astronomy Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest includ ...

, a superhump is a periodic brightness variation in a

cataclysmic variable star In astronomy, cataclysmic variable stars (CVs) are stars which irregularly increase in brightness by a large factor, then drop back down to a quiescent state. They were initially called novae (), since those with an outburst brightness visible t ...

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 Ursae Majoris, or SU UMa, is a close binary star in the northern circumpolar constellation of Ursa Major. It is a periodic cataclysmic variable that varies in magnitude from a peak of 10.8 down to a base of 14.96. The distance to thi ...

(SU UMa) stars, a subclass of

dwarf nova A dwarf nova (pl. wiktionary:nova, novae), or U Geminorum variable, 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 accretion disk, accretes matter ...

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 An accretion disk is a structure (often a circumstellar disk) formed by diffuse material in orbital motion around a massive central body. The central body is most frequently a star. Friction, uneven irradiance, magnetohydrodynamic effects, and ...

is elongated by the

tidal force The tidal force or tide-generating force is the difference in gravitational attraction between different points in a gravitational field, causing bodies to be pulled unevenly and as a result are being stretched towards the attraction. It is the ...

of the donor star. The elliptical disk precesses around the

white dwarf A white dwarf is a Compact star, stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very density, dense: in an Earth sized volume, it packs a mass that is comparable to the Sun. No nuclear fusion takes place i ...

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 Viscosity is a measure of a fluid's rate-dependent resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for example, syrup h ...

dissipation In thermodynamics, dissipation is the result of an irreversible process that affects a thermodynamic system. In a dissipative process, energy ( internal, bulk flow kinetic, or system potential) transforms from an initial form to a final form, wh ...

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

resonance Resonance is a phenomenon that occurs when an object or system is subjected to an external force or vibration whose frequency matches a resonant frequency (or resonance frequency) of the system, defined as a frequency that generates a maximu ...

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 In astronomy, the Roche lobe is the region around a star in a binary system within which orbiting material is gravitationally bound to that star. It is an approximately teardrop-shaped region bounded by a critical gravitational equipotential, wi ...

(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

{{Reflist, 30em, refs= {{cite journal , last1=Pearson , first1=K. J. , title=Are superhumps good measures of the mass ratio for AM CVn systems? , journal=

Monthly Notices of the Royal Astronomical Society ''Monthly Notices of the Royal Astronomical Society'' (MNRAS) is a peer-reviewed scientific journal in astronomy, astrophysics and related fields. It publishes original research in two formats: papers (of any length) and letters (limited to ...

, 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 , doi-access=free , 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 Englis ...

, 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'' (''ApJ'') is a peer-reviewed scientific journal of astrophysics and astronomy, established in 1895 by American astronomers George Ellery Hale and James Edward Keeler. The journal discontinued its print edition 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=

, bibcode = 2000MNRAS.319..510R , volume=319 , issue=2 , pages=510–516 , doi= 10.1111/j.1365-8711.2000.03931.x, arxiv=astro-ph/0007113 Dwarf novae Cataclysmic variable stars Stellar phenomena