¹Jinping Hu,¹Thomas G. Sharp
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.07.018]
¹School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA
Copyright Elsevier

Post-shock annealing of meteorites can destroy their shock-induced features, particularly high-pressure minerals, and complicate the estimation of impact pressure-temperature conditions. However, distinguishing post-shock annealing features from thermal metamorphism effects can be practically difficult. Here we report results from Mbale, a highly shocked L chondrite, to investigate the mechanisms, kinetics and identification criteria for post-shock annealing of high-pressure signatures. Olivine fragments within shock-melt veins in Mbale occur as chemically heterogeneous nanocrystalline aggregates that contain trace wadsleyite and ringwoodite. Their strong variation in fayalite content provides evidence of iron partitioning during transformation of olivine to wadsleyite, followed by back-transformation to olivine after decompression. Experimental studies of transformation kinetics show that wadsleyite transforms to olivine in seconds at temperatures above ∼1200 K and in hours at temperatures between 900 and 1200 K. Thermal models of shock-melt cooling show that shock veins in Mbale cooled to 1200 K in 1 s. The shock pulse must have been shorter than ∼1 s to provide the high temperature conditions for post-shock back-transformation of wadsleyite. Many highly shocked L chondrites, which have abundant high-pressure minerals, must have experienced relatively long shock durations combined with rapid cooling of shock-melt regions to preserve high-pressure phases. The most highly shocked samples, such as impact melt breccias, lack high-pressure phases because of post-shock back-transformations.

¹D.Perna et al. (>10)*
Astronomy & Astrophysics 599, L1 Link to Article [https://doi.org/10.1051/0004-6361/201630346]
¹LESIA – Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
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¹Yazhou Yang (杨亚洲), ^1,2Hao Zhang (张昊), ¹Ziwei Wang (王紫薇), ¹Ye Yuan (袁野), ³Shaolin Li (李少林), ³Weibiao Hsu (徐伟彪), ⁴Chujian Liu (刘初见)
Astronomy & Astrophysics 597, A50 Link to Article [https://doi.org/10.1051/0004-6361/201629327]
¹Planetary Science Institute, School of Earth Sciences, China University of Geosciences, Wuhan 430074, PR China
e-mail: um_zhanghao@yahoo.com; yangyazhou1@gmail.com
²Key Laboratory of Spectral Imaging Technology, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi’an 710119, PR China
³Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, PR China
⁴State Key Laboratory of Geological Process and Mineral Resources, China University of Geosciences, Wuhan, PR China

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¹S. Antonellini, ¹J. Bremer, ¹I. Kamp, ²P. Riviere-Marichalar, ^1,3F. Lahuis, ⁴W.-F. Thi, ⁵P. Woitke, ⁶R. Meijerink, ^1,7G. Aresu, ¹M. Spaans
Astronomy&Astrophysics 597, A72 Link to Article [https://doi.org/10.1051/0004-6361/201527820]
¹Kapteyn Astronomical Institute, Postbus 800, 9700 AV Groningen, The Netherlands
e-mail: antonellini@astro.rug.nl
²Centro de Astrobiología (INTA-CSIC) – Depto. Astrofísica, POB 78, ESAC Campus, 28691 Villanueva de la Cañada, Spain
³SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
⁴Max-Planck-Institut für extraterrestrische Physisk, Giessenbachstrasse 1, 85748 Garching, Germany
⁵St. Andrews University, School of Physics and Astronomy, St. Andrews KY16 9SS, UK
⁶Leiden Observatory, Leiden University, PO Box, 2300 RA Leiden, The Netherlands
⁷INAF–Osservatorio Astronomico di Cagliari, via della Scienza 5, 09047 Selargius, Italy

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¹Nan Liu, ¹Larry R. Nittler, ^2,3Marco Pignatari, ¹Conel M. O’D. Alexander, ¹Jianhua Wang
The Astrophysical Journal Letters 842 L1 Link to Article [https://doi.org/10.3847/2041-8213/aa74e5]
¹Department of Terrestrial Magnetism, Carnegie Institution for Science, Washington, DC 20015, USA
²E. A. Milne Centre for Astrophysics, Department of Physics & Mathematics, University of Hull, Hull HU6 7RX, UK
³NuGrid collaboration, http://www.nugridstars.org.

