One-armed oscillations in Be star discs

Aims. In this paper we study the effect of the quadrupole-term in the gravitational potential of a rotationally deformed central Be star on one armed density waves in the circumstellar disc. The aim is to explain the observed long-term violet over red (V/R) intensity variations of the double peaked Balmer emission-lines, not only in cool Be star systems, but also in the hot systems like gamma Cas. Methods. We have carried out semi-analytic and numerical studies of low-frequency one armed global oscillations in near Keplerian discs around Be stars. In these we have investigated surface density profiles for the circumstellar disc which have inner narrow low surface density or gap regions, just interior to global maxima close to the rapidly rotating star, as well as the mode inner boundary conditions. Results. Our results indicate that it is not necessary to invoke extra forces such as caused by line absorption from the stellar flux in order to explain the long-term V/R variations in the discs around massive Be stars. When there exists a narrow gap between the star and its circumstellar disc, with the result that the radial velocity perturbation is non-zero at the inner disc boundary, we find oscillation (and V/R) periods in the observed range for plausible magnitudes for the rotational quadrupole term.