar HU Aqr . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1103.2005 .
  10. ↑ SG Person ua: Orbital Period Variations in Eclipsing Post Common Envelope Binaries . In: Astrophysics. Solar and Stellar Astrophysics . 2010, arxiv : 1005.3958 .
  11. ↑ Jonathan Horner, Robert A Wittenmyer, Jonathan P Marshall, Chris G Tinney and Oliver W Butters: The Curious Case of HU Aquarii - Dynamically Testing Proposed Planetary Systems . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1201.5730v1 .