Formation of Narrow Dust Rings in Circumstellar Debris Disks

Narrow dust rings observed around some young stars (e.g., HR 4796A) need tobe confined. We present a possible explanation for the formation andconfinement of such rings in optically thin circumstellar disks, withoutinvoking shepherding planets. If an enhancement of dust grains (e.g., due to acatastrophic collision) occurs somewhere in the disk, photoelectric emissionfrom the grains can heat the gas to temperatures well above that of the dust.The gas orbits with super(sub)-Keplerian speeds inward (outward) of theassociated pressure maximum. This tends to concentrate the grains into a narrowregion. The rise in dust density leads to further heating and a strongerconcentration of grains. A narrow dust ring forms as a result of thisinstability. We show that this mechanism not only operates around early-typestars that have high UV fluxes, but also around stars with spectral types aslate as K. This implies that this process is generic and may have occurredduring the lifetime of each circumstellar disk. We examine the stringentupper-limit on the H2 column density in the HR 4796A disk and find it to becompatible with the presence of a significant amount of hydrogen gas in thedisk. We also compute the OI and CII infrared line fluxes expected from variousdebris disks and show that these will be easily detectable by the upcomingHerschel mission. Herschel will be instrumental in detecting and characterizinggas in these disks.Comment: Accepted for publication in ApJ; 14 pages, 7 figure