Super hump

from Wikipedia, the free encyclopedia
Light curve of the dwarf Nova HT Cas in the outbreak. In addition to the superhumps, there is a change in the overcast light . The lower light curve shows the scatter when observing unchangeable comparison stars

Superhumps (on German about Super bumps ) are an approximately sinusoidal modulation in the light curves of some dwarf novae low amplitude .

properties

The superhumps occur in all dwarf novae of the type SU UMa during the super maxima. They were discovered in the southern dwarf nova VW Hydri in December 1972. In the case of positive or ordinary superhumps, the period is a few percent longer than the orbital period in the close binary star systems , while in negative superhumps it is a few percent shorter. Positive and negative superhumps can occur at the same time.

Typically, superhumps are not detectable at the beginning of the outbreak, but develop only near the maximum and also reach the maximum amplitude at the maximum. The period of the superhumps is slightly shortened in the course of an eruption with a value that is characteristic of the respective dwarf nova. The amplitude then decreases continuously until it can no longer be observed after days or weeks. However, some stars also show permanent superhumps and, rarely, early superhumps that occur during the ascent to the super maximum. These different properties can occur alternately in some stars, as can superhumps with a double maximum.

The amplitudes than 0.3  may achieve, depend on the orbital inclination of the binary system. In addition to the SU UMa stars, superhumps could also be observed in permanent superhumpers. These are dwarf novae in constant eruption, the nova-like stars, as well as dwarf novae that show no eruptions. Another source of superhumps is the AM Canum Venaticorum stars . These are binary star systems consisting of two white dwarfs with an exchange of matter via an accretion disk. Outside the group of cataclysmic binary stars , superhumps could also be detected in X-ray binary stars of low mass. In X-ray binary stars, the compact star is a neutron star or a black hole .

In rare cases, superhumps also occur during normal outbreaks. With these the following outbreak is then a super maximum. The only known exception so far is the prototype of the SU-UMa star SU Ursae Majoris . Here superhumps could also be detected in an isolated normal maximum and the superhumps disappeared again before the next super maximum.

Interpretation of positive superhumps

Artist's impression of a cataclysmic mutable

Dwarf novae consist of a red dwarf that is so close to a white dwarf that it loses mass on them. The gas hits an accretion disk and spirals through it until it hits the white dwarf. The superhumps are caused by a 3: 1 orbit resonance at the edge of the accretion disk, the eccentricity of which was first demonstrated by photometric and spectroscopic observations during a super outbreak of the eccentric dwarf nova Z Chamaeleontis in 1978. The disk then does not rotate axially symmetrically . That is, the distance between the red dwarf and the outer edge of the accretion disk varies periodically during a super-eruption. This means that more or less potential energy is released when the gas hits the accretion disk. This leads to the observed fluctuations in brightness. Assuming that positive superhumps are caused by a 3: 1 orbit resonance, one can deduce from the observed changes in brightness the mass ratio of the stars in the cataclysmic variables. The result agrees quite well with the analyzes from the eclipsing light change of SU Ursae Majoris stars with the exception of the short-period systems of the WZ Sagittae type.

An alternative interpretation assumes an increased flow of matter from the companion to the accretion disk. A hot spot forms at the point in the accretion disk where the flow of matter hits. The luminosity of the hot spot increases due to the increased flow of matter and leads to the superhumps. The somewhat longer superhump period compared to the orbital period is the result of an expansion of the accretion disk during the eruption.

Negative superhumps

In negative superhumps, the period of the brightness variation in the light curve is a few percent shorter than the period of the binary star system. The cause lies in an accretion disc tilted towards the plane of the orbit , which precesses relative to it, and with a speed different from the orbital period of the two stars. The brightness change of Superhumps is a consequence of the resulting different depth in the gravitational field, wherein the mass flow from the companion incident on the precessing accretion disk and the kinetic energy is converted into electromagnetic radiation. Examples of negative superhumps are TV Col and V344 Lyr. The cycle length of infrahumps is also shorter than the orbital period of the cataclysmic double star. Infrahumps are sinusoidal modulations of the light curve with a very low amplitude and only occur temporarily. They are interpreted as tidal effects in the accretion disk that are not strong enough to trigger a super-eruption.

Cause of superhumps

There are two hypotheses regarding the triggering cause of the superhumps and super-eruptions that arise from the deviation from the axial symmetry of the accretion disc. Smak prefers the cause an increased mass transfer from the red dwarf, which is triggered by a warming of the companion as a result of a normal outbreak. Osaki and Kato, on the other hand, prefer the model of thermal instability. Thereafter, due to its thermal properties, the accretion disk expands in the event of a normal eruption and thus gets into an orbital resonance between the orbital period and the period of rotation of the disk.

