Discovery of an 86 AU radius debris ring around HP 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 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 black-body 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 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 ∼4″. Deeper 0.6 μm Hubble Space Telescope (HST) ACS PSF-subtracted coronagraphic observations reveal a faint (V ≈ 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 ≈1 μm of ≈4M_Moon.