  HD 179821 is an evolved star of unknown progenitor mass range (either
post-Asymptotic Giant Branch or post-Red Supergiant) exhibiting a double peaked
spectral energy distribution (SED) with a sharp rise from $\sim8-20$ $\mu$m.
Such features have been associated with ejected dust shells or inwardly
truncated circumstellar discs. In order to compare SEDs from both systems, we
employ a spherically symmetric radiative transfer code and compare it to a
radiative, inwardly truncated disc code. As a case study, we model the
broad-band SED of HD 179821 using both codes. Shortward of 40 $\mu$m, we find
that both models produce equivalent fits to the data. However, longward of 40
$\mu$m, the radial density distribution and corresponding broad range of disc
temperatures produce excess emission above our spherically symmetric solutions
and the observations. For HD 179821, our best fit consists of a $T_{eff}=7000$
K central source characterized by $\tau_V\sim1.95$ and surrounded by a
radiatively driven, spherically symmetric dust shell. The extinction of the
central source reddens the broad-band colours so that they resemble a
$T_{eff}=5750$ K photosphere. We believe that HD 179821 contains a hotter
central star than previously thought. Our results provide an initial step
towards a technique to distinguish geometric differences from spectral
modeling.
