茅原 弘毅

Abstract The presence of amorphous silicate particles in interstellar and circumstellar space has been suggested based on the observation of 9.7 and 18 μm emission bands. We have successfully synthesized amorphous silicate samples of an enstatite and forsterite composition by the mechanical milling of mixed powder consisting of SiO₂–MgO and SiO₂–Mg(OH)₂ reagent-grade particles under different rotation frequencies and milling times. These two types of starting materials are prepared to study the effect of the OH bond on synthesis and crystallization. The amorphous samples were characterized by X-ray diffraction and infrared spectroscopy. Amorphous samples with enstatite composition are synthesized from both SiO₂–MgO and SiO₂–Mg(OH)₂ at 300 rpm and for 300 hr. Amorphous samples with forsterite composition are synthesized from both SiO₂–MgO and SiO₂–Mg(OH)₂. The samples from SiO₂–Mg(OH)₂ require 400 rpm and a long milling time of 1600 hr. After crystallization, amorphous samples with an enstatite composition synthesized from SiO₂–Mg(OH)₂ mainly transform into forsterite with small amounts of amorphous silica SiO₂ and enstatite depending on the rotation frequencies and milling time, while those from SiO₂–MgO become enstatite. The amorphous samples with a forsterite composition are crystallized to forsterite from both starting materials. The presence of H₂O or OH bonds significantly affects the final products after the crystallization of amorphous silicates of enstatite composition.

We investigated the absorption spectra of crystalline plagioclase feldspar, using natural and synthetic samples with a wide compositional range, in the infrared region. And we also carried out cryogenic measurements for synthetic anorthite. In the absorption spectra, systematic spectroscopic variations that depend on the chemical composition were observed, in terms of characteristic features such as the number of absorption peaks, shift of peak location and variety of peak width and intensity. Since, plagioclase feldspar is considered as a reasonable mineral species formed in oxygen rich circumstellar environments from the viewpoint of the equilibrium condensation sequence, our spectral data obtained in this work is expected to be used widely in the field of astronomical mineralogy.

Context. We study the absorption and scattering of X-ray radiation by interstellar dust particles, which allows us to access the physical and chemical properties of dust. The interstellar dust composition is not well understood, especially on the densest sight lines of the Galactic plane. X-rays provide a powerful tool in this study. Aims. We present newly acquired laboratory measurements of silicate compounds taken at the Soleil synchrotron facility in Paris using the Lucia beamline. The dust absorption profiles resulting from this campaign were used in this pilot study to model the absorption by interstellar dust along the line of sight of the low-mass X-ray binary GX 5-1. Methods. The measured laboratory cross-sections were adapted for astrophysical data analysis and the resulting extinction profiles of the Si K-edge were implemented in the SPEX spectral fitting program. We derive the properties of the interstellar dust along the line of sight by fitting the Si K-edge seen in absorption in the spectrum of GX 5-1. Results. We measured the hydrogen column density towards GX 5-1 to be 3.40 +/- 0 : 1 x 10(22) cm(-2). The best fit of the silicon edge in the spectrum of GX 5-1 is obtained by a mixture of olivine and pyroxene. In this study, our modeling is limited to Si absorption by silicates with different Mg:Fe ratios. We obtained an abundance of silicon in dust of 4.0 +/- 0.3 x 10(-5) per H atom and a lower limit for total abundance, considering both gas and dust of >4.4 x 10(-5) per H atom, which leads to a gas to dust ratio of >0.22. Furthermore, an enhanced scattering feature in the Si K-edge may suggest the presence of large particles along the line of sight.

The existence of silica within several debris disks has been suggested. Data on both the spectroscopy and annealing conditions of the various polymorphs of silica need to be investigated, as these data are lacking and incomplete in the literature. We investigate the annealing conditions of silica and prepare various types of silica, including α-cristobalite, α-quartz, coesite, stishovite, and fused quartz, which are natural, synthetic, or commercial samples. This paper presents a new study of both the spectroscopy of relevant silica polymorphs and the conditions under which they form. We compare the results to previous studies and find that there are discrepancies. The interesting result of features similar to those of forsterite should be highlighted, where α-cristobalite and coesite showed similar peaks at 16, 33, and 69 μm as forsterite. The 69 μm band for α-cristobalite is especially very broad and strong and shifts largely to a shorter wavelengths under cooling to low temperatures. The band for coesite, however, is very sharp and shifts only a small amount to longer wavelengths under cooling to low temperatures. We discuss the possibility of silica detection around debris disks. © 2013. The American Astronomical Society. All rights reserved..

