Yuki Kimura

Direct Measurements of Hydrogen Atom Diffusion and the Spin Temperature of Nascent H2 Molecule on Amorphous Solid Water

Physicochemical processes (H-atom sticking, diffusion, recombination, and the nuclear spin temperature of nascent H2 molecules) important in the formation of molecular hydrogen have been experimentally investigated on amorphous solid water (ASW). A new type of experiment is performed to shed light on a longstanding dispute. The diffusion rate of H atom is directly measured at 8 K and is found to consist of a fast and a slow component due to the presence of at least two types of potential sites with the energy depths of ~20 and >50 meV, respectively. The fast diffusion at the shallow sites enables efficient H2 formation on interstellar ice dust even at 8 K, while H atoms trapped in the deeper sites hardly migrate. The spin temperature of nascent H2 formed by recombination on ASW has been obtained for the first time and is higher than approximately 200 K. After formation, H2 molecules are trapped and their spin temperature decreases due to the conversion of spin states on ASW.

The Mechanism of Surface Diffusion of H and D Atoms on Amorphous Solid Water: Existence of Various Potential Sites

To understand elementary processes leading to H{sub 2} formation, and the hydrogenation and deuteration reactions of adsorbed species on dust grains in dense clouds, we experimentally investigated the diffusion of atomic hydrogen and deuterium on amorphous solid water (ASW) at temperatures of 8-15 K. The present study extended our previous study for selective detections of H and D atoms, and of H{sub 2} (J = 0 and 1) and D{sub 2} (J = 0 and 1) molecules adsorbed on ASW using both photo-stimulated desorption and resonance-enhanced multiphoton ionization, to investigate potential sites on ASW for diffusion, recombination dynamics, and the diffusion mechanism of H and D atoms. Our results demonstrate that the ASW surface contains various potential sites that can be categorized into at least three groups: very shallow, middle-, and deep-potential sites, with diffusion activation energies of { =}30 meV, respectively. The present study pictured the outline of H{sub 2} formation on cosmic ice dust at low temperatures: H atoms landing on the dust will diffuse rapidly at the abundant shallow and middle sites on ASW, and finally become trapped at deep sites. The H atoms that arrive next recombine with suchmorexa0» trapped H atoms to yield H{sub 2} molecules. The small isotopic difference between the diffusion of H and D atoms on ASW indicates that the diffusion mechanism can be explained by thermal hopping, at least at middle-potential sites.«xa0less

NOVEL ROUTES FOR DIAMOND FORMATION IN INTERSTELLAR ICES AND METEORITIC PARENT BODIES

We have identified new formation routes of diamond in interstellar clouds and parent bodies of carbonaceous chondrites in laboratory experiments. The diamond precursor nucleated under UV photolysis of interstellar ice mixtures in molecular clouds and grew with further UV irradiation in diffuse clouds. The present study supports the occurrence of diamonds in interstellar clouds and suggests that diamond is ubiquitous in space. We also performed experiments on the processes of aqueous alteration and subsequent thermal metamorphism of organic materials formed in molecular clouds, and the results were consistent with the formation of diamonds by this process in the parent bodies of carbonaceous chondrites. The various characteristics of nanodiamonds in chondrites and interplanetary dust are well explained by these new formation routes.

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.

Is the 21-micron Feature Observed in Some Post-AGB Stars Caused by the Interaction Between Ti Atoms and Fullerenes?

Recent measurements of fullerenes and Ti atoms recorded in our laboratory have demonstrated the presence of an infrared feature near 21 μm. The feature observed has nearly the same shape and position as is observed for one of the most enigmatic features in post-asymptotic giant branch (AGB) stars. In our experimental system, large-cage carbon particles, such as large fullerenes, were produced from CO gas by the Boudouard reaction. Large-cage carbon particles intermixed with Ti atoms were produced by the evaporation of a Ti-metal-wrapped carbon electrode in CO gas. The infrared spectra of large fullerenes interacting with Ti atoms show a characteristic feature at 20.3 μm that closely corresponds to the 20.1 μm feature observed in post-AGB stars. Both the laboratory and stellar spectra also show a small but significant peak at 19.0 μm, which is attributed to fullerenes. Here we propose that the interaction between fullerenes and Ti atoms may be a plausible explanation for the 21 μm feature seen in some post-AGB stars.

