Chuanfei Dong^1,2 et al. (>10)
Astrophysical Journal Letters 859, L14 Link to Article [DOI: 10.3847/2041-8213/aac489]
¹Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA

In this Letter, we make use of sophisticated 3D numerical simulations to assess the extent of atmospheric ion and photochemical losses from Mars over time. We demonstrate that the atmospheric ion escape rates were significantly higher (by more than two orders of magnitude) in the past at ~4 Ga compared to the present-day value owing to the stronger solar wind and higher ultraviolet fluxes from the young Sun. We found that the photochemical loss of atomic hot oxygen dominates over the total ion loss at the current epoch, while the atmospheric ion loss is likely much more important at ancient times. We briefly discuss the ensuing implications of high atmospheric ion escape rates in the context of ancient Mars, and exoplanets with similar atmospheric compositions around young solar-type stars and M-dwarfs.

Cécile Favre¹ et al. (>10)
Astrophysical Journal Letters 862, L2 Link to Article [DOI: 10.3847/2041-8213/aad046]
¹INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125, Florence, Italy

The formation of asteroids, comets, and planets occurs in the interior of protoplanetary disks during the early phase of star formation. Consequently, the chemical composition of the disk might shape the properties of the emerging planetary system. In this context, it is crucial to understand whether and what organic molecules are synthesized in the disk. In this Letter, we report the first detection of formic acid (HCOOH) toward the TW Hydrae protoplanetary disk. The observations of the trans-HCOOH 6_{(1,6)–5(1,5)} transition were carried out at 129 GHz with Atacama Large Millimeter/Submillimeter Array (ALMA). We measured a disk-averaged gas-phase t-HCOOH column density of ~(2–4) × 10¹² cm⁻², namely as large as that of methanol. HCOOH is the first organic molecule containing two oxygen atoms detected in a protoplanetary disk, a proof that organic chemistry is very active, albeit difficult to observe, in these objects. Specifically, this simplest acid stands as the basis for synthesis of more complex carboxylic acids used by life on Earth.

Jiao He¹, SM Emtiaz, and Gianfranco Vidali
The Astrophysical Journal 863, 156 Link to Article [https://doi.org/10.3847/1538-4357/aad227]
Physics Department, Syracuse University, Syracuse, NY 13244, USA
¹Current address: Raymond and Beverly Sackler Laboratory for Astrophysics, Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands.

The diffusion of atoms and molecules in ices covering dust grains in dense clouds in interstellar space is an important but poorly characterized step in the formation of complex molecules in space. Here we report the measurement of diffusion of simple molecules in amorphous solid water (ASW), an analog of interstellar ices, which are amorphous and made mostly of water molecules. The new approach that we used relies on measuring, in situ, the change in band strength and position of mid-infrared features of OH dangling bonds as molecules move through pores and channels of ASW. We obtained the Arrhenius pre-exponents and activation energies for diffusion of CO, O₂, N₂, CH₄, and Ar in ASW. The diffusion energy barrier of H₂ and D₂ were also measured, but only upper limits were obtained. These values constitute the first comprehensive set of diffusion parameters of simple molecules on the pore surface of ASW and can be used in simulations of the chemical evolution of Interstellar Medium environments, thus replacing unsupported estimates. We also present a set of argon temperature programmed desorption experiments to determine the desorption energy distribution of argon on non-porous ASW.

Kanji Mori^1,2, Michael A. Famiano^3,2, Toshitaka Kajino^4,2,1, Toshio Suzuki^5,2, Peter M. Garnavich⁶, Grant J. Mathews^5,2, Roland Diehl^7,2, Shing-Chi Leung⁸, and Ken’ichi Nomoto⁸
The Astrophysical Journal 863, 176 Link to Article [https://doi.org/10.3847/1538-4357/aad233]
¹Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan
²National Astronomical Observatory of Japan 2-21-1 Osawa, Mitaka, Tokyo, 181-8588, Japan
³Department of Physics, Western Michigan University, Kalamazoo, MI 49008, USA
⁴School of Physics and Nuclear Energy Engineering, and Internationsl Research Center for Big-Bang Cosmology and Element Genesis, Beihang University, Beijing 100083, People’s Republic of China
⁵Department of Physics, College of Humanities and Sciences, Nihon University 3-25-40 Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan
⁶Departmant of Physics, Center for Astrophysics, University of Notre Dame, Notre Dame, IN 46556, USA
⁷Max Planck Institut für extraterrestrische Physik, D-85748 Garching, Germany
⁸Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo, Kashiwa, Chiba 277-8583, Japan

