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
the mechanism that causes the confinement of
one-armed 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.
Keywords:
global disc oscillation -- hydrodynamics
-- radiative transfer
-- stars: circumstellar matter
-- stars: emission line, Be
