Astronomy and Astrophysics – Astrophysics
Scientific paper
2008-01-18
Astronomy and Astrophysics
Astrophysics
18 pages, 4 figures, 3 tables; accepted to MNRAS
Scientific paper
10.1111/j.1365-2966.2008.13428.x
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.
Balick Bruce
Blackman Eric G.
de Marco Orsola
Forrest William
Frank Alejandro
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