NICMOS imaging of the HR 4796A circumstellar disk

We report the first near-infrared (NIR) imaging of a circumstellar annular disk around the young (similar to 8 Myr), Vega-like star HR 4796A. NICMOS c oronagraph observations at 1.1 and 1.6 mu m reveal a ringlike symmetrical s tructure that peaks in reflected intensity 1." 05 +/- 0." 02 (similar to 70 AU) from the central AO V star. The ring geometry, with an inclination of 73.degrees 1 +/- 1.degrees 2 and a major axis position angle of 26.degrees 8 +/- 0.degrees 6, is in good agreement with recent 12.5 and 20.8 mu m obse rvations of a truncated disk. The ring is resolved with a characteristic wi dth 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 similar to 60 AU appears to be relatively free of scattering material. The integrated fl ux density of the part of the disk that is visible (greater than 0." 65 fro m the star) is found to be 7.6 +/- 0.5 and 7.4 +/- 1.2 mJy at 1.1 and 1.6 m u m, 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 mag nitudes equal to 12.92 +/- 0.08 and 12.35 +/- 0.18). The NIR luminosity rat io is evaluated from these results and ground-based photometry of the star. At these wavelengths, L-disk(lambda)/L-*(lambda) is equal to 1.4 +/- 0.2 x 10(-3) and 2.4 +/- 0.5 x 10(-3), giving reasonable agreement between the s tellar 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 severa l microns, which is larger than typical interstellar grains. The confinemen t of material to a relatively narrow annular zone implies dynamical constra ints on the disk particles by one or more as yet unseen bodies.