Mid-Infrared Imaging of a Circumstellar Disk around HR 4796: Mapping the Debris of Planetary Formation

We report the discovery of a circumstellar disk around the young A0 star, HR4796, in thermal infrared imaging carried out at the W.M. Keck Observatory. Byfitting a model of the emission from a flat dusty disk to an image atlambda=20.8 microns, we derive a disk inclination, i = 72 +6/-9 deg from faceon, with the long axis of emission at PA 28 +/-6 deg. The intensity of emissiondoes not decrease with radius as expected for circumstellar disks but increasesoutward from the star, peaking near both ends of the elongated structure. Wesimulate this appearance by varying the inner radius in our model and find aninner hole in the disk with radius R_in = 55+/-15 AU. This value corresponds tothe radial distance of our own Kuiper belt and may suggest a source of dust inthe collision of cometesimals. By contrast with the appearance at 20.8 microns,excess emission at lambda = 12.5 microns is faint and concentrated at thestellar position. Similar emission is also detected at 20.8 microns in residualsubtraction of the best-fit model from the image. The intensity and ratio offlux densities at the two wavelengths could be accounted for by a tenuous dustcomponent that is confined within a few AU of the star with mean temperature ofa few hundred degrees K, similar to that of zodiacal dust in our own solarsystem. The morphology of dust emission from HR 4796 (age 10 Myr) suggests thatits disk is in a transitional planet-forming stage, between that of massivegaseous proto-stellar disks and more tenuous debris disks such as the onedetected around Vega.