astronomy Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, g ...

, a superhump is a periodic brightness variation in a cataclysmic variable star system, with a period within a few percent of the

orbital period The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets ...

of the system.

History

Superhumps were first seen in SU Ursae Majoris (SU UMa) stars, a subclass of

dwarf nova A U Geminorum-type variable star, or dwarf nova (pl. novae) is one of several types of cataclysmic variable star, consisting of a close binary star system in which one of the components is a white dwarf that accretes matter from its companion. Dwa ...

e, at times when the binary system underwent a superoutburst, which is an unusually strong outburst (increase in brightness) caused by an increased accretion rate.

Period excess

The period of the superhump variations can be either greater or less than the orbital period, known as positive or negative superhumps respectively. The period excess is the difference between the superhump period and the orbital period, expressed as a fraction of the orbital period.

Physical origin

The accretion disk is elongated by the

tidal force The tidal force is a gravitational effect that stretches a body along the line towards the center of mass of another body due to a gradient (difference in strength) in gravitational field from the other body; it is responsible for diverse phenomen ...

of the donor star. The elliptical disk precesses around the

white dwarf A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes ...

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. Superhumps in cataclysmic variable stars are the result of

viscous The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...

dissipation In thermodynamics, dissipation is the result of an irreversible process that takes place in homogeneous thermodynamic systems. In a dissipative process, energy ( internal, bulk flow kinetic, or system potential) transforms from an initial form to ...

by periodic deformations of the disk. These deformations are caused by the presence of a 3:1

resonance Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied Periodic function, periodic force (or a Fourier analysis, Fourier component of it) is equal or close to a natural frequency of the system ...

between the orbital periods of the accretion disk and the donor star. Retrograde precession of the disk causes negative superhumps, with periods slightly less than the orbital period. 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). The amplitude can be up to 0.6 magnitudes.

References

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Monthly Notices of the Royal Astronomical Society ''Monthly Notices of the Royal Astronomical Society'' (MNRAS) is a peer-reviewed scientific journal covering research in astronomy and astrophysics. It has been in continuous existence since 1827 and publishes letters and papers reporting orig ...

, volume=379 , issue=1 , pages=183–189 , year=2007 , bibcode=2007MNRAS.379..183P , arxiv=0705.0141 , doi=10.1111/j.1365-2966.2007.11932.x , s2cid=2685807 {{cite journal , title=Superhumps and their Amplitudes , year=2010 , last1=Smak , first1=J. , journal=

Acta Astronomica ''Acta Astronomica'' is a quarterly peer-reviewed scientific journal covering astronomy and astrophysics. It was established in 1925 by the Polish astronomer Tadeusz Banachiewicz. Initially, the journal published articles in Latin, later Eng ...

, bibcode = 2010AcA....60..357S , volume=60 , issue=4 , pages=357–371 , arxiv=1011.1090 {{cite journal , title=The physical origin of negative superhumps in Cataclysmic Variables , year=2007 , last1=Wood , first1=Matt A. , last2=Burke , first2=Christopher J. , journal=

The Astrophysical Journal ''The Astrophysical Journal'', often abbreviated ''ApJ'' (pronounced "ap jay") in references and speech, is a peer-reviewed scientific journal of astrophysics and astronomy, established in 1895 by American astronomers George Ellery Hale and ...

, bibcode = 2007ApJ...661.1042W , volume=661 , issue=2 , pages=1042–1047 , doi=10.1086/516723 , doi-access=free {{cite journal , title=Thermal stability and nova cycles in permanent superhump systems , year=2000 , last1=Retter , first1=A. , last2=Naylor , first2=T. , journal=

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