Reconstruction of the structure of the accretion disk

If superhump is interpreted as a deviation from a rotational symmetry, it should be possible to infer the structure of the accretion disk from the light curve with the help of tomographic methods . The change in color should contain information about the radial structure of the disc, since the temperature of the accretion disc drops towards its edges. The structure of the disk in the azimuthal direction should be drawn from the development of the superhump phase . The reconstruction of the accretion disk of V455 And in the early phase of a super-eruption shows an embedded spiral structure in a disk deformed by tidal forces. Such behavior was also concluded in other dwarf novae by the technique of Doppler tomography.

See also

Individual evidence

  1. ↑ C. Hoffmeister, G. Richter, W. Wenzel: Veränderliche Sterne . 3. Edition. Springer Verlag, Berlin 1990, ISBN 3-335-00224-5 .
  2. ^ N. Vogt: Photometric study of the dwarf nova VW Hydri . Astronomy and Astrophysics, Volume 36, 369-378 (1974)
  3. ^ Taichi Kato et al .: Survey of Period Variations of Superhumps in SU UMa-Type Dwarf Novae. III: The Fourth Year (2011-2012) . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1210.0678 .
  4. ^ Taichi Kato et al .: Survey of Period Variations of Superhumps in SU UMa-Type Dwarf Novae. III: The Third Year (2010-2011) . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1108.5252v1 .
  5. ^ Brian Warner: Cataclysmic Variable Stars . Cambridge University Press, Cambridge 2003, ISBN 0-521-54209-X ( Cambridge astrophysics series 28).
  6. ↑ David Levitan et al: PTF1 J071912.13 + 485834.0: AN OUTBURSTING AM CVN SYSTEM DISCOVERED BY A SYNOPTIC SURVEY . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1107.1209v1 .
  7. ^ Walter Lewin, Michael van der Klies: Compact Stellar X-ray Sources (Cambridge Astrophysics) . Cambridge University Press, Cambridge 2010, ISBN 978-0-521-15806-0 .
  8. ↑ Akira Imada et al .: Discovery of superhumps during a normal outburst of SU Ursae Majoris . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1210.1087 .
  9. ^ N. Vogt: Z Chamaeleontis: evidence for an eccentric disk during supermaximum? Astrophysical Journal, Vol. 252, 653-667 (1982)
  10. ↑ J. Smak: Superhumps and their Amplitudes . In: Acta Astronomica . tape 60 , no. 4 , 2010, ISSN  0567-7262 , p. 357-371 .
  11. ^ Taichi Kato, Yoji Osaki: New Method to Estimate Binary Mass Ratios by Using Superhumps . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1307.5588v1 .
  12. ↑ A. Olech, E. de Miguel, M. Otulakowska, JR Thorstensen, A. Rutkowski, R. Novak, G. Masi, M. Richmond, B. Staels, S. Lowther, W. Stein, T. Ak, D Boyd, R. Koff, J. Patterson, Z. Eker: SDSS J162520.29 + 120308.7 - a new SU UMa star in the period gap . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1103.5754 .
  13. ^ Matt A. Wood, Martin D. Still, Steve B. Howell, John K. Cannizzo, Alan P. Smale: V344 Lyrae: A Touchstone SU UMa Cataclysmic Variable in the Kepler Field . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1108.3083v1 .
  14. ^ R. Coyne, A. Shenoy, GA MacLachlan, TR Lewis, KS Dhuga, A. Eskandarian, BE Cobb, LC Maximon, and WC Parke: Observation of Infrahumps in V1504 Cygni . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1206.6762 .
  15. ↑ J. Smak: On the Periods of Negative Superhumps and the Nature of Superhumps . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1301.0187 .
  16. ↑ Yoji Osaki, Taichi Kato: The Cause of the Superoutburst in SU UMa Stars is Finally Revealed by Kepler Light Curve of V1504 Cygni . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1212.1516 .
  17. ↑ Makoto Uemura et al: Reconstruction of the Structure of Accretion Disks in Dwarf Novae from the Multi-Band Light Curves of Early Superhumps . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1203.1358v1 .
  18. ↑ J. Echevarrıa: Doppler Tomography in Cataclysmic Variables: an historical perspective . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1201.3075v1 .