Superhumps in AM Canum Venaticorum stars

Master Thesis


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University of Cape Town

The AM Canum Venaticorum stars are an in homogeneous group of six stars that are characterised by the absence of hydrogen lines in their spectra. The structure of these stars has been the subject of much debate, and single star models were considered as recently as 1992. The prevailing opinion, however, is that the A.Jf eVn stars are semi-detached binaries which consist of a low-mass, degenerate helium secondary which is transferring mass via an accretion disc to a DB white dwarf. Under this model, the orbital periods of the AM CVn systems are in the range 1000 - 3000 s. The AM CVn systems show many of the accretion disc-related phenomena that are observed in non-magnetic (hydrogen-rich) cataclysmic variables. Five of the six systems show low-amplitude optical oscillations with periods between 500 and 2000 s. The oscillations have a complex and variable harmonic structure. Since they are consistent with the predictions of the double-degenerate binary model, the oscillations were assumed to correspond to the orbital period in each system. However, the fundamental periods and their harmonics are unstable, showing amplitude and phase drifts on a time scale of weeks to months. The instability of the photometric periods has therefore been an obstacle to accepting that the AM CVn stars are binary systems. An alternative explanation was proposed by O'Donoghue & Kilkenny (1989): the photometric variations in the AM CVn stars are not their orbital periods, but are caused by the same mechanism that produces superhumps in SU UMa dwarf novae. Superhumps are optical variations observed during the superoutbursts of SU UMa stars with periods a few percent in excess of the orbital period. The superhumps usually disappear as the star declines to quiescence after a superoutburst. Superhumps are thought to be the result of periodic enhancements of tidal dissipation in the outer regions of the accretion disc. In addition to the superhumps in SU UMa dwarf novae, permanent superhumps have been observed in other hydrogen-rich cataclysmic variables. The aim of this dissertation is to present new photometric data and to explore the period structure of four of the six known AM CVn stars to see whether the observations are consistent with a superhump interpretation. The aim is also to draw comparisons between the AM CVn stars and examples of hydrogen-rich cataclysmic variables in which superhumps have been observed, and in this way to strengthen the evidence that similar mechanisms are operating in the hydrogen-rich superhumpers and the multi periodic AM CVn stars.

Bibliography: pages 205-214.