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.