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 , \iota = { 72 ^ { \circ } } ^ { +6 ^ { \circ } } _ { -9 ^ { \circ } } from face on , with the long axis of emission at PA { 28 ^ { \circ } } \pm 6 ^ { \circ } .
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 \pm 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 proto-stellar disks and more tenuous debris disks such as the one detected around Vega .