Radiation-Driven Warping of Circumbinary Disks Around Eccentric Young
  Star Binaries
  We study a warping instability of a geometrically thin, non-self-gravitating,
circumbinary disk around young binary stars on an eccentric orbit. Such a disk
is subject to both the tidal torques due to a time-dependent binary potential
and the radiative torques due to radiation emitted from each star. The tilt
angle between the circumbinary disk plane and the binary orbital plane is
assumed to be very small. We find that there is a radius within/beyond which
the circumbinary disk is unstable to radiation-driven warping, depending on the
disk density and temperature gradient indices. This marginally stable warping
radius is very sensitive to viscosity parameters, a fiducial disk radius and
the temperature measured there, the stellar luminosity, and the disk surface
density at a radius where the disk changes from the optically thick to thin for
the irradiation from the central stars. On the other hand, it is insensitive to
the orbital eccentricity and binary irradiation parameter, which is a function
of the binary mass ratio and luminosity of each star. Since the tidal torques
can suppress the warping in the inner part of the circumbinary disk, the disk
starts to be warped in the outer part. While the circumbinary disks are most
likely to be subject to the radiation-driven warping on a AU to kilo-AU scale
for binaries with young massive stars more luminous than 10^4Lsun, the
radiation driven warping does not work for those around young binaries with the
luminosity comparable to the solar luminosity.
