Alexey Potapov¹, Cornelia Jäger¹, Thomas Henning², Mindaugas Jonusas^3,4, and Lahouari Krim^3,4
Astrophysical Journal 846, 131 Link to Article [https://doi.org/10.3847/1538-4357/aa85e8]
¹Laboratory Astrophysics Group of the Max Planck Institute for Astronomy at the Friedrich Schiller University Jena, Institute of Solid State Physics, Helmholtzweg 3, D-07743 Jena, Germany
²Max Planck Institute for Astronomy, Königstuhl 17, D-69117 Heidelberg, Germany
³Department of Chemistry, Sorbonne Universités, UPMC Univ Paris 06, UMR 8233, MONARIS, Paris F-75005, France
⁴CNRS, UMR 8233, MONARIS, Paris F-75005, France

An understanding of possible scenarios for the formation of astrophysically relevant molecules, particularly complex organic molecules, will bring us one step closer to the understanding of our astrochemical heritage. In this context, formaldehyde is an important molecule as a precursor of methanol, which in turn is a starting point for the formation of more complex organic species. In the present experiments, for the first time, following the synthesis of CO, formaldehyde has been produced on the surface of interstellar grain analogs, hydrogenated fullerene-like carbon grains, by O and H atom bombardment. The formation of H₂CO is an indication for a possible methanol formation route in such systems.

O. Mousis¹ et al. (>10)
The Astrophysical Journal Letters 839 L4 Link to Article [https://doi.org/10.3847/2041-8213/aa6839]
¹Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, F-13388, Marseille, France

Because of the high fraction of refractory material present in comets, the heat produced by the radiogenic decay of elements such as aluminum and iron can be high enough to induce the loss of ultravolatile species such as nitrogen, argon, or carbon monoxide during their accretion phase in the protosolar nebula (PSN). Here, we investigate how heat generated by the radioactive decay of ²⁶Al and ⁶⁰Fe influences the formation of comet 67P/Churyumov–Gerasimenko, as a function of its accretion time and the size of its parent body. We use an existing thermal evolution model that includes various phase transitions, heat transfer in the ice-dust matrix, and gas diffusion throughout the porous material, based on thermodynamic parameters derived from Rosetta observations. Two possibilities are considered: either, to account for its bilobate shape, 67P/Churyumov–Gerasimenko was assembled from two primordial ~2 km sized planetesimals, or it resulted from the disruption of a larger parent body with a size corresponding to that of comet Hale–Bopp (~70 km). To fully preserve its volatile content, we find that either 67P/Churyumov–Gerasimenko’s formation was delayed between ~2.2 and 7.7 Myr after that of Ca–Al-rich Inclusions in the PSN or the comet’s accretion phase took place over the entire time interval, depending on the primordial size of its parent body and the composition of the icy material considered. Our calculations suggest that the formation of 67P/Churyumov–Gerasimenko is consistent with both its accretion from primordial building blocks formed in the nebula or from debris issued from the disruption of a Hale–Bopp-like body.

Tunahan Demirci¹, Jens Teiser¹, Tobias Steinpilz¹, Joachim Landers^1,2, Soma Salamon^1,2, Heiko Wende^1,2, and Gerhard Wurm¹
Astrophysical Journal 846, 48 Link to Article [https://doi.org/10.3847/1538-4357/aa816c]
¹Faculty of Physics, University of Duisburg-Essen, Lotharstr. 1, D-47057 Duisburg, Germany
²Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Carl-Benz-Str. 199, D-47057 Duisburg, Germany

Dust drifting inward in protoplanetary disks is subject to increasing temperatures. In laboratory experiments, we tempered basaltic dust between 873 K and 1273 K and find that the dust grains change in size and composition. These modifications influence the outcome of self-consistent low speed aggregation experiments showing a transition temperature of 1000 K. Dust tempered at lower temperatures grows to a maximum aggregate size of 2.02 ± 0.06 mm, which is 1.49 ± 0.08 times the value for dust tempered at higher temperatures. A similar size ratio of 1.75 ± 0.16 results for a different set of collision velocities. This transition temperature is in agreement with orbit temperatures deduced for observed extrasolar planets. Most terrestrial planets are observed at positions equivalent to less than 1000 K. Dust aggregation on the millimeter-scale at elevated temperatures might therefore be a key factor for terrestrial planet formation.

