2006ApJ...650..414S

HST NICMOS PSF-subtracted coronagraphic observations of HD 181327 have revealed the presence of a ringlike disk of circumstellar debris seen in 1.1 µm light scattered by the disk grains, surrounded by a diffuse outer region of lower surface brightness. The annular disk appears to be inclined by 31°.7±1°.6 from face-on, with the disk major-axis P.A. at 107°±2°. The total 1.1 µm flux density of the light scattered by the disk (at 1".2<r<5".0) of 9.6±0.8 mJy is 0.17%±0.015% of the starlight. Seventy percent of the light from the scattering grains appears to be confined in a 36 AU wide annulus centered on the peak of the radial surface brightness (SB) profile 86.3±3.9 AU from the star, well beyond the characteristic radius of thermal emission estimated from IRAS and Spitzer flux densities, assuming blackbody grains (~22 AU). The 1.1 µm light scattered by the ring (1) appears bilaterally symmetric, (2) exhibits directionally preferential scattering well represented by a Henyey-Greenstein scattering phase function with g_HG=0.30±0.03, and (3) has a median SB (over all azimuth angles) at the 86.3 AU radius of peak SB of 1.00±0.07 mJy/arcsec2. No photocentric offset is seen in the ring relative to the position of the central star. A low SB diffuse halo is seen in the NICMOS image to a distance of ∼4". Deeper 0.6 µm Hubble Space Telescope (HST) ACS PSF-subtracted coronagraphic observations reveal a faint (V~21.5 mag/arcsec2) outer nebulosity at 4''<r<9'', asymmetrically brighter to the north of the star. We discuss models of the disk and properties of its grains, from which we infer a maximum vertical scale height of 4-8 AU at the 87.6 AU radius of maximum surface density, and a total maximum dust mass of collisionally replenished grains with minimum grain sizes of ~1 µm of ~4M_Moon.