H. Chihara

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.

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.

Crystallization Experiments on Amorphous Silicates with Chondritic Composition: Quantitative Formulation of the Crystallization

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°-1200°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)2SiO4] 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 (5Fe2O3 9H2O) 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.

Shape and lattice distortion effects on infrared absorption spectra of olivine particles

We prepared sub-micron olivine (Fo92) 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.

Laboratory production of magnesium sulfide grains and their characteristic infrared spectra due to shape

Nanosized MgS grains, which have been considered the origin of the 30 µm 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 µm) with three shoulders at 460, 400 and 262 cm −1 (21.7, 25.0 and 38.2 µm). 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.

Mg/Fe FRACTIONATION IN CIRCUMSTELLAR SILICATE DUST INVOLVED IN CRYSTALLIZATION

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.

INTERSTELLAR ANALOGS FROM DEFECTIVE CARBON NANOSTRUCTURES ACCOUNT FOR INTERSTELLAR EXTINCTION

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 magnetic ordering of graphite grains and its application to astrophysical problems

Abstract 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 μm 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.

Magnetorotation Experiment of Diamagnetic Single-Crystal Grains Suspended in a Gas Medium for Examining Dust Alignment in the Interstellar Region

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 µm in diameter and 1.0 µm 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 in 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.