The far-infrared continuum opacity of cold dust is an important quantity for the study of debris disks in planetary systems and of protoplanetary disks. Olivine is considered the most abundant crystalline dust species in such environments. We present spectroscopic absorption measurements on olivine plates of the order of a millimeter thickness at wavelengths between 60 and 400 μm for temperatures down to 10 K. Our data reveal a strong temperature dependence of the continuum absorption coefficient, i.e. more than an order of magnitude decrease at 100 μm for 10 K compared to room temperature. The absolute values are generally much lower than those measured with olivine powders embedded into polyethylene pellets, even if the difference between plate and powder samples is taken into account by theoretical models. In contrast to this, the room temperature data are in relatively good agreement with simulations using optical constants determined from reflection measurements. At low temperatures, the absorption coefficient of olivine was measurable with sufficient accuracy only up to 90 μm for 10 K and up to 110 μm for 100 K. These data reveal a drastic change in the spectral slope (from β ∼ 2.0 to β &gt 5.0) for the continuum underlying the 69-μm band, which is not predicted by the low-temperature optical constants determined for forsterite. © The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS) .

Crystallization experiments of relatively SiO2-rich amorphous silicates using the mean chemical composition of the silicate portions in GEMS (glass with embedded metal and sulfide), which is a major component in anhydrous interplanetary dust particles and a primitive material of the early solar system, were performed to understand the presence of crystalline silica around young stars and crystallization in GEMS. Olivine crystallized at similar to 900-1400 K, probably prior to pyroxene. Three different polymorphs of pyroxene, protopyroxene, orthopyroxene, and clinopyroxene, were identified at >= 1000 K. Cristobalite, which is one of the silica polymorphs, crystallized only at high temperatures (>= 1500 K). We obtained time-temperature-transformation (TTT) crystallization diagrams. These results suggest that crystallization of a silica polymorph is kinetically difficult in a day or so at similar to 900-1400 K even for the SiO2-saturated composition, while the crystallization might be possible after metastable olivine crystallization if duration is long enough. The TTT diagram also indicates that the GEMS cooling timescale was similar to 10(5) s if they condensed at 1000 K as amorphous silicates and annealed during cooling after the condensation.

Infrared (IR) spectra provide a rich amount of information concerning chemical composition, lattice structure, size and shape of circumstellar dust. Accurate reference data are then required for the analysis of the various detected components. This study provides the IR characterization of one of the most frequently observed compounds, MgSiO3 ortho-enstatite, and shows that IR experiments and ab initio techniques can be used synergically to obtain high-quality data concerning crystalline materials. The IR reflectance spectrum of synthetic ortho-enstatite was collected and compared to ab initio results (Gaussian-type basis set, PBE0 hybrid density functional theory (DFT) functional). An excellent agreement is observed both for vibrational frequencies (v) and intensities, the latter estimated through the oscillator strength. The mean absolute difference between experimental and calculated. is of the order of 7-8 cm(-1) ( 43 out of 65 peaks differ by less than 10 cm(-1), only four peaks differ by 15-19 cm(-1)). The static dielectric tensor and its components (electronic and ionic contributions) were measured and compared to calculated data: differences are in the 2-5 per cent range.

Because interstellar dust is closely related to the evolution of matter in the galactic environment and many other astrophysical phenomena, the laboratory synthesis of interstellar dust analogs has received significant attention over the past decade. To simulate the ultraviolet (UV) interstellar extinction feature at 217.5 nm originating from carbonaceous interstellar dust, many reports focused on the UV absorption properties of laboratory-synthesized interstellar dust analogs. However, no general relation has been established between UV interstellar extinction and artificial interstellar dust analogs. Here, we show that defective carbon nanostructures prepared by high-energy collisions exhibit a UV absorption feature at 220 nm which we suggest accounts for the UV interstellar extinction at 217.5 nm. The morphology of some carbon nanostructures is similar to that of nanocarbons discovered in the Allende meteorite. The similarity between the absorption feature of the defective carbon nanostructures and UV interstellar extinction indicates a strong correlation between the defective carbon nanostructures and interstellar dust.

