2010MNRAS.405.1397M

In a recent work by the first author, we show that accretion discs in binary systems could retrogradely precess by the same physics that causes the Earth to retrogradely precess [i.e. tidal torques by the Moon and the Sun (or a secondary star) on a tilted, spinning, non-spherical Earth (or e.g. a primary star surrounded by an accretion disc)]. In addition, we show that the state of matter and the geometrical shape of the celestial object could significantly affect the precessional value. For example, a cataclysmic variable (CV) dwarf novae (DN) non-magnetic system that shows negative superhumps in its light curve can be described by a retrogradely precessing, differentially rotating, tilted disc. Because the disc is a fluid and because the gas stream overflows the tilted disc and particles can migrate into inner disc annuli, coupled to the disc could be a retrogradely precessing inner ring that is located near the innermost annuli of the disc. However, numerical simulations by Bisikalo et al. and this work show that an inner spiral density wave can be generated instead of an inner ring. Therefore, we show that retrograde precession in non-magnetic, spinning, tilted CV DN systems can equally be described by a retrogradely precessing and differentially rotating disc with an attached retrogradely precessing inner spiral density wave so long as the wave appears at the same radius as the ring and within the plane of the tilted disc. We find that the theoretical results generated in this work agree well with the theoretical results presented in Montgomery and thus with the numerical simulations and select CV DN systems in Montgomery that may have a main-sequence secondary. Therefore, pressure effects do need to be considered in CV DN systems that exhibit negative superhumps if the accretion discs are tilted and have an inner spiral density wave that is in the plane of the disc.