Scattered light models of the dust shell around HD 179821

The dust shell around the evolved star HD 179821 has been detected in scatt ered light in near-IR imaging polarimetry observations. Here, we subtract t he contribution of the unpolarized stellar light to obtain an intrinsic lin ear polarization of between 30 and 40 per cent in the shell that seems to i ncrease with radial offset from the star. The J- and K-band data are modell ed using a scattering code to determine the shell parameters and dust prope rties. We find that the observations are well described by a spherically-sy mmetric distribution of dust with an r(-2) density law, indicating that whe n mass-loss was occurring, the mass-loss rate was constant. The models pred ict that the detached nature of a spherically-symmetric, optically-thin dus t shell, with a distinct inner boundary, will only be apparent in polarized flux. This is in accordance with the observations of this and other optica lly-thin circumstellar shells, such as ERAS 17436 + 5003. By fitting the sh ell brightness we derive an optical depth to the star that is consistent wi th V-band observations and that, assuming a distance of 6 kpc, gives an inn er-shell radius of r(in) = 1.44 X 10(15) m, a dust number density of N-in = 2.70 X 10(-1) m(-3) at r(in) and a dust mass of M-d = 0.08 M.. We have exp lored axisymmetric shell models but conclude that any deviations from spher ical symmetry in the shell must be slight, with an equator-to-pole density contrast of less than 2:1. We have not been able to fit simultaneously the high linear polarizations and the small [E(J - K) = -0.3] colour excess of the shell and we attribute this to the unusual scattering properties of the dust. We suggest that the dust grains around HD 179821 either are highly e longated or consist of aggregates of smaller particles.