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 mu m 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.degrees 7 +/- 1.degrees 6 from face-on, with the disk major-axis P. A. at 107 degrees +/- 2 degrees. The total 1.1 mu m 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 black-body grains (approximate to 22 AU). The 1.1 mu m 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 arcsec(-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 similar to 4. Deeper 0.6 mu m Hubble Space Telescope (HST) ACS PSF-subtracted coronagraphic observations reveal a faint (V approximate to 21.5 mag arcsec(-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 approximate to 1 mu m of approximate to 4M(Moon).