Open AccessMid‐infrared imaging of the dust shell around the post‐asymptotic giant branch star HD 161796
Author(s) -
Gledhill T. M.,
Yates J. A.
Publication year - 2003
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1046/j.1365-8711.2003.06732.x
Subject(s) - physics , astrophysics , asymptotic giant branch , spectral energy distribution , astronomy , luminosity , infrared , equator , stellar mass , circumstellar dust , stars , star formation , galaxy , latitude
We present mid‐infrared (IR) images of HD 161796 (IRAS 17436+5003), taken with the OSCIR imager on the Gemini North Telescope, that resolve for the first time the thermal emission structure of the dust shell around this post‐asymptotic giant branch (AGB) star. As well as a basic axisymmetric structure, the observations show deviations from axisymmetry in the dust density and a twist in the symmetry axis. Modelling of the mid‐IR images and of the spectral energy distribution from ultraviolet to submillimetre wavelengths reproduces all of the axisymmetric features with an equator‐to‐pole density contrast of 6: 1 and an inclination of the symmetry axis of 10° to the plane of the sky. We find that a model incorporating small (0.01μm) grains and a steep (∝ a −6 ) power‐law size distribution can successfully account for the thermal emission and for the observed degrees of near‐IR polarization. Assuming a distance of 1.2 kpc to HD 161796, the stellar luminosity is 3.4 × 10 3 L ⊙ and the mass of the shell is ∼0.7 M ⊙ . This is consistent with a star of initial mass between 1 and 2 M ⊙ that has undergone an intensive (2.2 × 10 −4 M ⊙ yr −1 ) phase of mass loss lasting about 3000 yr at the end of the AGB. A current stellar mass of 0.56 M ⊙ , as indicated by the luminosity, suggests that HD 161796 is a few hundred years into its post‐AGB evolution and will take about 5000 yr to evolve from its present temperature of 7500 K to become the central star of an extended elliptical planetary nebula.