Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003mnras.343..880g&link_type=abstract
Monthly Notice of the Royal Astronomical Society, Volume 343, Issue 3, pp. 880-890.
Astronomy and Astrophysics
Astronomy
17
Radiative Transfer, Stars: Agb And Post-Agb, Circumstellar Matter, Stars: Individual: Hd 161796, Stars: Individual: Iras 17436+5003, Stars: Mass-Loss
Scientific paper
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 × 103 Lsolar and the mass of the shell is ~0.7 Msolar. This is consistent with a star of initial mass between 1 and 2 Msolar that has undergone an intensive (2.2 × 10-4 Msolar yr-1) phase of mass loss lasting about 3000 yr at the end of the AGB. A current stellar mass of 0.56 Msolar, 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.
Gledhill Tim M.
Yates Jeremy A.
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