@article{Nordhaus2008_0,<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Abstract = {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 dust shells or inwardly truncated circumstellar disks. In order to investigate and remove degeneracies in SED fits, we model HD 179821 using a spherically symmetric radiative transfer code and a radiative, inwardly truncated disk code. Shortward of 40 $mu$m, we find that both codes produce equivalent fits to the data. However, longward of 40 $mu$m, the broad range of temperatures in the disk 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 $tau_Vsim1.95$ and surrounded by a radiatively driven, spherically symmetric dust shell. The extinction of the central source reddens the broad-band colors 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 highlight the importance of sub-millimeter and millimeter observations for removing degeneracies in disk versus spherical shell geometries of unresolved sources.},<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Author = {Jason Nordhaus and Ivan  Minchev and Benjamin A.  Sargent and William J.  Forrest and Eric G.  Blackman and Orsola  De Marco and Joel H.  Kastner and Bruce  Balick and Adam  Frank},<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Journal = {Astrophysical Journal},<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Keywords = {hd 179821},<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Month = {May},<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Number = {},<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Pages = {},<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Title = {Spectral Determination of Material Geometry Around Evolved Stars: The Case of HD 179821},<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Url = {},<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Volume = {},<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Year = {2008}}