Issue |
A&A
Volume 378, Number 1, October IV 2001
|
|
---|---|---|
Page(s) | 228 - 238 | |
Section | The Sun | |
DOI | https://doi.org/10.1051/0004-6361:20011196 | |
Published online | 15 October 2001 |
Steps toward interstellar silicate mineralogy
VI. Dependence of crystalline olivine IR spectra on iron content and particle shape
1
Astrophysical Institute and University Observatory, University of Jena, Schillergässchen 3, 07745 Jena, Germany
2
Institute for Optics and Quantumelectronics, University of Jena, Max-Wien-Platz 1, 07745 Jena, Germany
Corresponding author: D. Fabian, fabian@astro.uni-jena.de
Received:
1
February
2001
Accepted:
22
August
2001
Crystalline olivines are an important component of silicate dust particles in space.
ISO observations revealed the presence of crystalline silicates in comets, protoplanetary accretion
disks, and outflows from evolved stars. For the interpretation of astronomical spectra, the
relevant material data at a variety of temperatures and over a broad wavelength range, are urgently
needed. In contrast to this need, optical properties of the astronomically interesting
olivines are scarcely available at present.
In order to close this gap, we studied the optical properties of three minerals of the olivine group
by reflection spectroscopy on single crystals in the infrared spectral range. We measured
the iron endmember (fayalite, ), an Mg-rich olivine (
), and the magnesium endmember
(forsterite,
) of the
series.
For a direct comparison with astronomical observations, we present calculated mass absorption coefficients
in the Rayleigh limit for different shapes and varying iron content of the dust particles.
The laboratory data together with a set of ISO data for envelopes around evolved stars (Molster 2000)
are used to constrain the properties of circumstellar silicates. We find that essentially all band
positions are shifted to larger wavelengths with increasing iron content. The particle shape
influences very significantly the strong bands such as the B
:
mode that appears as the
"11.4 μm" band of forsterite,
whereas e.g. the two FIR modes longward of 40 μm remain practically unaffected by the particle shape but shift due to
increasing iron content. The comparison with
the band positions in ISO spectra points to the presence of olivine crystals strongly elongated
along the crystallographic c-axis.
In addition, we apply the calculated mass absorption coefficients to evaluate
transmission measurements of particles embedded in a matrix -a technique which is frequently
used in laboratory astrophysics.
All data shown in this paper will be made available in digital form
via the electronic database
http://www.astro.uni-jena.de.
Key words: stars: circumstellar matter / stars: formation / stars: AGB and post-AGB / solar system: formation / methods: laboratory
© ESO, 2001
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