Takeshi Chigai

FORMATION OF COMPACT AMORPHOUS H2O ICE BY CODEPOSITION OF HYDROGEN ATOMS WITH OXYGEN MOLECULES ON GRAIN SURFACES

Formation of H2O molecules through the codeposition of oxygen molecules and hydrogen atoms is examined in situ using IR spectroscopy at 10-40 K under various O2 and H fluxes. It is found that H2O and H2O2 are continuously formed by reaction, even at 40 K. The H2O ice formed is amorphous, but has a compact (not microporous) structure compared to vapor-deposited amorphous H2O ice, because dangling OH bonds are not observed in the IR spectrum. This is consistent with astronomical observations in molecular clouds and theoretical predictions, which suggest that hydrogenation of O2 is one of the potential routes for reproducing these IR spectral characteristics. The composition of the ice formed by codeposition varies with the O2/H ratio and temperature. Although no data are available at present for the H2O/H2O2 ratio of ice in molecular clouds, this study suggests that hydrogenation of O2 has a potential to yield a H2O/H2O2 ratio of 5 or more in molecular clouds.

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.

Laboratory simulation of competition between hydrogenation and photolysis in the chemical evolution of H2O-CO ice mixtures

Hydrogenation and photolysis of H2O-CO binary ice mixtures at 10-50 K have been revisited in order to quantitatively evaluate their relative importance in the chemical evolution of interstellar dust icy mantles. The dominant product of photolysis was CO2, with lower yields of formaldehyde, methanol, and formic acid, while only formaldehyde and methanol were obtained by hydrogenation reactions. Hydrogenation has higher formation efficiencies and yields of formaldehyde and methanol than photolysis. However, the contribution of photolysis should not be negligible for the formation of these molecules in molecular clouds. The simultaneous irradiation of binary ice mixtures with hydrogen atoms and UV photons resulted in relative abundances of CO2, formaldehyde, methanol, and formic acid that are consistent with the observed abundances. Our results show that the composition and structure of ice are crucial in the chemical evolution of ice mantles, as much as the temperature and the type of irradiation.

New Rate Constants of Hydrogenation of CO on H2O-CO Ice Surfaces

The key route of the formation of solid H2CO and CH3OH on grain surfaces is the hydrogenation reactions. This route was confirmed recently by experiments carried out by Watanabe and his coworkers in 2002 and 2003. On the basis of these experimental results, we derive theoretical formulae and determine the rate constants of the formation of H2CO and CH3OH via the hydrogenation of CO molecules in amorphous H2O-CO ice. In order to reproduce the experimental results, we introduce a new model of the ice called the crack model. We find that the morphology of the ice plays a vital role for the calculations of the rate constants. The rate constants for hydrogenation of CO are k0nH = 0.58-0.52 minute-1 at temperature T = 10 K and 0.23-0.22 minute-1 at T = 15 K, where nH is the number density of hydrogen atoms. For H2CO, the rate constants are k2nH = 0.020-0.013 minute-1 at T = 10 K and 0.075-0.056 minute-1 at T = 15 K. At temperatures 10 and 15 K, the diffusion constant of hydrogen atoms into the amorphous H2O-CO ice is estimated to be 10-21 to 10-20 cm2 s-1, which has little effect on the calculations of the rate constant of the surface hydrogenation. We discuss the deviation of the theoretical results from the experimental ones at late times.

Formation Conditions of Presolar TiC Core-Graphite Mantle Spherules in the Murchison Meteorite

Formation of TiC and graphite grains in the gas outflows from carbon-rich asymptotic giant branch (AGB) stars is investigated to reveal the formation conditions of presolar TiC core-graphite mantle spherules extracted from the Murchison meteorite. We employ the nonequilibrium condensation theory involving chemical reactions in nucleation and grain growth to derive the formation conditions not only from the condensation sequence for realizing the core-mantle structure but also from the sizes of observed TiC cores and graphite mantles. The results of calculations show that the sizes of TiC cores constrain the mass-loss rate and the gas outflow velocity v at the formation sites to satisfy the relation 2.5×10−3<v2L½4/5<0.185, where L4 is the stellar luminosity in units of 104 L☉, v is in km s-1, and 5 is in 10−5 M☉ yr−1. The constraint on the condensation sequence limits the C/O abundance ratio to less than 1.26-1.48 for =10−6 to 10−4 M☉ yr−1. The more stringent constraint on and v at the formation sites expressed by 2.5×10−3<v2L½4/5<1.35×10−2 with <1.26-1.48 is imposed by the conditions necessary for reproducing the range of the observed sizes of TiC cores and graphite mantles simultaneously. The derived total gas pressure in the range of 2×10−3<P<0.1 dyn cm−2 and gas outflow velocity in the range 0.015<v<0.4 km s−1 at the formation sites of TiC cores seem to be consistent with the picture of steady state dust-driven winds around carbon-rich AGB stars. The discrepancy between the observed and the predicted size ratios of graphite mantles to TiC cores would be closely related to the detailed formation process of graphite mantles on TiC cores.

