NICMOS Imaging of the HR 4796A Circumstellar Disk

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Abstract
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 microns reveal a ring-like symmetrical structure peaking in reflected intensity 1.05 arcsec +/- 0.02 arcsec (~ 70 AU) from the central A0V star. The ring geometry, with an inclination of 73.1 deg +/- 1.2 deg and a major axis PA of 26.8 deg +/- 0.6 deg, is in good agreement with recent 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 0.26 arcsec (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 arcsec from the star) is found to be 7.5 +/- 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 these results and ground-based photometry of the star. At these wavelengths Ldisk(lambda)/L*(lambda) = 1.4 +/- 0.2E-3 and 2.4 +/- 0.5E-3, giving reasonable agreement between the stellar flux scattered in the NIR and that which is absorbed in the visible and re-radiated in the thermal infrared. The somewhat red reflectance of the disk at these wavelengths implies mean particle sizes in excess of several microns, 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.
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