Superhump

Lore & Background

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—an unusually strong outburst caused by an increased accretion rate. 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. The physical origin involves the accretion disk being 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, called the beat period, causing a slight change in the orientation of the disk over each orbit. Superhumps result from viscous dissipation by periodic deformations of the disk, caused by a 3:1 resonance 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 where the donor star has a mass at most 34 percent the mass of the accretor star, and the amplitude can be up to 0.6 magnitudes.

Reader's Guide

Superhumps are significant because they reveal the behavior of accretion disks in cataclysmic variable systems, particularly during superoutbursts. The discovery in SU Ursae Majoris stars linked superhumps to a specific subclass of dwarf novae, establishing them as a key observational feature. The period excess—whether positive or negative—provides a direct measure of disk precession and resonance conditions. The requirement that the donor star be at most 34 percent the mass of the accretor star sets a constraint on the binary systems that can exhibit superhumps. The amplitude of up to 0.6 magnitudes makes superhumps detectable and useful for studying disk dynamics. The 3:1 resonance mechanism explains the periodic deformations and viscous dissipation that produce the brightness variations. Negative superhumps, arising from retrograde precession, add complexity to the understanding of disk orientation. Overall, superhumps serve as a tool for probing the structure and evolution of accretion disks in interacting binary systems.

Did You Know?

• Negative superhumps are caused by retrograde precession of the accretion disk.
• The amplitude of a superhump can be up to 0.6 magnitudes.