Chiyoe Koike

Crystalline silicate dust around evolved stars - III. A correlations study of crystalline silicate features

We have carried out a quantitative trend analysis of the crystalline silicates observed in the ISO spectra of a sample of 14 stars with dierent evolutionary backgrounds. We have modeled the spectra using a simple dust radiative transfer model and have correlated the results with other known parameters. We conrm the abundance dierence of the crystalline silicates in disk and in outflow sources, as found by Molster et al. (1999a). We found some evidence that the enstatite over forsterite abundance ratio diers, it is slightly higher in the outflow sources with respect to the disk sources. It is clear that more data is required to fully test this hypothesis. We show that the 69.0 micron feature, attributed to forsterite, may be a very suitable temperature indicator. We found that the enstatite is more abundant than forsterite in almost all sources. The temperature of the enstatite grains is about equal to that of the forsterite grains in the disk sources but slightly lower in the outflow sources. Crystalline silicates are on average colder than amorphous silicates. This may be due to the dierence in Fe content of both materials. Finally we nd an indication that the ratio of ortho to clino enstatite, which is about 1:1 in disk sources, shifts towards ortho enstatite in the high luminosity (outflow) sources.

Compositional dependence of infrared absorption spectra of crystalline silicates - I. Mg–Fe pyroxenes

Crystalline Mg-Fe pyroxenes with dierent Mg=(Mg+ Fe) ratio (MgSiO3 Mg0:5Fe0:5SiO3 and FeSiO3 )w ere synthesized in laboratory and their absorption properties were investigated in the infrared region. The absorption spectrum of ferrosilite (FeSiO3) is reported for the first time in this work. Our study confirmed that the Mg end members have sharp and characteristic features in the far-IR region and in particular that it is easy to distinguish the two types of crystalline structures (orthorhombic and monoclinic) in this wavelength range. In addition we find that the absorption spectra depend on the chemistry and crystal structure of the material: (1) The far-IR features which are prominent in the Mg end members, vanish when the iron concentration is increased. However, even in the iron bearing pyroxenes, the variation of the mid-IR features with Fe concentration is less significant in comparison to that of the far-IR features. (2) Peak positions shifted to longer wavelength an increase in iron concentration; the dependence of the shift is approximately linear in wavenumber (cm 1 ). (3) Bandwidths of the far-IR bands for the end members are significantly smaller than those of solid solutions. These results suggest that the far-IR features of pyroxenes are very sensitive to chemical composition and crystal structure. Therefore, far-IR features are a very useful constraint on the chemical composition and crystal structure of circumstellar pyroxenes.

Temperature effects on the mid-and far-infrared spectra of olivine particles

The absorption spectra of the olivine particles of different Mg/Fe content were measured in the infrared spectral region between 5 and 100 µm, while the particles were continuously cooled down to 10 K. Measurements independently carried out on different samples of synthetic forsterite, natural olivine, and synthetic fayalite at laboratories in Kyoto and Jena. The positions of the olivine infrared bands were measured for these samples in detail at up to seven individual temperatures in the interval between 300 K and 10 K. According to the different widths of the olivine bands in different wavelength regions, spectral resolutions of 2, 1, 0.5, 0.25, 0.2, and 0.125 cm −1 were used in order to measure the band positions with high accuracy. While in general the band positions and their temperature-dependent shift agree very well for the Kyoto and Jena samples, the positions of some very strong bands differ, which is probably a consequence of different particle shapes. For the two long-wavelength forsterite bands at 49 and 69 µm, the sharpening and strengthening of the bands were quantified. The widths of these bands differ for the Kyoto and Jena samples, which is discussed in terms of different crystal quality and particle coagulation of the samples. Our new data can be used to derive dust temperatures from the observed peak positions for crystalline silicate dust in circumstellar regions.

Infrared reflection spectra of forsterite crystal

Polarized infrared reflectance of large single crystals of forsterite was measured for each crystal axis at frequencies between 5000 cm −1 and 100 cm −1 with a resolution of 1 cm −1 . The reflectance spectra were analyzed based on classical dispersion theory. A set of oscillator parameters for crystalline axes of forsterite was obtained. Those parameters are used to derive optical constants of forsterite, which are important for analyses of observed spectra of astronomical objects and laboratory spectra of particle samples. Calculations of absorption for small particles using the bulk optical constants are compared with mid-infrared and far-infrared absorption spectra measured in the laboratory.

Optical constants of olivine particles between wavelengths of 7 and 200 μm

Abstract Laboratory measurements of the optical constants of crystalline olivine particles are presented for wavelengths (λ) from 7 to 200μm. Assuming a spherical particle of olivine, the emission features are computed for the grain radii (s) of 0.03, 0.03, 3, and 30mum, it is found that for s ⪅ 0.3 μm, the emission peaks are seen at at λ = 16.3, 18.6, 23, 28, and 33.6 μm, as well as a twin-peaked structure in the 10-μm region, whereas for s ⪆ 3 μ m , these features become weak. If we apply the Halley dust size distribution to the olivine particles, not remarkable twin-peaked emission feature would be expected in spite of its appearance in Comet P/Halley. This implies the presence of only small olivine particles with radii less than roughly 1 μm or that the enhancement of such smaller olivines might pvvur in the comet whose spectrum shows a significant twin-peaked structure in the region of the 10-μm silicate band.

