A.T. Okazaki
College of General Education, Hokkai-Gakuen University,
Toyohira-ku, Sapporo 062, Japan
We consider the structure and time dependent behavior of the outflow in disks of Be stars in Be/X-ray binaries, based on the viscous decretion disk scenario (Lee et al. 1991). In this scenario, the matter ejected from the star with the Keplerian velocity at the equatorial surface of the star drifts outward because of the effects of viscosity, and forms the disk.
In the present study, we adopt the Shakura-Sunyaev's
-viscosity prescription,
and assume the disk to be isothermal.
The disk radius is fixed to be 
times
the mean binary separation, because
in a Be/X-ray binary the disk of the Be star is likely
truncated at the radius where
the tidally-induced eccentric instability occurs.
Figure 1(a) illustrates a typical structure
of viscous decretion disks around Be stars in Be/X-ray binaries:
The outflow velocity increases as r,
the surface density decreases as 
,
and the angular velocity of the disk decreases as 
.
In general, the viscous decretion discs are
overstable for m=1 perturbations.
The growth rate is of the order 
,
where H is the scale-height of the disk
and 
is the angular frequency of disk rotation.
Figure 1(b) shows the m=1 overstable mode
in the decretion disk shown in panel (a).
We note that the perturbation pattern is
of the leading, one-armed spiral.
We also note also that the characteristics of
the one-armed spiral modes are in agreement with
the periodicity and the profile variability
of the V/R variations of Balmer lines observed for some
Be/X-ray binaries.
Finally, we discuss the orbital modulations
of X-ray lightcurves observed for
Be/X-ray binaries A0535+26 and 4U0115+63.
Type I outbursts from A0535+26 occur close to
the periastron passages of the neutron star,
while those from 4U0115+63 are seen close to
orbital phase of 
(Negueruella et al. 1997).
The present model is consistent with these features.
For small amplitude m=1 perturbations,
the eccentricity of the disk is negligible.
Then, the Type I outbursts will occur close to the periastron
passages of the neutron star.
However, when the amplitude of the m=1 perturbation is large
enough to make the disk highly eccentric,
the phase of the Type I outburst will also depend on the phase
of the m=1 mode which does not necessarily coincide
with the periastron pasage of the neutron star.
In conclusion, the viscous decretion disk model for Be stars in Be/X-ray binaries agrees well with long-term V/R variations of Balmer lines and is consistent with the orbital modulations observed for some Be/X-ray binaries.