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 mu m. The spectra depend on particle shape. The spectra of the 11, 19, 23, and 33 mu m bands, in particular, are extremely sensitive to particle shape, whereas some peaks such as the 11.9, 49, and 69 mu m 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 degrees 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 degrees 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 mu m, may be due to spherical or spherical-like forsterite.

We prepared sub-micron olivine (Fo(92)) particles of different shapes and different degrees of crystal lattice distortion, and studied both effects on the infrared absorption spectra experimentally. We found that the peak positions of the spectra shifted to short wavelength when the number fraction of spherical grains became large, consistent with spectral calculations using the continuous distributions of ellipsoids (CDE) model. We also found for the first time that the spectral features become broader by crystal lattice distortion. The peak intensities are affected by both effects. The peak intensity decreases with lattice distortion in general, but some peaks are largely affected by particle shape. The spectral difference of olivine measured in different laboratories can be explained by both the shape and lattice distortion effects through sample preparation methods. Broadening features in the infrared spectra of forsterite ejecta from comet Tempel 1 obtained in the Deep Impact mission, and forsterite crystals in circumstellar regions, suggest that forsterite crystals undergo lattice distortion.

We carried out experiments of low-temperature infrared spectroscopy and transmission electron microscopy of enstatite (MgSiO(3)) synthesized by heating of amorphous magnesium silicate. There is a discrepancy between the infrared feature of enstatite obtained in this experiment and that of fine powdered single crystals. This reflects stacking disorder of enstatite. We show that circumstellar dust emission of enstatite is similar to the infrared feature measured in this experiment. This result strongly suggests that circumstellar enstatite has abundant stacking faults and is different from the single crystal.

Infrared astronomical observations of oxygen-rich young and evolved stars show that only magnesium-rich crystalline silicates exist in circumstellar regions, and iron, one of the most important dust-forming elements, is extremely depleted. The compositional characteristic of circumstellar crystalline silicates is fundamentally different from that of primitive extraterrestrial materials in our solar system, such as chondritic meteorites and interplanetary dust particles. Amorphous silicates are ubiquitous and abundant in space, and are a promising carrier of iron. However, since the first detection of crystalline silicates, there has been an unsolved inconsistency due to differing compositions of coexisting crystalline and amorphous phases, considering that amorphous silicates have been expected to be precursors of these crystals. Here we show the first experimental evidence that Fe-depleted olivine can be formed by crystallization via thermal heating of FeO-bearing amorphous silicates under subsolidus conditions. Mg/Fe fractionation involved in crystallization makes possible to coexist Mg-rich crystalline silicates with Fe-bearing amorphous silicates around stars.

In order to clarify the crystallization process of silicates in circumstellar environments around young and evolved stars, we performed a laboratory simulation of the crystallization of silicate materials by use of synthetic samples in the MgO-SiO(2) system with an Mg/Si ratio of 1.07. The starting amorphous material was synthesized by a sol-gel method. The samples were heated at 1023-1073 K in an electric furnace in the atmosphere. The crystallization process of enstatite (MgSiO(3)) shows nucleation delay in the early stage of the infrared spectral evolution. We quantitatively evaluated the time constant of the enstatite crystallization, and determined the activation energy to be 1.12 x 10(5) K. The activation energy is much affected by the temperature dependence of nucleation. This large value indicates the kinetic inhibition of crystallization in cold circumstellar regions.