Abrar H. Quadery¹, Baochi D. Doan², William C. Tucker¹, Adrienne R. Dove¹, and Patrick K. Schelling^1,3
Astrophysical Journal 844, 105 Link to Article [https://doi.org/10.3847/1538-4357/aa7890]
¹Department of Physics, University of Central Florida, Orlando, FL 32816-2385, USA
²Department of Materials Science and Engineering, University of Central Florida, Orlando, FL 32816-2385, USA
³Advanced Materials Processing and Analysis Center, University of Central Florida, Orlando, FL 32804, USA

The accretion of dust grains to form larger objects, including planetesimals, is a central problem in planetary science. It is generally thought that weak van der Waals interactions play a role in accretion at small scales where gravitational attraction is negligible. However, it is likely that in many instances, chemical reactions also play an important role, and the particular chemical environment on the surface could determine the outcomes of dust grain collisions. Using atomic-scale simulations of collisional aggregation of nanometer-sized silica (SiO₂) grains, we demonstrate that surface hydroxylation can act to weaken adhesive forces and reduce the ability of mineral grains to dissipate kinetic energy during collisions. The results suggest that surface passivation of dangling bonds, which generally is quite complete in an Earth environment, should tend to render mineral grains less likely to adhere during collisions. It is shown that during collisions, interactions scale with interparticle distance in a manner consistent with the formation of strong chemical bonds. Finally, it is demonstrated that in the case of collisions of nanometer-scale grains with no angular momentum, adhesion can occur even for relative velocities of several kilometers per second. These results have significant implications for early planet formation processes, potentially expanding the range of collision velocities over which larger dust grains can form.

Ninette Abou Mrad, Fabrice Duvernay, Robin Isnard, Thierry Chiavassa, and Grégoire Danger
Astrophysical Journal 846, 124 Link to Article [https://doi.org/10.3847/1538-4357/aa7cf0]
Aix-Marseille Université, PIIM UMR-CNRS 7345, F-13397 Marseille, France

In support of space missions and spectroscopic observations, laboratory experiments on ice analogs enable a better understanding of organic matter formation and evolution in astrophysical environments. Herein, we report the monitoring of the gaseous phase of processed astrophysical ice analogs to determine if the gaseous phase can elucidate the chemical mechanisms and dominant reaction pathways occurring in the solid ice subjected to vacuum ultra-violet (VUV) irradiation at low temperature and subsequently warmed. Simple (CH₃OH), binary (H₂O:CH₃OH, CH₃OH:NH₃), and ternary ice analogs (H₂O:CH₃OH:NH₃) were VUV-processed and warmed. The evolution of volatile organic compounds in the gaseous phase shows a direct link between their relative abundances in the gaseous phase, and the radical and thermal chemistries modifying the initial ice composition. The correlation between the gaseous and solid phases may play a crucial role in deciphering the organic composition of astrophysical objects. As an example, possible solid compositions of the comet Lovejoy are suggested using the abundances of organics in its comae.

María J. Jiménez-Donaire1¹ et al. (>10)
The Astrophysical Journal Letters 836 L29 Link to Article [https://doi.org/10.3847/2041-8213/836/2/L29]
¹Institut für theoretische Astrophysik, Zentrum für Astronomie der Universität Heidelberg, Albert-Ueberle Str. 2, D-69120 Heidelberg, Germany

We use the IRAM Large Program EMPIRE and new high-resolution ALMA data to measure ¹³CO(1-0)/C¹⁸O(1-0) intensity ratios across nine nearby spiral galaxies. These isotopologues of ¹²CO are typically optically thin across most of the area in galaxy disks, and this ratio allows us to gauge their relative abundance due to chemistry or stellar nucleosynthesis effects. Resolved ¹³CO/C¹⁸O gradients across normal galaxies have been rare due to the faintness of these lines. We find a mean ¹³CO/C¹⁸O ratio of 6.0 ± 0.9 for the central regions of our galaxies. This agrees well with results in the Milky Way, but differs from results for starburst galaxies (3.4 ± 0.9) and ultraluminous infrared galaxies (1.1 ± 0.4). In our sample, the ¹³CO/C¹⁸O ratio consistently increases with increasing galactocentric radius and decreases with increasing star formation rate surface density. These trends could be explained if the isotopic abundances are altered by fractionation; the sense of the trends also agrees with those expected for carbon and oxygen isotopic abundance variations due to selective enrichment by massive stars.

Sota Arakawa
Astrophysical Journal 846, 2 Link to Article [https://doi.org/10.3847/1538-4357/aa8564]
Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro, Tokyo, 152-8551, Japan

Chondritic meteorites primarily contain millimeter-sized spherical objects, chondrules; however, the co-accretion process of chondrules and matrix grains is not yet understood. In this study, we investigate the ejection process of chondrules via collisions of fluffy aggregates composed of chondrules and matrices. We reveal that fluffy aggregates cannot grow into planetesimals without losing chondrules if we assume that the chondrite parent bodies are formed via direct aggregation of similar-sized aggregates. Therefore, an examination of other growth pathways is necessary to explain the formation of rocky planetesimals in our solar system.

Cayman T. Unterborn^1,3 and Wendy R. Panero²
Astrophysical Journal 845, 61 Link to Article [https://doi.org/10.3847/1538-4357/aa7f79]
¹School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA
²School of Earth Sciences, The Ohio State University, Columbus, OH 43210, USA
³SESE Exploration Fellow.