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¹Simone Gerber, ¹Christoph Burkhardt, ¹Gerrit Budde, ¹Knut Metzler, ¹Thorsten Kleine
The Astrophysical Journal Letters 841, L17 Link to Article [https://doi.org/10.3847/2041-8213/aa72a2]
¹Institut für Planetologie, University of Münster, Wilhelm Klemm-Straße 10, D-48149 Münster, Germany

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^1,2Alexis Bouquet, ²Christopher R. Glein, ²Danielle Wyrick, ^1,2J. Hunter Waite
The Astrophysical Journal Letters 840 L8, Link to Article [https://doi.org/10.3847/2041-8213/aa6d56]
¹Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, TX, USA
²Southwest Research Institute, Space Science and Engineering Division, San Antonio, TX, USA

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¹S. Xu (许偲艺), ²B. Zuckerman, ³P. Dufour, ⁴E. D. Young, ²B. Klein, ²M. Jura
The Astrophysical Journal 836, L7 Link to Article [https://doi.org/10.3847/2041-8213/836/1/L7]
¹European Southern Observatory, Karl-Schwarzschild-Straße 2, D-85748 Garching, Germany
²Department of Physics and Astronomy, University of California, Los Angeles, CA 90095-1562, USA
³Institut de Recherche sur les Exoplanètes (iREx) and Département de physique, Université de Montréal, Montréal, QC H3C 3J7, Canada
⁴Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095, USA

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¹R.A.Arneson et al. (>10)*
The Astrophysical Journal 843, 51 Link to Article [https://doi.org/10.3847/1538-4357/aa75cf]
¹Minnesota Institute for Astrophysics, School of Physics and Astronomy, University of Minnesota, 106 Pleasant Street S.E., Minneapolis, MN 55455, USA
*Find the extensive, full author and affiliation list on the publishers website

We present a SOFIA FORCAST grism spectroscopic survey to examine the mineralogy of the circumstellar dust in a sample of post-asymptotic giant branch (post-AGB) yellow supergiants that are believed to be the precursors of planetary nebulae. Our mineralogical model of each star indicates the presence of both carbon-rich and oxygen-rich dust species—contrary to simple dredge-up models—with a majority of the dust in the form of amorphous carbon and graphite. The oxygen-rich dust is primarily in the form of amorphous silicates. The spectra do not exhibit any prominent crystalline silicate emission features. For most of the systems, our analysis suggests that the grains are relatively large and have undergone significant processing, supporting the hypothesis that the dust is confined to a Keplerian disk and that we are viewing the heavily processed, central regions of the disk from a nearly face-on orientation. These results help to determine the physical properties of the post-AGB circumstellar environment and to constrain models of post-AGB mass loss and planetary nebula formation.

^1,4David Gobrecht, ¹Sergio Cristallo, ¹Luciano Piersanti, ^2,3Stefan T. Bromley
The Astrophysical Journal 840, 2 Link to Article [https://doi.org/10.3847/1538-4357/aa6db0]
¹Osservatorio Astronomico di Teramo, INAF, I-64100 Teramo, Italy
²Departament de Cincia de Materials i Química Fisica and Institut de Química Terica i Computacional (IQTCUB),Universitat de Barcelona, E-08028 Barcelona, Spain
³Institucio Catalana de Recerca i Estudis Avancats (ICREA), E-08010 Barcelona, Spain
⁴Instituut voor Sterrenkunde, Celestijnenlaan 200 D, B-3001 Heverlee (Leuven), Belgium

Silicon carbide (SiC) grains are a major dust component in carbon-rich asymptotic giant branch stars. However, the formation pathways of these grains are not fully understood. We calculate ground states and energetically low-lying structures of (SiC) n , n = 1, 16 clusters by means of simulated annealing and Monte Carlo simulations of seed structures and subsequent quantum-mechanical calculations on the density functional level of theory. We derive the infrared (IR) spectra of these clusters and compare the IR signatures to observational and laboratory data. According to energetic considerations, we evaluate the viability of SiC cluster growth at several densities and temperatures, characterizing various locations and evolutionary states in circumstellar envelopes. We discover new, energetically low-lying structures for Si4C4, Si5C5, Si15C15, and Si16C16 and new ground states for Si10C10 and Si15C15. The clusters with carbon-segregated substructures tend to be more stable by 4–9 eV than their bulk-like isomers with alternating Si–C bonds. However, we find ground states with cage geometries resembling buckminsterfullerens (“bucky-like”) for Si12C12 and Si16C16 and low-lying stable cage structures for n ≥ 12. The latter findings thus indicate a regime of cluster sizes that differ from small clusters as well as from large-scale crystals. Thus—and owing to their stability and geometry—the latter clusters may mark a transition from a quantum-confined cluster regime to a crystalline, solid bulk-material. The calculated vibrational IR spectra of the ground-state SiC clusters show significant emission. They include the 10–13 μm wavelength range and the 11.3 μm feature inferred from laboratory measurements and observations, respectively, although the overall intensities are rather low.