Discovery of an 86 AU Radius Debris Ring around HD 181327

HST NICMOS PSF-subtracted coronagraphic observations of HD 181327 have revealed the presence of a ringlike disk of circumstellar debris seen in 1.1 km 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 c 1.6 from face-on, with the disk major-axis P.A. at 107 c 2. The total 1.1 km flux density of the light scattered by the disk (at 1".2 < r < 5".0) of 9.6 c 0.8 mJy is 0.17%c 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 c3.9 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 km 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 sub(HG) = 0.30 c 0.03, and (3) has a median SB (overall azimuth angles) at the 86.3 AU radius of peak SB of 1.00 c 0.07 mJy arcsec super(-2). 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 64n. Deeper 0.6 km Hubble Space Telescope (HST) ACS PSF-subtracted coronagraphic observations reveal a faint (V-21.5 mag arcsec super(-2)) 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 km of -4 M sub(Moon).