T.-C. Peng¹, E. M. L. Humphreys¹, L. Testi^1,2,3, A. Baudry^4,5, M. Wittkowski¹, M. G. Rawlings⁶, I. de Gregorio-Monsalvo^1,8, W. Vlemmings⁷, L.-A. Nyman⁸, M. D. Gray⁹ and C. de Breuck¹

¹ESO Garching, Karl-Schwarzschild Str. 2 85748 Garching, Germany
²Excellence Cluster Universe, Boltzmannstr. 2, 85748 Garching, Germany
³INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁴Univ. Bordeaux, LAB, UMR 5804, 33270 Floirac, France
⁵CNRS, LAB, UMR 5804, 33270 Floirac, France
⁶National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
⁷Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
⁸Joint ALMA Observatory (JAO) and European Southern Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile
⁹JBCA, Alan Turing Building, School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK

Aims. Galactic chemical evolution (GCE) is important for understanding the composition of the present-day interstellar medium (ISM) and of our solar system. In this paper, we aim to track the GCE by using the ²⁹Si/³⁰Si ratios in evolved stars and tentatively relate this to presolar grain composition.
Methods. We used the APEX telescope to detect thermal SiO isotopologue emission toward four oxygen-rich M-type stars. Together with the data retrieved from the Herschelscience archive and from the literature, we were able to obtain the ²⁹Si/³⁰Si ratios for a total of 15 evolved stars inferred from their optically thin ²⁹SiO and ³⁰SiO emission. These stars cover a range of masses and ages, and because they do not significantly alter ²⁹Si/³⁰Si during their lifetimes, they provide excellent probes of the ISM metallicity (or ²⁹Si/³⁰Si ratio) as a function of time.
Results. The ²⁹Si/³⁰Si ratios inferred from the thermal SiO emission tend to be lower toward low-mass oxygen-rich stars (e.g., down to about unity for W Hya), and close to an interstellar or solar value of 1.5 for the higher-mass carbon star IRC+10216 and two red supergiants. There is a tentative correlation between the ²⁹Si/³⁰Si ratios and the mass-loss rates of evolved stars, where we take the mass-loss rate as a proxy for the initial stellar mass or current stellar age. This is consistent with the different abundance ratios found in presolar grains. Before the formation of the Sun, the presolar grains indicate that the bulk of presolar grains already had ²⁹Si/³⁰Si ratios of about 1.5, which is also the ratio we found for the objects younger than the Sun, such as VY CMa and IRC+10216. However, we found that older objects (up to possibly 10 Gyr old) in our sample trace a previous, lower ²⁹Si/³⁰Si value of about 1. Material with this isotopic ratio is present in two subclasses of presolar grains, providing independent evidence of the lower ratio. Therefore, the ²⁹Si/³⁰Si ratio derived from the SiO emission of evolved stars is a useful diagnostic tool for the study of the GCE and presolar grains.

Reference
Peng T-C, Humphreys EML, Testi L, Baudry A, Wittkowski M Rawlings, MG, de Gregorio-Monsalvo I, Vlemmings W, Nyman L-A, Gray MD and de Breuck C (2013) Silicon isotopic abundance toward evolved stars and its application for presolar grains. Astronomy & Astrophysics 559:L8.
[doi:10.1051/0004-6361/201322466]
Reproduced with permission © ESO

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Katherine R. Compaan^1,2, Jay Agarwal¹, Bryson E. Dye¹, Yukio Yamaguchi¹ and Henry F. Schaefer, III¹

¹Center for Computational Quantum Chemistry, University of Georgia, Athens, GA 30602, USA
²Research Focus Area for Chemical Resource Beneficiation, North-West University, Hoffman Street, Potchefstroom 2520, South Africa

