Superhump

In astronomy, a superhump is a periodic brightening in a cataclysmic variable star system, in which the period is a few percent longer than the orbital period of the system.

History

Superhumps were first seen in SU Ursae Majoris (SU UMa) stars, a subclass of dwarf novae, at times when the binary system underwent a superoutburst, which is an unusually strong outburst (increase in brightness) caused by an increased accretion rate.[1]

Physical origin

The accretion disk is elongated by the tidal force of the donor star. The elliptical disk precesses around the white dwarf accretor over a time interval much longer than the orbital period, the beat period, causing a slight change in the orientation of the disk over each orbit.[2] Superhumps in cataclysmic variable stars are the result of viscous dissipation by periodic deformations of the disk. These deformations are caused by the presence of a 3:1 resonance between the orbital periods of the accretion disk and the donor star.

Superhumps can occur in dwarf nova systems in which the donor star (mass-losing star) has a mass that is at most 34 percent the mass of the accretor star (mass-gaining star).[3] The amplitude can be up to 0.6 magnitudes.[4]

Negative superhumps

In some systems, the superhump period is observed to be slightly shorter than the orbital period; such modulations are called negative superhumps. Negative superhumps are caused by the retrograde precession of an accretion disk that is tilted with respect to the orbital plane of the binary system.[3]

References

  1. ↑ Retter, A.; Naylor, T. (2000). "Thermal stability and nova cycles in permanent superhump systems". Monthly Notices of the Royal Astronomical Society. 319 (2): 510–516. arXiv:astro-ph/0007113. Bibcode:2000MNRAS.319..510R. doi:10.1046/j.1365-8711.2000.03931.x.
  2. ↑ Pearson, K. J. (2007). "Are superhumps good measures of the mass ratio for AM CVn systems?". Monthly Notices of the Royal Astronomical Society. 379 (1): 183–189. arXiv:0705.0141. Bibcode:2007MNRAS.379..183P. doi:10.1111/j.1365-2966.2007.11932.x.
  3. 1 2 Wood, Matt A.; Burke, Christopher J. (2007). "The physical origin of negative superhumps in Cataclysmic Variables". The Astrophysical Journal. 661 (2): 1042–1047. Bibcode:2007ApJ...661.1042W. doi:10.1086/516723.
  4. ↑ Smak, J. (2010). "Superhumps and their Amplitudes". Acta Astronomica. 60 (4): 357–371. arXiv:1011.1090. Bibcode:2010AcA....60..357S.
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