  The merger of a binary system composed of a black hole and a neutron star may
leave behind a torus of hot, dense matter orbiting around the black hole. While
numerical-relativity simulations are necessary to simulate this process
accurately, they are also computationally expensive and unable at present to
cover the large space of possible parameters, which include the relative mass
ratio, the stellar compactness, and the black hole spin. To mitigate this and
provide a first reasonable coverage of the space of parameters, we have
developed a method for estimating the mass of the remnant torus from black
hole-neutron star mergers. The toy model makes use of an improved relativistic
affine model to describe the tidal deformations of an extended tri-axial
ellipsoid orbiting around a Kerr black hole and measures the mass of the
remnant torus by considering which of the fluid particles composing the star
are on bound orbits at the time of the tidal disruption. We tune the toy model
by using the results of fully general-relativistic simulations obtaining
relative precisions of a few percent and use it to extensively investigate the
space of parameters. In this way we find that the torus mass is largest for
systems with highly spinning black holes, small stellar compactnesses, and
large mass ratios. As an example, tori as massive as ~1.33 solar masses can be
produced for a very extended star with compactness of ~0.1 inspiralling around
a black hole with dimensionless spin equal to 0.85 and mass ratio of about 0.3.
However, for a more astrophysically reasonable mass ratio of ~0.14 and a
canonical value of the stellar compactness of ~0.145, the toy model sets a
considerably smaller upper limit to the torus mass of less than ~0.34 solar
masses.