In order to make clear crystallization process of silicates in circumstellar environments of oxygen-rich young stars, we have performed laboratory experiments on crystallization of a silicate material by use of a synthetic sample with the chondritic composition for the first time. The aim of this work is to analyze the crystallization process quantitatively using the amorphous material with the chondritic composition. The starting amorphous material was synthesized by the sol-gel method. The sample was heated at 660 degrees-1200 degrees C for 0.5-12 hr to investigate the temperature and time dependence of the crystallization. The run products were analyzed using infrared absorption spectroscopy, X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Olivine [(Mg, Fe)(2)SiO4] was mainly crystallized from the starting amorphous material. We performed infrared spectral fittings of the heated samples using individual spectra of olivine and amorphous silicate, and estimated the degree of crystallization quantitatively. The time-dependent crystallization process could be formulated using the Johnson-Mehl-Avrami equation with the power of about 1.2, which is consistent with theoretical crystallization model of three-dimensional diffusion-controlled growth from a state that a number of nuclei is constant. The constant number of nuclei corresponds to the starting material, which contains crystallites of magnetite (Fe3O4) and ferrihydrite (5Fe(2)O(3)center dot 9H(2)O) as nucleation sites of olivine crystals. From the quantitative analyses, we suggest that crystallization processes in circumstellar regions should depend on properties of the interstellar amorphous silicates such as existence of crystallites and/or FeO content.

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 similar to 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.

We have performed heating experiments on amorphous silicates with the chondritic composition. The amorphous samples were heated to investigate the temperature and time dependence of crystallization. In IR spectra, the absorption peaks of olivine grew gradually with increasing heating duration at a constant temperature or with increasing temperature for a constant duration. We performed an IR spectral fit of the heated samples and estimated the degree of crystallization quantitatively.

To obtain information on the chemical and physical conditions of circumstellar dust and its regions of formation, we have investigated the behavior of the far-infrared absorption spectra of olivines with chemical compositions close to the Mg-end member by use of newly synthesized polycrystalline samples. In the future, we intend to put some constraints on the range of chemical composition and circumstellar dust temperature by comparison of observational results with laboratory spectroscopic data.

The infrared reflectivities of crystalline forsterite (Mg2SiO4) were measured for the temperature range 295-50 K for each crystal axis, between wavenumber 5000 and 100 cm(-1). The reflection spectra show clear dependence of temperature; most of the bands become more intense, sharper and their peak positions shift to higher wavenumber with decreasing temperature. Reflection spectra were fitted with dispersion formula of damped oscillator model of the dielectric constants and the oscillator parameters in the model were derived. The absorption spectra of forsterite particle are calculated with the derived dielectric constants to show that the forsterite features are good thermal indicator for cold temperature range below 295 K.

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.

The absorption spectra of the olivine particles of different Mg/Fe content were measured in the infrared spectral region between 5 and 100 mu m, 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 mu m, 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.

Interstellar and circumstellar matter is known to be strongly irradiated by cosmic radiation and several types of cosmic ray particles. The effects of irradiation on simulated interstellar and circumstellar matter such as CaCO3, MgCO3, SiO2 and Al2O3 are investigated. Especially, thermoluminescence (TL) spectra after gamma-ray and neutron irradiation are compared carefully. It is shown that the thermoluminescence after neutron irradiation appears significantly in the wavelength of blue region. On the reflectance in infrared region, the irradiation effect appears scarcely. (c) 2006 Elsevier Ltd. All rights reserved.

Nanosized MgS grains, which have been considered the origin of the 30 mu m emission feature of carbon-rich evolved objects, were produced from the gas phase using an advanced gas evaporation method. The far-infrared spectrum of cubic MgS grains showed a characteristic absorption peak at 311 cm(-1) (32.1 mu m) with three shoulders at 460, 400 and 262 cm(-1) (21.7, 25.0 and 38.2 mu m). On the other hand, when the grains were roundish or network-like, the absorption peak at 250 cm(-1) became predominant. The cubic MgS grains were produced by direct nucleation from the gas phase. In the case of production via a gas-solid reaction, the MgS grains were network-like. Therefore, the formation environments of MgS grains around carbon-rich evolved objects may be predicted from the intensity of 310 and 250 cm(-1) bands. We suggest that the origins of the absorption band at 310 and 250 cm(-1) are (100), (110) and/or (111) surfaces of MgS grains, respectively.

A new method of producing pyrrhotite grains, which are most commonly found in cometary material and interplanetary dust particles, was developed. Pyrrhotite grains in the monophase having a 7C structure were predominately produced using a solid-solid reaction between iron and sulfur grains at room temperature. The characteristic infrared peaks were observed at 602, 563, and 397 cm(-1) (16.6, 17.8, and 25.2 mu m). (c) 2005 Elsevier Inc. All rights reserved.

