  The dust shell around the evolved star HD 179821 has been detected in
scattered light in near-IR imaging polarimetry observations. Here, we subtract
the contribution of the unpolarized stellar light to obtain an intrinsic linear
polarization of between 30 and 40 per cent in the shell which seems to increase
with radial offset from the star. The J and K band data are modelled using a
scattering code to determine the shell parameters and dust properties. We find
that the observations are well described by a spherically symmetric
distribution of dust with an inverse square density law, indicating that when
mass-loss was occurring, the mass-loss rate was constant. The models predict
that the detached nature of a spherically symmetric, optically thin dust shell,
with a distinct inner boundary, will only be apparent in polarized flux. This
is in accordance with the observations of this and other optically thin
circumstellar shells, such as IRAS 17436+5003. By fitting the shell brightness
we derive an optical depth to the star which is consistent with V band
observations and which, assuming a distance of 6 kpc, gives an inner shell
radius of 1.44 X 10^{15} m, a dust number density of 2.70 X 10^{-1} m^{-3}. and
a dust mass of 0.08 Msun. We have explored axisymmetric shell models but
conclude that any deviations from spherical 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 simultaneously fit 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 are either highly elongated or consist of aggregates of smaller
particles.
