Issue |
A&A
Volume 389, Number 2, July II 2002
|
|
---|---|---|
Page(s) | 547 - 555 | |
Section | Stellar structure and evolution | |
DOI | https://doi.org/10.1051/0004-6361:20020403 | |
Published online | 27 June 2002 |
The circumstellar dust shell of the post-AGB star HD 161796*
1
Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
2
ESTEC/ESA, RSSD-ST, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
3
Instituut voor Sterrenkunde, K.U. Leuven, Celestijnenlaan 200B, 3001 Heverlee, Belgium
4
Department of Physics and Astronomy, University College London, Gower Street, London WC1E6BT, UK
Corresponding author: L. B. F. M. Waters, rensw@astro.uva.nl
Received:
13
December
2001
Accepted:
14
March
2002
We have modeled the complete optical to millimeter spectrum
of the Post-Asymptotic Giant Branch (Post-AGB) star HD 161796 and its
circumstellar dust shell. A full 2–200 μm spectrum taken with the
Infrared Space Observatory was used to constrain the dust properties.
A good fit is achieved using only 4 dust components: amorphous
silicates, the crystalline silicates forsterite and enstatite, and
crystalline water ice, contributing respectively about 63, 4, 6 and 27% to the total dust mass. The different dust species were assumed
to be co-spatial but distinct, resulting in different temperatures for
the different grain populations. We find a temperature for the
crystalline H2O ice of 70 K, which is higher than thermal
equilibrium calculations of pure H2O ice would give. This implies
that the ice must be formed as a mantle on top of an (amorphous)
silicate core. In order to form H2O ice mantles the
mass loss rate must exceed some
yr-1. With a
water-ice fraction of 27% a lower limit for the gas to dust mass
ratio of 270 is found. At a distance of 1.2 kpc (Skinner et al. [CITE])
and adopting an outflow velocity of 15 km s-1
(Likkel et al. [CITE]) an AGB mass loss rate of
(
yr-1) is found, which lasted
900 years and ended 430 years ago. During this phase a total
of 0.46
was expelled. The mass loss rate was high
enough to account for the presence of the H2O ice.
Key words: stars: circumstellar matter, AGB and post-AGB / stars: individual: HD 161796 / infrared: ISM / ISM: lines and bands
© ESO, 2002
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