Highly reliable molecular properties have been computed for AlCH₂ and AlCH₂⁺ at the CCSD(T)/cc-pwCVQZ level of theory. These simple aluminum species are relevant to the chemistry of the interstellar medium (ISM) and circumstellar medium, and are proposed to form in regions where aluminum and methylene exist in appreciable concentrations because AlCH₂ and AlCH₂⁺ are thermodynamically stable with respect to dissociation. Herein, dissociation energies were computed for both species through extrapolation to the complete basis set limit using focal point analysis. For the neutral species, 86 kcal mol^-1 is required to heterolytically cleave the Al–C single bond, while the cationic species requires 38 kcal mol^-1. To aid in identification within the ISM, the anharmonic (fundamental) frequencies, spectroscopic constants, and vibrationally averaged properties for AlCH₂ and the corresponding cation are also reported.

Reference
Compaan KR, Agarwal J, Dye BE, Yamaguchi Y and Schaefer, III HF (2013) Toward Detection of AlCH2 and AlCH2+ in the Interstellar Medium. The Astrophysical Journal 778:125.
[doi:10.1088/0004-637X/778/2/125]

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Robin T. Garrod

Center for Radiophysics and Space Research, Cornell University, Ithaca, NY 14853-6801, USA

The first off-lattice Monte Carlo kinetics model of interstellar dust grain surface chemistry is presented. The positions of all surface particles are determined explicitly, according to the local potential minima resulting from the pair-wise interactions of contiguous atoms and molecules, rather than by a pre-defined lattice structure. The model is capable of simulating chemical kinetics on any arbitrary dust grain morphology, as determined by the user-defined positions of each individual dust grain atom. A simple method is devised for the determination of the most likely diffusion pathways and their associated energy barriers for surface species. The model is applied to a small, idealized dust grain, adopting various gas densities and using a small chemical network. Hydrogen and oxygen atoms accrete onto the grain to produce H₂O, H₂, O₂, and H₂O₂. The off-lattice method allows the ice structure to evolve freely; the ice mantle porosity is found to be dependent on the gas density, which controls the accretion rate. A gas density of 2 × 10⁴cm^-3, appropriate for dark interstellar clouds, is found to produce a fairly smooth and non-porous ice mantle. At all densities, H₂ molecules formed on the grains collect within the crevices that divide nodules of ice and within micropores (whose extreme inward curvature produces strong local potential minima). The larger pores produced in the high-density models are not typically filled with H₂. Direct deposition of water molecules onto the grain indicates that amorphous ices formed in this way may be significantly more porous than interstellar ices that are formed by surface chemistry.

Reference
Garrod RT (2013) Three-dimensional, Off-lattice Monte Carlo Kinetics Simulations of Interstellar Grain Chemistry and Ice Structure. The Astrophysical Journal 778:158.
[doi:10.1088/0004-637X/778/2/158]

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Refers To

Cyrena A. Goodrich, Richard D. Ash, James A. Van Orman, Kenneth Domanik, William F. McDonough
Metallic phases and siderophile elements in main group ureilites: Implications for ureilite petrogenesis
Geochimica et Cosmochimica Acta, Volume 112, 1 July 2013, Pages 340-373

The publisher regrets the article is incorrectly labelled as “Response” when it is a full length research article. The publisher would like to apologise for any inconvenience caused.

Christoph Loesche¹, Gerhard Wurm¹, Jens Teiser¹, Jon M. Friedrich^2,3, and Addi Bischoff⁴

¹Faculty of Physics, University of Duisburg-Essen, Lotharstr. 1, D-47057 Duisburg, Germany
²Department of Chemistry, Fordham University, Bronx, NY 10458, USA
³Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024, USA
⁴Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, D-48149 Münster, Germany