The infrared reflectivities or single crystalline forsterite (Mg2SiO4) were measured for the temperature range 300 - 50 K for each crystal axis, between wavenumber 5000 - 100 cm(-1) ( wavelength : 2 - 100 mu m). The spectra show significant dependence of temperature, most of die bands becomes more intense, sharper, and thier peak positions shift to higher wavenumber with temperature decreased. Reflection spectra were fitted with dispersion formula of damped oscillator model to derive the oscillator parameters that. are handy and useful in assessing optical constants of forsterite in low temperature. The absorption spectra of forsterite, powder are calculated for spherical shape, CDE, and hollow-sphere shape distributions under Rayleigh approximation to predict emission spectra in low temperature environment.

IRAS 16279-4757 belongs to a group of post-asymptotic giant branch (post-AGB) stars showing both polycyclic aromatic hydrocarbon (PAH) bands and crystalline silicates. We present mid-infrared images that resolve the object for the first time. The morphology is similar to that of the "Red Rectangle" (HD 44179), the prototype object with PAHs and crystalline silicates. A two-component model and images suggest a dense oxygen-rich torus, an inner, low-density, carbon-rich region, and a carbon-rich bipolar outflow. The PAH bands are enhanced at the outflow, while the continuum emission is concentrated toward the center. Our findings support the suggestion that mixed chemistry and morphology are closely related. We discuss the Infrared Space Observatory Short-Wavelength Spectrometer (ISO SWS) spectra of IRAS 16279-4757. Several bands in the ISO SWS spectrum show a match with anorthite: this would be the first detection of this mineral outside the solar system. Compared to HD 44179, the shapes of PAH bands are closer to those of planetary nebulae, possibly related to a population of small PAHs present in HD 44179 but absent around IRAS 16279-4757. Detailed examination of the spectra shows the individual character of these two objects. The comparison suggests that the torus found in IRAS 16279-4757 may have formed more recently than that in HD 44179.

The mass absorption spectra of crystalline olivine particles with different Mg/(Mg+Fe) ratios, between forsterite (Mg2SiO4, Fo) and fayalite (Fe2SiO4, Fa), were measured for synthetic and natural samples in the mid- and far-infrared regions. The main strong peaks show a systematic shift to longer wavelengths as the Mg/(Mg+Fe) ratio decreases. In the 10-17 mum region, these trends are very clear, and the peak positions are a good indicator of the composition of dust grains. In the 20-100 mum region, the trends are also rather clear, though they are slightly complicated; the intensity and width of the various bands become weak and broad, respectively, or the bands disappear as the concentration of Fo decreases. However, the trends are clear only in a limited composition range near Fo and Fa; the peak positions of 30, 50, and 69 mum bands of forsterite shift linearly as the Fo concentration decreases to about Fo(70). Those of the double band in the 50 mum region for Fe-rich olivine shift linearly as the Fa concentration increases from Fo(20) to Fa. When the spectral signature of Mg-rich or Fe-rich olivine can be observed, the peak positions in the 20-100 mum region are a good indicator of the composition of dust grains.

Crystalline Mg-Fe pyroxene solid solutions with different Mg/(Mg + Fe) ratio (MgSiO3 similar to Mg0.5Fe0.5SiO3 and FeSiO3) were synthesized in the laboratory and their absorption properties were investigated in the IR region. 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 as 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 Mg and Fe 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 the chemical composition and crystal structure of circumstellar pyroxenes.

The mass absorption spectra of crystalline olivine particles with different Mg/(Mg+Fe) ratios, between forsterite Mg2SiO4 (Fo) and fayalite Fe2SiO4 (Fa), were measured for synthetic and natural samples in the mid- and far-infrared regions. The positions of main strong peaks shift to longer wavelength systematically as the Mg/(Mg+Fe) ratio decreases. In the 10-17 mum region, these trends are very clear and the positions are a good indicator to know the chemistry of dust grains. In the 20-100 mum region, the trends are also rather clear, but a little complicated, because the intensity and width of each band become weak and broad, or bands disappear as the concentration of Fo decreases. However, the trends are clear only in the limited composition range at near Fo and Fa: the peak positions of 33, 50, and 69 mum bands of forsterite shift linearly as the Fo concentration decreases to about Fo(70). Those of double band at 50 mum region for Fe-rich olivine shift linearly as the Fa concentration increases from Fo(20) to Fa. Only when the spectral signature of Mg-rich or Fe-rich olivine might be observed, are the positions in the 20-100 mum region a good indicator to know the chemistry of dust grains.

