3,425 research outputs found
Program computes equilibrium normal shock and stagnation point solutions for arbitrary gas mixtures
Program computes solutions for flow parameters in arbitrary gas mixtures behind a normal and a reflected normal shock, for in-flight and shock-tube stagnation conditions. Equilibrium flow calculations are made by a free-energy minimization technique coupled with the steady-flow conservation equations and a modified Newton-Raphson iterative scheme
Microorganism study - Bacterial isolants from harsh environments Final report
Soil bacterial isolants from harsh environment
Determining the forsterite abundance of the dust around Asymptotic Giant Branch stars
Aims. We present a diagnostic tool to determine the abundance of the
crystalline silicate forsterite in AGB stars surrounded by a thick shell of
silicate dust. Using six infrared spectra of high mass-loss oxygen rich AGB
stars we obtain the forsterite abundance of their dust shells.
Methods. We use a monte carlo radiative transfer code to calculate infrared
spectra of dust enshrouded AGB stars. We vary the dust composition, mass-loss
rate and outer radius. We focus on the strength of the 11.3 and the 33.6 \mu m
forsterite bands, that probe the most recent (11.3 \mu m) and older (33.6 \mu
m) mass-loss history of the star. Simple diagnostic diagrams are derived,
allowing direct comparison to observed band strengths.
Results. Our analysis shows that the 11.3 \mu m forsterite band is a robust
indicator for the forsterite abundance of the current mass-loss period for AGB
stars with an optically thick dust shell. The 33.6 \mu m band of forsterite is
sensitive to changes in the density and the geometry of the emitting dust
shell, and so a less robust indicator. Applying our method to six high
mass-loss rate AGB stars shows that AGB stars can have forsterite abundances of
12% by mass and higher, which is more than the previously found maximum
abundance of 5%.Comment: Accepted for publication in A&
Crystallinity versus mass-loss rate in Asymptotic Giant Branch stars
Infrared Space Observatory (ISO) observations have shown that O-rich
Asymptotic Giant Branch (AGB) stars exhibit crystalline silicate features in
their spectra only if their mass-loss rate is higher than a certain threshold
value. Usually, this is interpreted as evidence that crystalline silicates are
not present in the dust shells of low mass-loss rate objects. In this study,
radiative transfer calculations have been performed to search for an
alternative explanation to the lack of crystalline silicate features in the
spectrum of low mass-loss rate AGB stars. It is shown that due to a temperature
difference between amorphous and crystalline silicates it is possible to
include up to 40% of crystalline silicate material in the circumstellar dust
shell, without the spectra showing the characteristic spectral features. Since
this implies that low mass-loss rate AGB stars might also form crystalline
silicates and deposit them into the Interstellar Medium (ISM), the described
observational selection effect may put the process of dust formation around AGB
stars and the composition of the predominantly amorphous dust in the
Interstellar Medium in a different light. Our model calculations result in a
diagnostic tool to determine the crystallinity of an AGB star with a known
mass-loss rate.Comment: accepted by A&A, 10 pages, 11 figure
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