Superhumps and their Amplitudes

TL;DR

This paper investigates how superhump amplitudes in dwarf novae vary with orbital inclination and beat phase, providing evidence against the tidal-resonance model and supporting a stream dissipation origin.

Contribution

It demonstrates that superhump amplitudes depend on inclination and beat phase, challenging existing models and favoring a stream dissipation explanation.

Findings

Superhump amplitudes increase with orbital inclination.

Modulation of amplitudes with beat phase indicates azimuth-dependent obscuration.

Superhump amplitudes are smaller in permanent superhumpers than during superoutbursts.

Abstract

Superhump amplitudes observed in dwarf novae during their superoutbursts depend on orbital inclination: the maximum amplitudes in systems with low inclinations are $A_\circ \approx 0.25$ mag., while at higher inclinations they increase from $A_\circ \sim 0.3$ to $A_\circ \sim 0.6$ mag. The mean maximum superhump amplitudes normalized to the average luminosity of the disk are: $<A_n>=0.34\pm 0.02$ in low inclination systems and only $<A_n>=0.17\pm 0.01$ in high inclination systems. This shows that at high inclinations the superhump lIght source is {\it partly} obscured by the disk edge and implies that it is located close to the disk surface but extends sufficiently high above that surface to avoid full obscuration. Superhump amplitudes in high inclination systems show modulation with beat phase ($\phi_b$), interpreted as being due to azimuth-dependent obscuration effects in a non-axisymmetric disk. In addition they show modulation with $2\phi_b$ which implies that the orientation of the superhump light source is correlated with the direction of the stream. The dependence of superhump amplitudes on orbital inclination and their modulation with beat phase eliminate the tidal-resonance model for superhumps. Instead they support an alternative interpretation of superhumps as being due to periodically modulated dissipation of the kinetic energy of the stream. Superhump amplitudes in permanent superhumpers are $<A>=0.12$, i.e. much smaller than the maximum amplitudes observed during superoutbursts.