EVIDENCE FOR ACCRETION DISK PRECESSION IN THE CATACLYSMIC BINARY AM-CANUM-VENATICORUM

We have found a 13.38 hr period in the helium absorption-line profiles of the cataclysmic variable AM Canum Venaticorum. The lines display blue and red troughs which alternate in depth on this period. Comparison of the data with theoretical line profiles of elliptical accretion disks suggests that this is the signature of an elliptical disk, with the line of apsides advancing slowly around the white dwarf. This interpretation of the 13.38 hr spectroscopic period as apsidal precession of the disk supports the idea that the familiar 1051 s photometric signal is the orbital frequency's precessional sideband, which is also believed to be present in the ''superhumps'' of dwarf novae. The exact orbital period is inferred to be 1028.77s +/- 0.18s. More important, this kinematic evidence for accretion disk precession in a noneruptive star strongly suggests that superhumps and tidal instabilities may be a very widespread and essentially permanent feature of the accretion disks in many CVs. The variation in the skewness of the absorption lines is best seen near line center, as expected since low velocities tend to arise in the outer disk, where tidal effects are more important. But the same periodic signal is found in the wings of the absorption lines, with a phase displaced by 120-degrees. This unexpected phase shift may arise from a dependence of tidal lag on orbital radius in the disk.