Hiroshi Chihara

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.

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.

Compositional dependence of infrared absorption spectra of crystalline silicates III. Melilite solid solution

The infrared optical properties and absorption spectra of the melilite solid solution series are reported. Melilite is a high-temperature condensate, and is expected to condense during the early phase of the condensation sequence. In this study, samples of the melilite solid solution series were newly synthesized between the aluminium end member (gehlenite) and the magnesium end member (akermanite) at ∼10% intervals in chemical composition, and the infrared absorption spectra of their samples were measured. Variation in the absorption features were detected: such variations include changes in numbers of absorption peaks, peak intensity, peak position, and peak width. The prominent absorption peaks appeared at 10-13 micron, 14, 15, 17, 19, 21, 24, 30, 37 and in the 60 micron region. In particular, the 10 micron feature complex and the 60 micron broad features are very sensitive to chemical composition. For application to and implications for astronomical data, we focused on the 60 micron feature, and carried out a comparison with ISO data of NGC 6302. Our data set will supply the spectroscopic basis for the interpretation of astronomical data accumulated by space and ground based observatories such as ISO, the Spitzer Space Telescope, the AKARI (ASTRO-F) and the Subaru telescope.

Thermoluminescence of simulated interstellar matter after gamma-ray irradiation - Forsterite, enstatite and carbonates

Interstellar matter is known to be strongly irradiated by radiation and several types of cosmic ray particles. Simulated interstellar matter, such as forsterite

CRYSTALLIZATION EXPERIMENTS ON AMORPHOUS MAGNESIUM SILICATE. I. ESTIMATION OF THE ACTIVATION ENERGY OF ENSTATITE CRYSTALLIZATION

\rm Mg_{2}SiO_{4}

Crystallization Experiments on Amorphous Magnesium Silicate. II. Effect of Stacking Faults on Infrared Spectra of Enstatite

, enstatite

Absorption and scattering by interstellar dust in the silicon K-edge of GX 5-1

\rm MgSiO_{3}

Compositional dependence of infrared absorption spectra of crystalline silicate: II. Natural and synthetic olivines

and magnesite

Absorption spectra of Mg-rich Mg-Fe and Ca pyroxenes in the mid- and far-infrared regions

\rm MgCO_{3}