  We study the interaction between an unequal mass binary with an isothermal
circumbinary disk, motivated by the theoretical and observational evidence that
after a major merger of gas-rich galaxies, a massive gaseous disk with a SMBH
binary will be formed in the nuclear region. We focus on the gravitational
torques that the binary exerts onto the disk and how these torques can drive
the formation of a gap in the disk. This exchange of angular momentum between
the binary and the disk is mainly driven by the gravitational interaction
between the binary and a strong non-axisymmetric density perturbation that is
produced in the disk, as response to the presence of the binary. Using SPH
numerical simulations we tested two gap-opening criterion, one that assumes
that the geometry of the density perturbation is an ellipsoid/thick-spirals and
another that assumes a geometry of flat-spirals for the density perturbation.
We find that the flat-spirals gap opening criterion successfully predicts which
simulations will have a gap on the disk and which simulations will not have a
gap on the disk. We also study the limiting cases predicted by the gap-opening
criteria. Since the viscosity in our simulations is considerably smaller than
the expected value in the nuclear regions of gas-rich merging galaxies, we
conclude that in such environments the formation of a circumbinary gap is
unlikely.
