NICMOS Imaging of the HR 4796A Circumstellar Disk

We report the first near infrared (NIR) imaging of a circumstellar annulardisk around the young (~8 Myr), Vega-like star, HR 4796A. NICMOS coronagraphobservations at 1.1 and 1.6 microns reveal a ring-like symmetrical structurepeaking in reflected intensity 1.05 arcsec +/- 0.02 arcsec (~ 70 AU) from thecentral A0V star. The ring geometry, with an inclination of 73.1 deg +/- 1.2deg and a major axis PA of 26.8 deg +/- 0.6 deg, is in good agreement withrecent 12.5 and 20.8 micron observations of a truncated disk (Koerner, et al.1998). The ring is resolved with a characteristic width of less than 0.26arcsec (17 AU) and appears abruptly truncated at both the inner and outeredges. The region of the disk-plane inward of ~60 AU appears to be relativelyfree of scattering material. The integrated flux density of the part of thedisk that is visible (greater than 0.65 arcsec from the star) is found to be7.5 +/- 0.5 mJy and 7.4 +/- 1.2 mJy at 1.1 and 1.6 microns, respectively.Correcting for the unseen area of the ring yields total flux densities of 12.8+/- 1.0 mJy and 12.5 +/- 2.0 mJy, respectively (Vega magnitudes = 12.92 /+-0.08 and 12.35 +/-0.18). The NIR luminosity ratio is evaluated from theseresults and ground-based photometry of the star. At these wavelengthsLdisk(lambda)/L*(lambda) = 1.4 +/- 0.2E-3 and 2.4 +/- 0.5E-3, giving reasonableagreement between the stellar flux scattered in the NIR and that which isabsorbed in the visible and re-radiated in the thermal infrared. The somewhatred reflectance of the disk at these wavelengths implies mean particle sizes inexcess of several microns, larger than typical interstellar grains. Theconfinement of material to a relatively narrow annular zone implies dynamicalconstraints on the disk particles by one or more as yet unseen bodies.