Photophoresis is a physical process that transports particles in optically thin parts of protoplanetary disks, especially at the inner edge and at the optical surface. To model the transport and resulting effects in detail, it is necessary to quantify the strength of photophoresis for different particle classes as a fundamental input. Here, we explore photophoresis for a set of chondrules. The composition and surface morphology of these chondrules were measured by X-ray tomography. Based on the three-dimensional models, heat transfer through illuminated chondrules was calculated. The resulting surface temperature map was then used to calculate the photophoretic strength. We found that irregularities in particle shape and variations in composition induce variations in the photophoretic force. These depend on the orientation of a particle with respect to the light source. The variation of the absolute value of the photophoretic force on average over all chondrules is 4.17%. The deviation between the direction of the photophoretic force and illumination is 30 ± 15. The average photophoretic force can be well approximated and calculated analytically assuming a homogeneous sphere with a volume equivalent mean radius and an effective thermal conductivity. We found an analytic expression for the effective thermal conductivity. The expression depends on the two main phases of a chondrule and decreases with the amount of fine-grained devitrified, plagioclase-normative mesostasis up to factor of three. For the chondrule sample studied (Bjurböle chondrite), we found a dependence of the photophoretic force on chondrule size.

Reference
Loesche C, Wurm G, Teiser J, Friedrich JM and Bischoff A (2013) Photophoretic Strength on Chondrules. 1. Modeling. The Astrophysical Journal 778:2.
[doi:10.1088/0004-637X/778/2/101]

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Jiří Borovička¹, Pavel Spurný¹, Peter Brown^2,3, Paul Wiegert^2,3, Pavel Kalenda⁴, David Clark^2,3 and Lukáš Shrbený¹

¹Astronomical Institute, Academy of Sciences of the Czech Republic, CZ-251 65 Ondřejov, Czech Republic
²Department of Physics and Astronomy, University of Western Ontario, London, Ontario N6A 3K7, Canada
³Centre for Planetary Science and Exploration, University of Western Ontario, London, Ontario N6A 5B7, Canada
⁴Institute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic, V Holešovičkách 41, CZ-18209 Praha 8, Czech Republic

We currently seek a copyright agreement with Nature Geoscience to display abstracts of their cosmochemistry related publications.

Reference
Borovička J, Spurný P, Brown P, Wiegert P, Kalenda P, Clark D and Shrbený L (2013) The trajectory, structure and origin of the Chelyabinsk asteroidal impactor. Nature 503:235–237.
[doi:10.1038/nature12671]

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Brown et al. (>>10)*
*Find the extensive, full author and affiliation list on the publishers website.

We currently seek a copyright agreement with Nature Geoscience to display abstracts of their cosmochemistry related publications.

Reference
Brown et al. (2013) A 500-kiloton airburst over Chelyabinsk and an enhanced hazard from small impactors. Nature 503:238–241.
[doi:10.1038/nature12741]

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M. Minissale¹, E. Congiu¹, G. Manicò², V. Pirronello² and F. Dulieu¹

¹LERMA, UMR8112 du CNRS, de l’Observatoire de Paris et de l’Université de Cergy Pontoise, 5 mail Gay Lussac, 95000 Cergy Pontoise Cedex, France
²Dipartimento di Fisica ed Astronomia, Universitá degli Studi di Catania, via Santa Sofia 64, 95123 Catania, Italy

Context. The formation of carbon dioxide in quiescent regions of molecular clouds has not yet been fully understood, even though CO₂ is one of the most abundant species in interstellar ices.
Aims. CO₂ formation is studied via oxidation of CO molecules on cold surfaces under conditions close to those encountered in quiescent molecular clouds.
Methods. Carbon monoxide and oxygen atoms are codeposited using two differentially pumped beam lines on two different surfaces (amorphous water ice or oxydized graphite) held at given temperatures between 10 and 60 K. The products are probed via mass spectroscopy by using the temperature-programmed desorption technique.
Results. We show that the reaction CO + O can form carbon dioxide in solid phase with an efficiency that depends on the temperature of the surface. The activation barrier for the reaction, based on modelling results, is estimated to be in the range of 780−475 K/k_b. Our model also allows us to distinguish the mechanisms (Eley Rideal or Langmuir-Hinshelwood) at play in different temperature regimes. Our results suggest that competition between CO₂ formation via CO + O and other surface reactions of O is a key factor in the yields of CO₂ obtained experimentally.
Conclusions. CO₂ can be formed by the CO + O reaction on cold surfaces via processes that mimic carbon dioxide formation in the interstellar medium. Astrophysically, the presence of CO₂ in quiescent molecular clouds could be explained by the reaction CO + O occurring on interstellar dust grains.