The Formation of Graphite Whiskers in the Primitive Solar Nebula

It has been suggested that carbonaceous grains are efficiently destroyed in the interstellar medium and must either reform in situ at very low pressures and temperatures or in an alternative environment more conducive to grain growth. Graphite whiskers have been discovered associated with high-temperature phases in meteorites such as calcium aluminum inclusions and chondrules, and it has been suggested that the expulsion of such material from protostellar nebulae could significantly affect the optical properties of the average interstellar grain population. We have experimentally studied the potential for Fischer-Tropsch and Haber-Bosch type reactions to produce organic materials in protostellar systems from the abundant H{sub 2}, CO, and N{sub 2} reacting on the surfaces of available silicate grains. When graphite grains are repeatedly exposed to H{sub 2}, CO, and N{sub 2} at 875 K abundant graphite whiskers are observed to form on or from the surfaces of the graphite grains. In a dense, turbulent nebula, such extended whiskers are very likely to be broken off, and fragments could be ejected either in polar jets or by photon pressure after transport to the outer reaches of the nebula.

What is the driving force to form refractory oxide grains? Silicate spectra depend on their formation environment

We discuss room-temperature condensation experiments using either an electrical discharge or ultraviolet radiation to initiate gas-phase reactions resulting in silicate smokes. This formation process could represent processes occurring in low-density environments, because it is possible that the gases, which can condense at higher temperatures, remain after it cools, e.g., following a shock. In these environments, many condensates could be formed simultaneously. However, in the case of our iron silicate experiments, many of the smoke particles are iron silicate with a uniform composition that reflects the composition of the ambient gas atmosphere where they were produced. In these experiments, smoke particles of other materials such as iron oxide and silica were not formed. In the case of our Si–O experiments, hydro-silicate smoke particles are produced together with anhydrous silicate particles directly from the gas phase without later hydro alteration. The infrared spectra of these silicate particles show a very strong 11.36 μm feature attributed to H2Si2O4 and possibly to Si2O3 compared with a simultaneously observed 9.2 μm feature due to the Si–O vibration. We believe that finding the driving force for grain growth under a wide range of environmental conditions is important if we are to understand grain formation, because silicate grains, which formed in a plasma field or under UV irradiation, show different compositions, structures, shapes, and spectra from thermally condensed grains.

Laboratory Synthesized Calcium Oxide and Calcium Hydroxide Grains: A Candidate to Explain the 6.8 μm Band

We demonstrate that CaO and Ca(OH)2 are excellent candidates to explain the 6.8 μm feature, which is one of the most obscure features in young stellar objects. We discuss the condensation of CaO grains and the potential formation of a Ca(OH)2 surface layer. The infrared spectra of these grains are compared with the spectra of 15 young stellar objects. We note that CaO-rich grains are seen in all meteoritic CAIs (calcium-aluminum-rich inclusions) and that the 6.8 μm feature has only been observed in young stellar objects. Therefore, we consider CaO grains to be a plausible candidate to explain the 6.8 μm feature and hypothesize that they are produced in the hot interiors of young stellar environments.

Phenomena of Nanoparticles in Relation to the Solar System

The physical properties of nanometer-sized particles are strongly affected by their surface due to larger surface to volume ratio and very different from bulk materials. For example, the melting point decreases as small as the half value and the diffusion coefficients become as large as nine orders. Surface free energy considerations can be as important or even more important than bulk thermodynamic quantities. Because the diameters of newly formed dust particles are of the order of nanometers, their physical properties, which induce a fusion growth as nanoparticles never occur in bulk, must be considered when considering the formation of cosmic dust. In this chapter, we exhibit the properties of nanoparticles that have a great potential for understanding phenomena in the current solar system and its evolution.

A SEED OF SOLAR FORSTERITE AND POSSIBLE NEW EVOLUTIONAL SCENARIO OF COSMIC SILICATES

Laboratory experiments suggest that magnesium silicide (Mg2Si) grains could be produced in the hydrogen dominant gas outflow from evolved stars in addition to amorphous oxide minerals. If the magnesium silicide grains were incorporated into the primitive solar nebula, the magnesium silicide would easily become forsterite (Mg2SiO4) by oxidation as it reacted with the relatively oxygen-rich, solar composition gas. This hypothesis can explain the existence of abundant forsterite grains with solar oxygen composition in meteorites, i.e., magnesium silicide could be the precursor of much of the forsterite found in our solar system. In addition, if a significant fraction of the solar forsterite is derived from magnesium silicide, it could explain the apparent low abundance of presolar forsterite. Furthermore, the lower degree of crystallinity observed in silicates formed in outflows of lower mass-loss-rate stars might be caused by the formation of magnesium silicide in this relatively hydrogen-rich environment.