  We study the effect of density distribution evolution on the global one-armed
oscillation modes in low viscosity disks around isolated and binary Be stars.
Observations show that some Be stars exhibit evidence of formation and
dissipation of the equatorial disk. In this paper, we first calculate the
density evolution in disks around isolated Be stars. To model the formation
stage of the disk, we inject mass at a radius just outside the star at a
constant rate for 30-50 years. As the disk develops, the density distribution
approaches the form of the steady disk solution. Then, we turn off the mass
injection to model the disk dissipation stage. The innermost part of the disk
starts accretion, and a gap forms between the star and the disk. Next, we
calculate the one-armed modes at several epochs. We neglect the effect of
viscosity because the time-scale of oscillations is much shorter than the disk
evolution time-scale for low viscosity. In the disk formation stage, the
eigenfrequency increases with time toward the value for the steady state disk.
On the other hand, one-armed eigenmodes in dissipating Be disks have
significantly higher eigenfrequencies and narrower propagation regions.
Observationally, such a change of mode characteristics can be taken as an
evidence for gap opening around the star. In binary Be stars, the
characteristics of the disk evolution and the eigenmodes are qualitatively the
same as in isolated Be stars, but quantitatively they have shorter evolution
time-scales and higher eigenfrequencies, which is in agreement with the
observed trend.
