  Recent images of the disks of dust around the young stars HR 4796A and
Fomalhaut show, in each case, a double-lobed feature that may be asymmetric
(one lobe may be brighter than the other). A symmetric double-lobed structure
is that expected from a disk of dust with a central hole that is observed
nearly edge-on (i.e., close to the plane of the disk). This paper shows how the
gravitational influence of a second body in the system with an eccentric orbit
would cause a brightness asymmetry in such a disk by imposing a "forced
eccentricity" on the orbits of the constituent dust particles, thus shifting
the center of symmetry of the disk away from the star and causing the dust near
the forced pericenter of the perturbed disk to glow. Dynamic modeling of the HR
4796 disk shows that its 5% brightness asymmetry could be the result of a
forced eccentricity as small as 0.02 imposed on the disk by either the binary
companion HR 4796B, or by an unseen planet close to the inner edge of the disk.
Since it is likely that a forced eccentricity of 0.01 or higher would be
imposed on a disk in a system in which there are planets, but no binary
companion, the corresponding asymmetry in the disk's structure could serve as a
sensitive indicator of these planets that might otherwise remain undetected.
