NICMOS Imaging of the HR 4796A Circumstellar Disk

We report the first near-infrared (NIR) imaging of a circumstellar annular disk around the young (~8 Myr), Vega-like star HR 4796A. NICMOS coronagraph observations at 1.1 and 1.6 μm reveal a ringlike symmetrical structure that peaks in reflected intensity 1.″05±0.″02 (~70 AU) from the central A0 V star. The ring geometry, with an inclination of 73.°1±1.°2 and a major axis position angle of 26.°8±0.°6, is in good agreement with recent 12.5 and 20.8 μm observations of a truncated disk. The ring is resolved with a characteristic width of less than 0.″26 (17 AU) and appears abruptly truncated at both the inner and outer edges. The region of the disk-plane inward of ~60 AU appears to be relatively free of scattering material. The integrated flux density of the part of the disk that is visible (greater than 0.″65 from the star) is found to be 7.6±0.5 and 7.4±1.2 mJy at 1.1 and 1.6 μm, respectively. Correcting for the unseen area of the ring yields total flux densities of 12.8±1.0 and 12.5±2.0 mJy, 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-based photometry of the star. At these wavelengths, L(λ)/L(λ) is equal to 1.4±0.2×10 and 2.4±0.5×10, giving reasonable agreement between the stellar flux scattered in the NIR and that which is absorbed in the visible and reradiated in the thermal infrared. The somewhat red reflectance of the disk at these wavelengths implies a mean particle size in excess of several microns, which is larger than typical interstellar grains. The confinement of material to a relatively narrow annular zone implies dynamical constraints on the disk particles by one or more as yet unseen bodies.