EFFECTS OF FORSTERITE GRAIN SHAPE ON INFRARED SPECTRA

The Infrared Space Observatory (ISO) detected several sharp infrared features around young stars, comets, and evolved stars. These sharp features were identified as Mg-rich crystalline silicates of forsterite and enstatite by comparison with spectra from laboratory data. However, certain infrared emission bands in the observed spectra cannot be identified because they appear at slightly shorter wavelengths than the peaks in forsterite laboratory spectra, where the shapes of forsterite particles are irregular. To solve this problem, we measured infrared spectra of forsterite grains of various shapes (irregular, plate-like with no sharp edges, elliptical, cauliflower, and spherical) in the infrared spectral region between 5 and 100 μm. The spectra depend on particle shape. The spectra of the 11, 19, 23, and 33 μm bands, in particular, are extremely sensitive to particle shape, whereas some peaks such as the 11.9, 49, and 69 μm bands remained almost unchanged despite different particle shapes. This becomes most evident from the spectra of near-spherical particles produced by annealing an originally amorphous silicate sample at temperature from 600 to 1150 ◦ C. The spectra of these samples differ strongly from those of other ones, showing peaks at much shorter wavelengths. At a higher annealing temperature of 1200 ◦ C, the particle shapes changed drastically from spherical to irregular and the spectra became similar to those of forsterite particles with irregular shapes. Based on ISO data and other observational data, the spectra of outflow sources and disk sources may correspond to differences in forsterite shape, and further some unidentified peaks, such as those at 32.8 or 32.5 μm, may be due to spherical or spherical-like forsterite.

The extinction coefficients in mid- and farinfrared of silicate and iron-oxide minerals of interest for astronomical observations

Extinction measurements were made for some silicate and iron-oxide mineral grains in mid- and far-infrared region. For far-infrared region, high temperature magnesium silicates such as olivine and pyroxenes show the absorption spectra of steep dependence as λ−3 (λ being the wavelength) with some peak structure, but the spectrum of magnetite shows λ−1 dependence.

Morphological effects on IR band profiles - Experimental spectroscopic analysis with application to observed spectra of oxygen-rich AGB stars

Aims. To trace the source of the unique 13, 19.5, and 28 μm emission features in the spectra of oxygen-rich circumstellar shells around AGB stars, we have compared dust extinction spectra obtained by aerosol measurements. Methods. We have measured the extinction spectra for 19 oxide powder samples of eight different types, such as Ti-compounds (TiO, TiO2 ,T i 2O3 ,T i 3O5 ,A l 2TiO5 ,C aTiO 3), α-, γ-, χ-δ-κ-Al2O3 ,a nd MgAl 2O4 in the infrared region (10–50 μm) paying special attention to the morphological (size, shape, and agglomeration) effects and the differences in crystal structure. Results. Anatase (TiO2) particles with rounded edges are the possible 13, 19.5 and 28 μm band carriers as the main contributor in the spectra of AGB stars, and spherically shaped nano-sized spinel and Al2TiO5 dust grains are possibly associated with the anatase, enhancing the prominence of the 13 μm feature and providing additional features at 28 μm. The extinction data sets obtained by the aerosol and CsI pellet measurements have been made available for public use at http://elbe.astro.uni-jena.de.

Infrared spectra of fayalite crystal

Crystalline silicate is an important matter since the ISO discovery of crystalline silicate features in circumstellar dusty environments. Polarized infrared reflection spectra of crystal fayalite (Fe2SiO4), an end member of olivine solid solution, were measured for each crystal axis in infrared region up to 100 m, and the oscillator parameters of the dielectric constants for each crystal axis are derived by tting the reflection spectra. The optical constants derived by the oscillator parameters provide a useful standard for the optical characterization of fayalite.

Mid- and far-infrared extinction coefficients of hydrous silicate minerals

Extinction coefficients were measured for three kind of hydrous silicate minerals, montmorillonite, chlorite and serpentine, from 7 to 140 μm. The infrared extinction coefficients of these minerals show (1) a few broad bands in the mid-infrared region and (2) a less steep wavelength-dependence in the far-infrared region, in contrast to those of high-temperature magnesium silicates. In the far-infrared region, montmorillonite shows a λ−0.8±0.1 dependence (λ, the wavelength) without any band structure, chlorite has a double maxima structure around 80 μm, and serpentine shows a rather steep dependence with a small peak at 77 μm.The changes of mid-infrared spectra by heating were measured. Change in chlorite spectrum is the most significant. Many fine features appear by heating and then they disappear. Above 900°C one broad feature remains around 10 μm. Fine features of the montmorillonite spectrum disappear by heating. For serpentine, many new peaks appear and the spectrum resembles the spectrum of olivin. In near-infrared a band around 2.72 μm disappears by heating.Extinction coefficients at very low temperatures were measured in the far-infrared region. For montmorillonite and serpentine, the spectrum is the same as that at room temperature. The double peaks of chlorite around 80 μm become higher.