We investigated thermoluminescence of silicates that are of interest in the interstellar and circumstellar medium after irradiation by gamma-rays and fast neutrons. The silicates are forsterite, orthoenstatite, olivine, quartz, and crystalline silicon. The irradiated enstatite shows weak and broad peaks at 545 and 760 nm. In contrast, irradiated bulk and powder samples of forsterite show strong and broad peaks at 640-660 nm. Although thermoluminescence of bulk forsterite is very similar to the extended red emission (ERE) of the Red Rectangle nebula, irradiated powdered forsterite did not reveal any sharp emission features over the broad band. Further, we investigated the possibility of thermoluminescence of crystalline silicon and found that luminescence scarcely appears. It is emphasized that the prominent carrier of ERE is forsterite and fused quartz.

Interstellar matter is known to be strongly irradiated by cosmic radiation and several types of cosmic ray particles. Simulated interstellar matter, such as synthesized forsterite (Mg2SiO4), enstatite (MgSiO3) and magnesite (MgCO3), has been irradiated with Co-60 gamma-rays in liquid nitrogen, and also irradiated with fast neutrons at 10 K and 70 K by making use of the low-temperature irradiation facility of the Kyoto University Reactor (KUR-LTL. Maximum fast neutron dose is 10(17)n(f)/cm(2)). After irradiation, samples are stored in liquid nitrogen for several months to allow the decay of induced radioactivity. We measured the luminescence spectra of the gamma ray irradiated samples during warming to 370 K using a spectrophotometer. For the forsterite and magnesite, the spectra exhibit a rather intense peak at about 645-655 nm and 660 nm respectively, whereas luminescence scarcely appeared in the natural olivine sample. The spectra of forsterite is very similar to the ERE of the Red Rectangle.

Crystalline Mg-Fe pyroxenes with different Mg/(Mg + Fe) ratio (MgSiO3 similar to Mg0.5Fe0.5SiO3 and FeSiO3) were 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.

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 mum, and the oscillator parameters of the dielectric constants for each crystal axis are derived by fitting the reflection spectra. The optical constants derived by the oscillator parameters provide a useful standard for the optical characterization of fayalite.

We have carried out a quantitative trend analysis of the crystalline silicates observed in the ISO spectra of a sample of 14 stars with different evolutionary backgrounds. We have modeled the spectra using a simple dust radiative transfer model and have correlated the results with other known parameters. We confirm the abundance difference 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 differs, 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 difference in Fe content of both materials. Finally we find 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.

The magnetic alignment of diamagnetic grains suspended in a gas medium is observed for the first time on micron-sized single crystals of graphite by the use of a system newly developed for this purpose. Previously magnetic alignment was studied on grains suspended in a liquid medium. The field of full orientation H-s at T = 293 K was as small as 140 G when the crystal size was 3.5 mum in diameter and 1.0 mum in thickness. It was examined experimentally that the alignment in the gas medium depends on the balance between the thermal agitation energy and the diamagnetic anisotropy energy induced in the particle. This experiment is necessary ill order to examine the grain alignment of the interstellar region. which is the basis to observe the interstellar magnetic field. The experiment is necessary also to determine the diamagnetic anisotropy for various inorganic materials through high-temperature measurements.

