Heating experiments of carbon grains: Implication for the origin of interstellar dust

Amorphous carbonaceous grains produced in a methane gas atmosphere were analyzed by high resolution transmission electron microscope (HRTEM). Structural alteration of the amorphous carbonaceous grains was directly observed by in situ HRTEM observation. The specimen was heated to a maximum of 550℃. The HRTEM image of a prepared specimen showed the structure of the onion-like carbon. The onion-like structure distorted at 100℃, which suggested the evaporation of -OH groups included in the grains. Upon heating to 260℃, the centers of the onion-like grains formed holes of the order of 3 nm in size. On further heating to 550℃, the structure of the chained grains with a size of 10 nm changed to graphite sheets with a length of 50 nm by the surface melting coalescence among chained grains. The structural changes at the above temperatures have been observed only in amorphous carbonaceous grains produced in a methane gas atmosphere and have been attributed to the existence of hydrogen in the grains

A Variation of the Present Star Formation Activity of Spiral Galaxies

The star formation rate in spiral galaxies is considered to be decreasing continuously with time in a time scale of $10^{9}$ yr. The present star formation activity, on the other hand, shows various degrees among galaxies. We make a new data set of 1681 nearby spiral galaxies from available databases and study the statistics of the present star formation activity. We analyze far-infrared and optical B-band surface brightnesses of the H II regions and the non-H II regions in M~31 and show that a far-infrared-to-optical B-band flux ratio, f(FIR)/f(B), is a useful indicator of the present star formation activity of spiral galaxies. For the sample galaxies, we make the distribution diagram of log (f(FIR)/f(B)) versus log L(B) for each morphological type. The distribution of f(FIR)/f(B) has a dispersion of one to two orders of magnitude even within the same morphological type of galaxies, implying that the star formation activities of spiral galaxies changes discontinuously in a short time scale. Analyzing the log (f(FIR)/f(B)) versus log L(B) correlation, we suggest that the most active star formation in galaxies does not continue longer than 10^{8} yr. We also construct a universal distribution histogram of log (f(FIR)/f(B)) for each morphological type. The earlier-type spirals tend to show larger variation of the present star formation activity. We discuss the correlation between the present star formation activity and the structures and environments of galaxies. We suggest that the short-term variation occurs primarily due to internal processes which may change with the morphological type of galaxies.Comment: LaTeX file, 32 pages including tables without figs. 30 figures at ftp://ftp.kusastro.kyoto-u.ac.jp/pub/atomita/preprint/p1489/p1489.tar.gz . To appear in PASJ, Vol. 48, No. 2, April 1996 issu

Tetrataenite in chondrites and experimental demonstration on formation of tetrataenite fine grains

Tetrataenite (Fe_Ni_γ" phase) is a unique metallic mineral in meteorites. A magnetic granulometry analysis of chondrites on the basis of magnetic hysteresis parameters at various temperatures has shown that some LL chondrites contain very fine grains of tetrataenite metal, the grain sizes of which are 10nm or smaller in diameter. The coalescence growth process of a joint cloud of fine smoke particles evaporated from Fe and that from Ni produces fine grains of tetrataenite (FeNiγ"), in addition to those of ordered kamacite (Fe_3Ni), awaruite (FeNi_3) as well as disordered taenites. An outline of the experimental procedures of coalescence growth formation of tetrataenite and the other ordered crystal grains of Fe-Ni alloy and main results of the experiments is described

Caracterização e análise do Programa de Aquisição de Alimentos (PAA) em Santa Catarina

Orientador : Airton SpiesMonografia (especialização) - Universidade Federal do Paraná, Setor de Ciências Agrárias, Curso de Especialização em AgronegócioInclui referência

Experimental demonstration of formation of magnetite and wustitefine grains

Production method of magnetite and wustite grains have been introduced. Magnetite and wustite grains were produced in an Ar gas pressure range of 25 to 100 Torr and 17 to 20 Torr respectively by evaporating FeO powder from a tantalum boat. The growth of these grains has been discussed as the result of oxidation of Fe fine grains and coalescence growth among the oxidized grains

Structural Study of SiO_x Amorphous Thin Films by the Grazing Incidence X-ray Scattering (GIXS) Method

Atomic structures of SiO_x amorphous thin films of 200 nm thick were analyzed by the grazing incident x-ray scattering (GIXS) method. The radial distribution functions (RDFs) were experimentally determined in two SiO_x amorphous thin films grown in the atmosphere with and without N_2 gas. The SiO_x amorphous film grown with N_2 gas forms the network structure consisting of SiO_4 tetrahedra which are connected each other by oxygen atoms at their vertices. This network structure is similar to the one observed in SiO_2 glass. On the other hand, in the SiO_x amorphous film grown without N_2 gas, the atomic distance of Si-O pairs is a few percent longer and the coordination number of O-O pairs is smaller than the other. This suggests that some of oxygen atoms in a SiO_4 tetrahedron are not connected to a next neighboring tetrahedron. Namely, some part of the network structure is disconnected in the SiO_x amorphous film grown without N_2 gas. Due to this imperfection of the network structure, it is expected that the SiO_x film grown without N_2 gas would be inferior to the other one grown with N_2 gas in some electrical properties as an insulator