Issue |
A&A
Volume 439, Number 1, August III 2005
|
|
---|---|---|
Page(s) | 171 - 182 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20042296 | |
Published online | 22 July 2005 |
Oxygen-rich AGB stars with optically thin dust envelopes
Astrophysics Missions Division, Research and Scientific Support Department of ESA, ESTEC/SCI-SA, Postbus 299, 2200 AG Noordwijk, The Netherlands e-mail: aheras@rssd.esa.int
Received:
2
November
2004
Accepted:
26
April
2005
The dust composition and dynamics of
the circumstellar envelopes of oxygen-rich AGB stars with low
mass-loss rates (5 10-8–10
yr-1)
have been investigated. We have analyzed the ISO-SWS spectra of twenty-eight oxygen-rich AGB stars with optically thin shells, and modelled the observations with the radiative transfer code
DUSTY using the optical constants from laboratory dust analogues. This
has allowed us to determine the composition of the dust and the
physical conditions at the inner edge of the shell. Moreover, by
comparing with CO observations available in the
literature, we have determined the gas-to-dust mass ratios and the
mass-loss rates of these sources, and analyzed the wind-driving
mechanism. The results show that the small amounts of dust
present in these envelopes, characterized by visual optical depths in the
0.03–0.6 range, are enough to drive the wind by radiation
pressure on the grains. In some sources there are indications of
circumstellar dust that does not contribute to the wind-driving, and that may
distributed in a disk or clumps. Other sources show signs of variable mass-loss
rates. A grain mixture in the shell consisting of aluminium oxide,
melilite, olivine, spinel and Mg0.1Fe0.9O fit the
observed spectra well. From these species, only melilite is required
to have a fractional abundance greater than 25% in all cases. Although spinel
reproduces the 13 μm feature, the absence of the 16.8 μm
peak in our SWS spectra casts doubts on this
identification. The outcome of the modelling reveals that the olivine
content in these CSEs increases with pressure and temperature
at the inner edge. Moreover, the aluminium oxide percentage in the dust
of the envelopes shows a positive correlation with the gas-to-dust mass
ratio. These results, together with the derived dust compositions, are
consistent with the thermodynamic dust condensation sequence scenario
and its freezing-out due to kinetics. However, the temperatures at
the inner edge of the shell are substantially lower than those predicted by theory.
Key words: stars: AGB and post-AGB / circumstellar matter / stars: late-type / stars: mass-loss / infrared: stars / astrochemistry
© ESO, 2005
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