Reference
Minissale M, Congiu E, Manicò G, Pirronello V and Dulieu F (2013) CO2 formation on interstellar dust grains: a detailed study of the barrier of the CO + O channel. Astronomy & Astrophysics 559:A49.
[doi:10.1051/0004-6361/201321453]
Reproduced with permission © ESO

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Dirk Elbeshausen^1,*, Kai Wünnemann¹, Gareth S. Collins²

¹Museum für Naturkunde, Leibniz-Institute for Research on Evolution and Biodiversity, Berlin, Germany
²Impacts and Astromaterials Research Centre, Department of Earth Science and Engineering, Imperial College London, London, UK

Elliptical impact craters are rare among the generally symmetric shape of impact structures on planetary surfaces. Nevertheless, a better understanding of the formation of these craters may significantly contribute to our overall understanding of hypervelocity impact cratering. The existence of elliptical craters raises a number of questions: Why do some impacts result in a circular crater whereas others form elliptical shapes? What conditions promote the formation of elliptical craters? How does the formation of elliptical craters differ from those of circular craters? Is the formation process comparable to those of elliptical craters formed at subsonic speeds? How does crater formation work at the transition from circular to elliptical craters? By conducting more than 800 three-dimensional (3-D) hydrocode simulations, we have investigated these questions in a quantitative manner. We show that the threshold angle for elliptical crater generation depends on cratering efficiency. We have analyzed and quantified the influence of projectile size and material strength (cohesion and coefficient of internal friction) independently from each other. We show that elliptical craters are formed by shock-induced excavation, the same process that forms circular craters and reveal that the transition from circular to elliptical craters is characterized by the dominance of two processes: A directed and momentum-controlled energy transfer in the beginning and a subsequent symmetric, nearly instantaneous energy release.

Reference
Elbeshausen D, Wünnemann K and Collins GS (in press) The transition from circular to elliptical impact craters. Journal of Geophysical Research – Planets
[doi:10.1002/2013JE004477]
Published by arrangement with John Wiley & Sons

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Sherry K. Fieber-Beyer^* and Michael J. Gaffey

Department of Space Studies, University Stop 9008, University of North Dakota, 58202

The 3:1 Kirkwood gap asteroids are a mineralogically diverse set of asteroids located in a region that delivers meteoroids into Earth-crossing orbits. Mineralogical characterizations of asteroids in/near the 3:1 Kirkwood Gap can be used as a tool to “map” conditions and processes in the early solar system. The chronological studies of the meteorite types provide a “clock” for the relative timing of those events and processes. By identifying the source asteroids of particular meteorite types, the “map” and “clock” can be combined to provide a much more sophisticated understanding of the history and evolution of the late solar nebula and the early solar system.
A mineralogical assessment of twelve 3:1 Kirkwood Gap asteroids has been carried out using near-infrared spectral data obtained from 2010-2011 combined with visible spectral data (when available) to cover the spectral interval of 0.4 – 2.5 μm. Eight of these asteroids have surfaces with basaltic-type silicate assemblages, indicating at least partial melting within their parent bodies. Although HED-like mineralogies are present these objects exhibit subdued features indicating the presence of an additional phase (e.g., NiFe metal) or process (e.g., space weathering). Four of these asteroids appear to be ordinary chondrite assemblages. Three of these are plausibly linked to the probable H-chondrite parent body, (6) Hebe.

Reference
Fieber-Beyer SK and Gaffey MJ (in press) Near-infrared Spectroscopy of 3:1 Kirkwood Gap Asteroids II: Probable and Plausible Parent Bodies; Primitive and Differentiated. Icarus
[doi:10.1016/j.icarus.2013.11.001]
Copyright Elsevier

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