Reaction kinetics and isotope effect of water formation by the surface reaction of solid H2O2 with H atoms at low temperatures

We performed laboratory experiments on the formation of water and its isotopologues by surface reactions of hydrogen peroxide (H2O2) with hydrogen (H) atoms and their deuterated counterparts (D2O2, D) at 10-30 K. High-purity H2O2 (> 95%) was prepared in situ by the codeposition of molecular oxygen and H atoms at relatively high temperatures (45-50 K). We determined that the high-purity H2O2 solid reacts with both H and deuterium (D) atoms at 10-30 K despite the large activation barriers (-2000 K). Moreover, the reaction rate for H atoms is approximately 45 times faster than that for D atoms at 15 K. Thus, the observed large isotope effect indicates that these reactions occurred through quantum tunneling. We propose that the observed HDO/H2O ratio in molecular clouds might be a good tool for the estimation of the atomic D/H ratio in those environments.

Mid-infrared spectra of cometary dust : the evasion of its silicate mineralogy

Infrared spectra of dust in cometary comae provide a way to identify its silicate constituents, and this is crucial for correctly understanding the condition under which our planetary system is formed. Recent studies assign a newly detected peak at a wavelength of 9.3 µm to pyroxenes and regard them as the most abundant silicate minerals in comets. Here we dispense with this pyroxene hypothesis to numerically reproduce the infrared features of cometary dust in the framework of our interstellar dust models. Presolar interstellar dust in a comet is modeled as fluffy aggregates consisting of submicrometer-sized organic grains with an amorphous-silicate core that undergoes nonthermal crystallization in a coma. We assert that forsterite (Mg2SiO4) is the carrier of all the observed features, including the 9.3 µm peak and that the major phase of iron is sulfides rather than iron-rich silicates.

Are TiC Grains a Carrier of the 21 Micron Emission Band Observed around Post-Asymptotic Giant Branch Objects?

The carrier of the 21 μm band observed in post-asymptotic giant branch (post-AGB) stars is examined. We analyze the infrared spectra of the TiC clusters measured by von Helden et al. in 2000 and determine the absorption efficiency Q in the 21 μm band. Using Q, we estimate the Ti/Si abundance ratios needed to realize the flux ratios of the 21 and 11 μm emission observed in the infrared spectra of the post-AGB stars exhibiting both 21 and 11 μm emission. In view of the nature of the TiC condensation by which TiC grains are quickly mantled by graphite, we calculate the emission spectra of the graphite-coated TiC grains and other possible types of core-mantle grains and compare with the observed spectra. Both the abundance and condensation considerations strongly suggest that TiC is an implausible carrier of the observed infrared 21 μm feature around carbon-rich post-AGB stars.

INFRARED SPECTRA OF DUST AGGREGATES IN COMETARY COMAE: CALCULATION WITH OLIVINE FORMED BY EXOTHERMIC CHEMICAL REACTIONS

Mineralogy of cometary dust plays an important role in understanding the formation and evolution of comets, the most primitive objects in the solar system. A correct interpretation of infrared spectra observed for cometary comae is the key to success in identifying mineral constituents of cometary dust. However, the composition, size, and structure of cometary dust might have been misinterpreted in previous studies, owing to a lack of a unique solution in their analyses of infrared spectra. We present a semianalytic method to compute infrared spectra for large aggregate particles consisting of submicrometer size grains inclusive of crystalline minerals. The method is applied to calculate the absorption cross section of primordial interstellar dust that is processed in a cometary coma. The processed interstellar dust is here modeled as clusters of concentrically stratified spheres consisting of an organic refractory outer mantle, an olivine inner mantle, and an amorphous silicate core. Spectral variations in the absorption cross sections for porous aggregates with a forsterite layer exhibit noticeable features at all the wavelengths where mineral features are observed in the infrared spectra of cometary comae. In contrast, the observed infrared spectra of cometary comae show no evidence for the presence of fayalite in cometary dust. Infrared observations of cometary comae are consistent with the picture that cometary nuclei contain primordial interstellar dust as well as interstellar ices.

Kinetic theory of steady chemical nucleation in the gas phase

We develop a kinetic theory of nucleation involving chemical reactions in the gas phase. For the basis of deriving the chemical nucleation rate, chemical kinetic considerations are presented on the steady current density and the eective rate constants of the overall reaction, which is a sum of a sequential elementary reactions. We formulate the steady rate of chemical nucleation in a multi-component vapor, in which nucleation occurs via the chemical reactions yielding a condensate having a stoichiometric composition. An exact expression of the steady nucleation rate is given together with its approximate formulas for practical applications. The present formulation is not concerned with any particular cluster model. The supersaturation ratio for a many-component vapor is dened so as to be a natural extension of that for a one-component vapor. It is shown that the transition probabilities due to growth and decay of the clusters are of the same form as the growth and evaporation rates in a one-component vapor.