Midori Saito

ORIGIN OF ORGANIC GLOBULES IN METEORITES: LABORATORY SIMULATION USING AROMATIC HYDROCARBONS

Analogs of organic hollow globules, which have been found in carbonaceous chondrite meteorites and interplanetary dust particles, were synthesized in our laboratory from benzene and anthracene using plasma. Our results suggest that organic globules could be made from aromatic rings in circumstellar envelopes around evolved stars. The hollow interior could be formed by coagulation of vacancies, formed by electronic excitation and/or knock-out of carbon atoms following irradiation by plasma particles such as protons and He+ ions. This experimental result suggests that organic globules are possibly the final products in the evolution of carbonaceous matter from acetylene and benzene to polycyclic aromatic hydrocarbons in ejecta gas from evolved stars.

Characteristic Low-Temperature Crystallization of Amorphous Mg-bearing Silicate Grains under Electron Irradiation

Amorphous Mg-bearing silicate grains, which were produced by the coalescence between MgO and SiOx smoke particles, were crystallized to forsterite (Mg2SiO4) by electron-beam irradiation in a transmission electron microscope at room temperature. The crystallization induced by electron beams was accelerated by the presence of CH4 adsorbed on the surface and incorporated interior of the grains. This experimental result implies the possibility of low-temperature crystallization in a silicate carbon star. In the case of binary stars, since the materials that flow from the stars stationarily exist around the star, the formed amorphous silicate grains will be irradiated by electrons from the star for a long duration. As a result, a significant amount of crystalline silicates can be produced.

Experimental Demonstration of Condensation of Mg-bearing Silicate Grains around Evolved Stars

Mg-bearing silicate grains were produced directly from the vapor phase from magnesium and silicon oxide in a mixed atmosphere of Ar and O2 in the laboratory. It was found that the crystallinity, which was deduced from the shape of the 10 μm feature, of the grains depends on the ratio of magnesium to silicon oxide in the vapor phase. When the Mg/SiOx ratio was high, crystalline forsterite grains were produced owing to annealing of the silicate accompanied by large exothermic energy due to the oxidation of magnesium. The experimental result suggests that the crystallinity of circumstellar silicates could be determined by the balance between heat generation by magnesium oxidation and heat dissipation due to radiation. In this situation, later annealing of the silicate fraction or the use of a warm substrate for condensation of crystalline silicate is unnecessary. Crystalline silicates found in young stars are also able to be produced following our hypothesis after simultaneous evaporation of silicates and ices during energetic shocks in protostellar nebulae.

Structural Alteration of Nanostructure Carbon Particles Carrying Pt Clusters in H2 and O2 Gases

The effects of H2 and O2 gases on the typical fuel cell catalyst of Pt clusters on carbon particles (Pt on C) were examined using a special side-entry sample holder for high-resolution transmission electron microscopy. The holder can be used to transfer the specimen without exposure to air. The effects of H2 and O2 were detected after the specimen was exposed to the gases at 60 °C for 15 h. The predominant effect of H2 was the coagulation of the Pt clusters. The effect of O2 was to alter the structure of the carbon particles by oxidation.

Structural Alterations of Carbon Particles in Saturated Water Vapor

High-resolution transmission electron microscopy (HRTEM) of carbon particles has been carried out at 80 °C in saturated water vapor, to evaluate any structural alterations. Amorphous particles or microcrystallites of carbon based on the graphitic structure were altered to more stable graphitic structures such as ribbons or onion-like structures. The stable graphitic structure was altered again to the amorphous structure. Results are interpreted in terms of the oxidation of carbon particles in air. The differences in alterations among carbon particles, Pt clusters on carbon particles, and Pt clusters in carbon particles are presented on the basis of HRTEM images.

Properties of Circumstellar PAHs in Various Radiation Environments

A series of emission bands in the mid‐infrared, mainly at 3.3, 6.2, 7.6–7.8, 8.6 and 11.2 μm, have been ubiquitously observed in various astrophysical environments. They are originally termed unidentified infrared (UIR) bands and many previous studies based on laboratory experiments and theoretical quantum chemical calculations have lead us to the general consensus that the carriers of these features are carbonaceous materials, including polycyclic aromatic hydrocarbons (PAHs). The relative band strengths, the profiles and the peak positions of these features are expected to vary according to the different physical conditions of the carriers. Recent observational studies have revealed the variations in the spectral properties of the UIR bands among sources with different stellar spectral types. In this proceedings we plan to give a brief introduction of our latest observational studies on the properties and the spatial distribution of the UIR bands in several objects, including both the early‐type (05–06) and the late‐type (K2) stars based on observations with Cooled Mid‐Infrared Camera and Spectrometer (COMICS) on board Subaru telescope.