On the confinement of one-armed oscillations in discs of Be stars

We discuss the effects due to the rotational deformation and the optically-thin line force on the confinement of one-armed oscillations in the inner part of Be-star discs. The period of these oscillations is identified with the observed V/R variations in Balmer emission lines. We take into account the effect of rotation by including the quadrupole contribution to the potential around the rotationally-distorted central star. For the radiative force due to an ensemble of optically thin lines, we adopt the parametric form proposed by Chen & Marlborough (1994). The disc is assumed to be isothermal. Based on these assumptions, we examine the linear, one-armed eigenmodes confined to the inner part of the disc. Our study strongly suggests that themechanism that causes the confinement of onearmed oscillations in early-type Be stars is different from that in late-type Be stars. In late-type Be stars, the confinement occurs because of the deviation from the point-mass potential around the rotationally deformed star.On this point,we confirm the conclusions obtained by Papaloizou et al. (1992). In early-type Be stars, however, it is the weak-line force that mainly contributes to the confinement. The rotational effect plays a much smaller role for these stars. The period of the eigenmode confined to the disc depends sensitively on the effect by rotation or radiation. This sensitiveness, together with the range of the period of observed V/R variations, places rather narrow constraints on the parameters characterizing these effects. We compare our results with observed V/R properties, for which a list of 53 stars has been compiled.