How Observations of Circumstellar Disk Asymmetries Can Reveal Hidden Planets: Pericenter Glow and Its Application to the HR 4796 Disk

Recent images of the disks of dust around the young stars HR 4796A andFomalhaut show, in each case, a double-lobed feature that may be asymmetric(one lobe may be brighter than the other). A symmetric double-lobed structureis that expected from a disk of dust with a central hole that is observednearly edge-on (i.e., close to the plane of the disk). This paper shows how thegravitational influence of a second body in the system with an eccentric orbitwould cause a brightness asymmetry in such a disk by imposing a "forcedeccentricity" on the orbits of the constituent dust particles, thus shiftingthe center of symmetry of the disk away from the star and causing the dust nearthe forced pericenter of the perturbed disk to glow. Dynamic modeling of the HR4796 disk shows that its 5% brightness asymmetry could be the result of aforced eccentricity as small as 0.02 imposed on the disk by either the binarycompanion 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 beimposed on a disk in a system in which there are planets, but no binarycompanion, the corresponding asymmetry in the disk's structure could serve as asensitive indicator of these planets that might otherwise remain undetected.Comment: 61 pages, 10 figures, accepted for publication in the Astrophysical Journal (scheduled for January 10, 2000