Tidal-Force-Induced Precessions of Accretion Disks

The preccession of an accretion disk around a compact star in a close binary has been studied. When the accretion disk tilts, the tidal force from the companion star induces a torque on it, which causes a preccession of the disk. We firstly consider the properties of a preccessing motion of a ring, which is circularly rotating around a compact star, and is preccessing with a slightly tilting angle under the influence of a tidal force from a companion star. We next compare the predicted behaviors of the preccessing ring with observations, and find that several observational facts from Her X-1, SS 433, and some other X-ray binaries can be explained by a tidal-force-induced precession scheme quite reasonably. We further examine the energetics of the preccessing ring as a function of the tilting angle. It is shown that the kinetic and potential energies of the orbiting motions of the ring matter around the compact star increases as the tilting angle increases, while the thermal and effective potential energies for hydro-static balance in the meridian cross section of the ring decreases through adiabatic expansion. Quantitative estimations have shown that when the ring has sufficient thermal energy, the decrease of the energy for the hydro-static balance can be larger than the increase of the energy for circular motion around the compact star until the tilting angle reaches a certain value. It is strongly suggested that preccessions of accretion disks are often realized in close binaries.