NICMOS Imaging of the HR 4796A Circumstellar Disk

Abstract

We report the first near-infrared (NIR) imaging of a circumstellarannular disk around the young (~8 Myr), Vega-like star HR 4796A.NICMOS coronagraph observations at 1.1 and 1.6 μm reveal a ringlikesymmetrical structure that peaks in reflected intensity 1.05"+/-0.02"(~70 AU) from the central A0 V star. The ring geometry, with an inclinationof 73.1d+/-1.2d and a major axis position angle of 26.8d+/-0.6d,is in good agreement with recent 12.5 and 20.8 μm observations ofa truncated disk. The ring is resolved with a characteristic widthof less than 0.26" (17 AU) and appears abruptly truncated at boththe inner and outer edges. The region of the disk-plane inward of~60 AU appears to be relatively free of scattering material. Theintegrated flux density of the part of the disk that is visible (greaterthan 0.65" from the star) is found to be 7.6+/-0.5 and 7.4+/-1.2mJy at 1.1 and 1.6 mum, respectively. Correcting for the unseen areaof the ring yields total flux densities of 12.8+/-1.0 and 12.5+/-2.0mJy, respectively (Vega magnitudes equal to 12.92+/-0.08 and 12.35+/-0.18).The NIR luminosity ratio is evaluated from these results and ground-basedphotometry of the star. At these wavelengths, L_disk(lambda)/L_*(lambda)is equal to 1.4+/-0.2x10^-3 and 2.4+/-0.5x10^-3, giving reasonableagreement between the stellar flux scattered in the NIR and thatwhich is absorbed in the visible and reradiated in the thermal infrared.The somewhat red reflectance of the disk at these wavelengths impliesa mean particle size in excess of several microns, which is largerthan typical interstellar grains. The confinement of material toa relatively narrow annular zone implies dynamical constraints onthe disk particles by one or more as yet unseen bodies.