-viscosity prescription.
Atsuo T.Okazaki
College of General Education,
Hokkai-Gakuen University,
Toyohira-ku, Sapporo 062-8605, Japan
We examine long-term, cyclic variabilities of the outflow in disks of Be stars in Be/X-ray binaries, based on the viscous decretion disk scenario. In this scenario, the matter ejected from the equatorial surface of the star with the Keplerian rotation velocity drifts outward because of the viscous effect and forms a disk (Lee et al. 1991).
We consider an isothermal disk truncated at a radius where the
tidally-induced eccentric instability occurs. We adopt the
Shakura-Sunyaev's -viscosity prescription.
We find that the outflow in disks formed by viscous decretion is
highly subsonic. Roughly, the outflow velocity increases as r and
the surface density decreases as 
. We also find that the
decretion disks are in general overstable for one-armed (m=1)
spiral modes. The growth rate is of the order of 
, which is about 1-10yr for 
, where H is
the scale-height of the disk and 
is the angular frequency of
disk rotation. Note that disks perturbed by these modes become
eccentric. The characteristics of these modes agree with the
long-term, cyclic variabilities of the Balmer line profiles observed
for some Be/X-ray binaries, which are similar to those of isolated Be
stars, except that the periods of the variabilities are much shorter
for Be/X-ray binaries than for isolated Be stars (Negueruela et al.\
1997). The short periods of line-profile variabilities might be
related to the compactness of Be-star disks in Be/X-ray binaries.
In our model, the orbital phase of the Type I outbursts depends on the amplitude of one-armed modes in Be-star disks. For a small purturbation amplitude, the Type I outbursts will occur close to the periastron passages of the neutron star. When the disk is strongly perturbed, however, the phase of the Type I outbursts will deviate from the periastron, depending on the phase of the one-armed mode.