The far-infrared absorption spectra of crystalline circumstellar dust analogues (orthoenstatite, clinoenstatite, diopside, and forsterite) were measured at room temperature (295 K) and liquid-helium temperature (4.2 K). The wavelength ranges of the measurements were 20-80 μm and 30-80 μm at 295 K and 4.2 K, respectively. The absorption bands were detected very clearly not only at room temperature, but also at liquid-helium temperature. The temperature effects were clearly observed at liquid-helium temperature, such as (1) an increase in the spectral index of the continuum, (2) an enhancement in the absorption band strength, (3) a decrease in the band width, and (4) the absorption peaks of all bands of all samples shifted toward a shorter wavelength. These results suggest that a reconsideration of the identification should be required for some absorption bands which have been recognized as unidentified bands because of a disagreement in the peak position between the observed data and the laboratory data obtained at room temperature. Our experimental data can be obtained from the web-site (http://www.kyoto-phu.ac.jp/labo/butsuri/).

Absorption spectra of the pyroxene group have been measured in mid and far infrared wavelengths. The samples are crystalline pyroxenes (enstatite, diopside, and natural pyroxene) and amorphous pyroxenes (enstatite and diopside glass). Especially, the synthetic pyroxenes such as orthoenstatite, clinoenstatite and diopside are pure and high-quality crystalline samples. For the first time we have detected very strong and sharp peaks at about 60-70 mum in the pyroxenes. The spectra of the pyroxenes are useful to identify the observed peaks, which have been detected by ISO on top of the broad amorphous silicates spectra of comets, circumstellar dust shells around both young stars and evolved stars. Compared to the broad features of amorphous pyroxenes, the crystalline pyroxenes show many sharp peaks in the mid and far infrared regions. Chemical composition, crystal structure, hydration, and temperature affect these spectra. The diopside may contribute to the emission feature of the planetary nebulae NGC 6302 in the far infrared region.

For natural and synthetic crystalline spinel (MgAl2O4), the mass absorption spectra of the fine particles and the reflectance of the bulk crystals were measured in the mid- and far-infrared regions. The absorption spectra showed the prominent peaks at 14.7, 19.7, and 32.4 mum The reflectance also showed three features at 13.8, 18.5, and 32.4 mum. From the reflectance, the optical constants (n,lc) were derived on the natural spinels.

A method to measure the magnetic anisotropy by the use of field-induced harmonic oscillation has been improved, and a sensitivity of 1.6 X 10(-12) emu has been achieved for small quartz single crystal of 0.8 mg in weight. In the condition where the restoring force of the string suspending the crystal becomes negligible compared to the force due to the held-induced anisotropic energy, the period of oscillation becomes proportional to the reciprocal of the applied field. The method is effective to investigate the diamagnetic anisotropies of the sheet silicates, since it is difficult to obtain large single crystals. The precise study of diamagnetic anisotropy for various sheet silicates is essential for investigating the origin of the anisotropy. (C) 1998 Elsevier Science B.V. All rights reserved.

The temperature dependence of diamagnetic grain orientation is reported for the first time concerning micron-sized graphite samples. An experimental method was developed for the purpose of studying the mechanism of interstellar grain alignment. The experiment was carried out in the temperature range from room temperature to 180 K, using ethanol as the suspending medium. The field of full orientation H-S decreased from 380 G to 260 G as the temperature decreased from room temperature to 180 K. The average sizes of the samples were 2.5 +/- 0.2 mu m and 3.8 +/- 0.2 mu m in diameter, and the thickness was 0.3 +/- 0.2 mu m. This temperature dependence of H-S agrees with a theoretical relation, which was derived from the theory of the Boltzmann distribution of suspended particles. A quantitative consistency in applying the conventional theory on interstellar grain alignment has not yet been achieved for the dense region, such as molecular clouds. The possibility of applying a diamagnetic-orientation experiment to the mechanism of interstellar grain alignment is discussed. The method for observing grain alignment established in the present work, can be applied for a quantitative evaluation of the star-light polarization mechanism.

The size dependence of diamagnetic orientation is reported on micron-sized graphite grains suspended in liquid ethanol at room temperature. The experiment was performed as a preliminary step to reproduce the magnetic alignment of astronomical dust particles in space. The mean grain sizes of the two samples were 2.5 and 3.8 mu m in diameter with the field of full orientation H-S being 35.9 and 29.0 mT, respectively. The present H-S obtained for graphite grains are the smallest values compared to any of the reported data on micron-sized particles. The diamagnetic orientation may be applicable to explain the grain alignment in the cosmic dense cloud of a star formation region. (C) 1998 Elsevier Science B.V. All rights reserved.