Formation of hydrogen peroxide and water from the reaction of cold hydrogen atoms with solid oxygen at 10K

The reactions of cold H atoms with solid O2 molecules were investigated at 10 K. The formation of H2O2 and H2O has been confirmed by in-situ infrared spectroscopy. We found that the reaction proceeds very efficiently and obtained the effective reaction rates. This is the first clear experimental evidence of the formation of water molecules under conditions mimicking those found in cold interstellar molecular clouds. Based on the experimental results, we discuss the reaction mechanism and astrophysical implications.Comment: 12 pages, 3 Postscript figures, use package amsmath, amssymb, graphic

The 21 micron and 30 micron circumstellar dust features in evolved C-rich objects

The 21micron and 30micron bands are the strongest dust emission features detected in evolved low- and intermediate-mass C-rich stars (i.e. asymptotic giant branch [AGB] stars, proto-planetary nebulae [PPN], and planetary nebulae [PN]). While the 21micron feature is rare and exists only in the transient PPN phase, the 30micron feature is more common and seen in the entire late stage of stellar evolution, from AGB to PPN and PN phases, as well as in the low-metallicity galaxies: the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC). The carriers of these features remain unidentified. Eleven of the twelve well-identified 21micron sources also emit in the 30micron band, suggesting that their carriers may be somewhat related.Comment: 7 pages, 1 figure, uses eps.cls. Accepted for publication in "Earth, Planets and Space" (special issue on Cosmic Dust

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 effective 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 defined 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

Heterogeneous condensation of presolar titanium carbide core-graphite mantle spherules

We investigate heterogeneous nucleation and growth of graphite on precondensed TiC grains in the gas outflows from carbon-rich asymptotic giant branch (AGB) stars employing a newly-derived heterogeneous nucleation rate taking into account of the chemical reactions at condensation. Competition between heterogeneous and homogeneous nucleations and growths of graphite is investigated to reveal the formation conditions of the TiC core-graphite mantle spherules found in the Murchison meteorite. It is shown that no homogeneous graphite grain condenses whenever TiC condenses prior to graphite in the plausible ranges of the stellar parameters. Heterogeneous condensation of graphite occurs on the surfaces of growing TiC grains, and prevents the TiC cores from reaching the sizes realized if all available Ti atoms were incorporated into TiC grains. The physical conditions at the formation sites of the TiC core-graphite mantle spherules observed in the Murchison meteorite are expressed by the relation 0.2 < v0.1 (M5 / zeta)^(-1/2) L4^(1/4) < 0.7, where v0.1 is the gas outflow velocity at the formation site in units of 0.1 km s^(-1), M^(-5), the mass loss rate in 10^(-5) M solar year^(-1), L4 the stellar luminosity in 10^4 L(solar), and M / zeta is the effective mass loss rate taking account of non-spherical symmetry of the gas outflows. The total gas pressures Pc at the formation sites for the effective mass loss rates M / zeta = 10^(-5) - 10^(-3) M solar year-1 correspond to 0.01 < Pc < 0.9 dyn cm-2, implying that the observed TiC core-graphite mantle spherules are formed not only at the superwind stage but also at the earlier stage of low mass loss rates. The constraint on the degrees C/O abundance ratio, 1 < eta which is less than or approximately equal to 1.03, is imposed to reproduce the observed sizes of the TiC cores. The derived upper limit of the degrees C/O ratio is lower than the values estimated from the calculations without taking into account of heterogeneous condensation of graphite, and is close to the lower end of the degrees C/O ratios inferred from the astronomical observations of carbon-rich AGB stars. Brief discussion is given on other types of graphite spherules.The Meteoritics & Planetary Science archives are made available by the Meteoritical Society and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202

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

Evolution of Morphological and Physical Properties of Laboratory Interstellar Organic Residues with Ultraviolet Irradiation

Refractory organic compounds formed in molecular clouds are among the building blocks of the solar system objects and could be the precursors of organic matter found in primitive meteorites and cometary materials. However, little is known about the evolutionary pathways of molecular cloud organics from dense molecular clouds to planetary systems. In this study, we focus on the evolution of the morphological and viscoelastic properties of molecular cloud refractory organic matter. We found that the organic residue, experimentally synthesized at ~10 K from UV-irradiated H2O-CH3OH-NH3 ice, changed significantly in terms of its nanometer- to micrometer-scale morphology and viscoelastic properties after UV irradiation at room temperature. The dose of this irradiation was equivalent to that experienced after short residence in diffuse clouds (≤104 years) or irradiation in outer protoplanetary disks. The irradiated organic residues became highly porous and more rigid and formed amorphous nanospherules. These nanospherules are morphologically similar to organic nanoglobules observed in the least-altered chondrites, chondritic porous interplanetary dust particles, and cometary samples, suggesting that irradiation of refractory organics could be a possible formation pathway for such nanoglobules. The storage modulus (elasticity) of photo-irradiated organic residues is ~100 MPa irrespective of vibrational frequency, a value that is lower than the storage moduli of minerals and ice. Dust grains coated with such irradiated organics would therefore stick together efficiently, but growth to larger grains might be suppressed due to an increase in aggregate brittleness caused by the strong connections between grains