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 HR 4796 in thermal infrared imaging carried out at the W. M. Keck Observatory. By fitting a model of the emission from a flat dusty disk to an image at lambda = 20.8 mu m, we derive a disk inclination, i = 72 degrees(-9 degrees)(+6 degrees) from face-on, with the long axis of emission at P.A. 28 degrees +/- 6 degrees. The intensity of emission does not decrease with radius, as expected for circumstellar disks, but increases outward from the star, peaking near both ends of the elongated structure. We simulate this appearance by varying the inner radius in our model and find an inner hole in the disk with radius R-in = 55 +/- 15 AU. This value corresponds to the radial distance of our own Kuiper belt and may suggest a source of dust in the collision of cometesimals. By contrast with the appearance at 20.8 mu m, excess emission at lambda = 12.5 mu m is faint and concentrated at the stellar position. Similar emission is also detected at 20.8 mu m in residual subtraction of the best-fit model from the image. The intensity and ratio of flux densities at the two wavelengths could be accounted for by a tenuous dust component that is confined within a few AU of the star with mean temperature of a few hundred degrees K, similar to that of zodiacal dust in our own solar system. The morphology of dust emission from HR 4796 (age 10 Myr) suggests that its disk is in a transitional planet-forming stage, between that of massive gaseous protostellar disks and more tenuous debris disks such as the one detected around Vega.