Chihara Hiroki

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.

The estimation of the temperature and mass of dust in high-redshift galaxies is essential for discussions of the origin of dust in the early Universe. However, this is made difficult by limited sampling of the infrared spectral-energy distribution. Here, we present an algorithm for deriving the temperature and mass of dust in a galaxy, assuming dust to be in radiative equilibrium. We formulate the algorithm for three geometries: a thin spherical shell, a homogeneous sphere and a clumpy sphere. We also discuss the effects of the mass absorption coefficients of dust at ultraviolet and infrared wavelengths, κUV and κIR, respectively. As an example, we apply the algorithm to a normal, dusty star-forming galaxy at z = 7.5, A1689zD1, for which three data points in the dust continuum are available. Using κUV = 5.0 × 104 and κIR = 30(λ/100 μm)−β cm2 g−1 with β = 2.0, we obtain dust temperatures of 38–70 K and masses of 106.5–7.3 M☉ for the three geometries considered. We obtain similar temperatures and masses from just a single data point in the dust continuum, suggesting that the algorithm is useful for high-redshift galaxies with limited infrared observations. In the case of the clumpy sphere, the temperature becomes equal to that of the usual modified black-body fit, because an additional parameter describing the clumpiness works as an adjuster. The best-fitting clumpiness parameter is ξcl = 0.1, corresponding to ∼10 per cent of the volume filling factor of the clumps in this high-redshift galaxy if the clump size is ∼10 pc, similar to that of giant molecular clouds in the local Universe.

Interplanetary dust (IPD) is thought to be recently supplied from asteroids and comets. Grain properties of the IPD can give us information about the environment in the protosolar system, and can be traced from the shapes of silicate features around 10 mu m seen in the zodiacal emission spectra. We analyzed mid-infrared slit-spectroscopic data of the zodiacal emission in various sky directions obtained with the Infrared Camera on board the Japanese AKARI satellite. After we subtracted the contamination due to instrumental artifacts, we successfully obtained high signal-to-noise spectra and have determined detailed shapes of excess emission features in the 9-12 mu m range in all sky directions. According to a comparison between the feature shapes averaged over all directions and the absorption coefficients of candidate minerals, the IPD was found to typically include small silicate crystals, especially enstatite grains. We also found variations in the feature shapes and the related grain properties among the different sky directions. From investigations of the correlation between feature shapes and the brightness contributions from dust bands, the IPD in dust bands seems to have a size frequency distribution biased toward large grains and shows indications of hydrated minerals. The spectra at higher ecliptic latitudes showed a stronger excess, which indicates an increase in the fraction of small grains included in the line of sight at higher ecliptic latitudes. If we focus on the dependence of detailed feature shapes on ecliptic latitudes, the IPD at higher ecliptic latitudes was found to have a lower olivine/(olivine + pyroxene) ratio for small amorphous grains. The variation of the mineral composition of the IPD in different sky directions may imply different properties of the IPD from different types of parent bodies, because the spatial distribution of the IPD depends on the type of the parent body.

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.