Solar photospheric abundances of refractory elements mirror the Earth’s to within ~10 mol% when normalized to the dominant terrestrial-planet-forming elements Mg, Si, and Fe. This allows for the adoption of solar composition as an order-of-magnitude proxy for Earth’s. It is not known, however, the degree to which this mirroring of stellar and terrestrial planet abundances holds true for other star–planet systems without determination of the composition of initial planetesimals via condensation sequence calculations and post condensation processes. We present the open-source Arbitrary Composition Condensation Sequence calculator (ArCCoS) to assess how the elemental composition of a parent star affects that of the planet-building material, including the extent of oxidation within the planetesimals. We demonstrate the utility of ArCCoS by showing how variations in the abundance of the stellar refractory elements Mg and Si affect the condensation of oxygen, a controlling factor in the relative proportions of planetary core and silicate mantle material. This thereby removes significant degeneracy in the interpretation of the structures of exoplanets, as well as provides observational tests for the validity of this model.

E. M. Penteado¹, C. Walsh^2,3, and H. M. Cuppen¹
Astrophysical Journal 844, 71 Link to Article [https://doi.org/10.3847/1538-4357/aa78f9]
¹Radboud University, Institute for Molecules and Materials, Heyendaalseweg 135, NL-6525 AJ Nijmegen, The Netherlands
²School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
³Leiden Observatory, Leiden University P.O. Box 9513, 2300 RA Leiden, The Netherlands

Advanced telescopes, such as ALMA and the James Webb Space Telescope, are likely to show that the chemical universe may be even more complex than currently observed, requiring astrochemical modelers to improve their models to account for the impact of new data. However, essential input information for gas−grain models, such as binding energies of molecules to the surface, have been derived experimentally only for a handful of species, leaving hundreds of species with highly uncertain estimates. We present in this paper a systematic study of the effect of uncertainties in the binding energies on an astrochemical two-phase model of a dark molecular cloud, using the rate equations approach. A list of recommended binding energy values based on a literature search of published data is presented. Thousands of simulations of dark cloud models were run, and in each simulation a value for the binding energy of hundreds of species was randomly chosen from a normal distribution. Our results show that the binding energy of H₂ is critical for the surface chemistry. For high binding energies, H₂ freezes out on the grain forming an H₂ ice. This is not physically realistic, and we suggest a change in the rate equations. The abundance ranges found are in reasonable agreement with astronomical ice observations. Pearson correlation coefficients revealed that the binding energy of HCO, HNO, CH₂, and C correlate most strongly with the abundance of dominant ice species. Finally, the formation route of complex organic molecules was found to be sensitive to the branching ratios of H₂CO hydrogenation.

Dominik C. Hezel^a, Markus Harak^b, Guy Libourel^c
Chemie der Erde (in Press) Link to Article [https://doi.org/10.1016/j.chemer.2017.05.003]
^aUniversity of Cologne, Department of Geology and Mineralogy, Zülpicher Str. 49b, 50674, Köln, Germany
^bNatural History Museum, Department of Mineralogy, Cromwell Road, SW7 5BD, London, UK
^cLaboratoire Lagrange, UMR7293, Université de la Côte d’Azur, CNRS, Observatoire de la Côte d’Azur, F-06304, Nice Cedex 4, France
Copyright Elsevier

Chondrules and matrix are the major components of chondritic meteorites and represent a significant evolutionary step in planet formation. The formation and evolution of chondrules and matrix and, in particular, the mechanics of chondrule formation remain the biggest unsolved challenge in meteoritics. A large number of studies of these major components not only helped to understand these in ever greater detail, but also produced a remarkably large body of data. Studying all available data has become known as ‹big data› analyses and promises deep insights – in this case – to chondrule and matrix formation and relationships. Looking at all data may also allow one to better understand the mechanism of chondrule formation or, equally important, what information we might be missing to identify this process. A database of all available chondrule and matrix data further provides an overview and quick visualisation, which will not only help to solve actual problems, but also enable students and future researchers to quickly access and understand all we know about these components. We collected all available data on elemental bulk chondrule and matrix compositions in a database that we call ChondriteDB. The database also contains petrographic and petrologic information on chondrules. Currently, ChondriteDB contains about 2388 chondrule and 1064 matrix data from 70 different publications and 161 different chondrites. Future iterations of ChondriteDB will include isotope data and information on other chondrite components. Data quality is of critical importance. However, as we discuss, quality is not an objective category, but a subjective judgement. Quantifiable data acquisition categories are required that allow selecting the appropriate data from a database in the context of a given research problem. We provide a comprehensive overview on the contents of ChondriteDB. The database is available as an Excel file upon request from the senior author of this paper, or can be accessed through MetBase.