Discovery of an 86 AU Radius Debris Ring around HD 181327

Abstract

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 31.7d+/-1.6d from face-on, with the disk major-axis P.A. at 107^deg+/-2^deg. 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 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.