Observations of type Ia supernovae (SNe Ia) include information about the characteristic nucleosynthesis associated with these thermonuclear explosions. We consider observational constraints from iron-group elemental and isotopic ratios, to compare with various models obtained with the most realistic recent treatment of electron captures (ECs). The nucleosynthesis is sensitive to the highest white-dwarf central densities. Hence, nucleosynthesis yields can distinguish high-density Chandrasekhar-mass models from lower-density burning models such as white-dwarf mergers. We discuss new results of post-processing nucleosynthesis for two spherical models (deflagration and/or delayed-detonation models) based upon new EC rates. We also consider cylindrical and 3D explosion models (including deflagration, delayed-detonation, or a violent merger model). Although there are uncertainties in the observational constraints, we identify some trends in the observations and the models. We make a new comparison of the models with elemental and isotopic ratios from five observed supernovae and three supernova remnants. We find that the models and data tend to fall into two groups. In one group, low-density cores such as in a 3D merger model are slightly more consistent with the nucleosynthesis data, while the other group is slightly better identified with higher-density cores such as in single-degenerate 1D–3D deflagration models. Hence, we postulate that both types of environments appear to contribute nearly equally to observed SN Ia. We also note that observational constraints on the yields of ⁵⁴Cr and ⁵⁴Fe, if available, might be used as a means to clarify the degree of geometrical symmetry of SN Ia explosions.

Shigeru Wakita^1,2, Yasuhiro Hasegawa³, and Takaya Nozawa⁴
The Astrophysical Journal 863, 100 Link to Article [https://doi.org/10.3847/1538-4357/aad0a2]
¹Center for Computational Astrophysics, National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan
²Earth-Life Science Institute, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8550, Japan
³Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
⁴Division of Theoretical Astronomy, National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan

Chondrites are some of the most primitive objects in the solar system, and they maintain a record of the degree of thermal metamorphism experienced in their parent bodies. This thermal history can be classified by the petrologic type. We investigate the thermal evolution of planetesimals to account for the current abundances (known as the fall statistics) of petrologic types 3–6 of ordinary chondrites. We carry out a number of numerical calculations in which formation times and sizes of planetesimals are taken as parameters. We find that planetesimals that form within 2.0 Myr after the formation of Ca-Al-rich inclusions (CAIs) can contain all petrologic types of ordinary chondrites. Our results also indicate that plausible scenarios of planetesimal formation, which are consistent with the fall statistics, are that planetesimals with radii larger than 60 km start to form around 2.0 Myr after CAIs and/or that ones with radii less than 50 km should be formed within 1.5 Myr after CAIs. Thus, thermal modeling of planetesimals is important for revealing the occurrence and amount of metamorphosed chondrites and for providing invaluable insights into planetesimal formation.

D. Vescovi^1,2 et al. (>10)
The Astrophysical Journal 863, 115 Link to Article [https://doi.org/10.3847/1538-4357/aad191]
¹Gran Sasso Science Institute, Viale Francesco Crispi, 7, I-67100 L’Aquila, Italy

Recent improvements in stellar models for intermediate-mass stars and massive stars (MSs) are recalled, together with their expectations for the synthesis of radioactive nuclei of lifetimes τ  25 Myr, in order to re-examine the origins of now extinct radioactivities that were alive in the solar nebula. The Galactic inheritance broadly explains most of them, especially if r-process nuclei are produced by neutron star merging, according to recent models. Instead, ²⁶Al, ⁴¹Ca, ¹³⁵Cs, and possibly ⁶⁰Fe require nucleosynthetic events close to the solar formation. We outline the persisting difficulties to account for these nuclei by intermediate-mass stars (2  M/M _⊙  7–8). Models of their final stages now predict the ubiquitous formation of a ¹³C reservoir as a neutron capture source; hence, even in the presence of ²⁶Al production from deep mixing or hot bottom burning, the ratio ²⁶Al/¹⁰⁷Pd remains incompatible with measured data, with a large excess in ¹⁰⁷Pd. This is shown for two recent approaches to deep mixing. Even a late contamination by an MS encounters problems. In fact, the inhomogeneous addition of supernova debris predicts nonmeasured excesses on stable isotopes. Revisions invoking specific low-mass supernovae and/or the sequential contamination of the presolar molecular cloud might be affected by similar problems, although our conclusions here are weakened by our schematic approach to the addition of SN ejecta. The limited parameter space that remains to be explored for solving this puzzle is discussed.

Xiaojun Xu^1,2, Qing Chang¹, Qi Xu¹, Vassilis Angelopoulos³, Yi Wang⁴, and Pingbing Zuo⁴
The Astrophysical Journal 863, 80 Link to Article [https://doi.org/10.3847/1538-4357/aad282]
¹Space Science Institute, Macau University of Science and Technology, Macao, People’s Republic of China
²Institute of Space Science and Technology, Nanchang University, Nanchang, People’s Republic of China
³Department of Earth, Planetary, and Space Sciences, and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California, USA
⁴Institute of Science and Applied Technology, Harbin Institute of Technology, Shenzhen, People’s Republic of China

Energetic particles from Earth’s bow shock can frequently access the lunar orbit. These energetic particles are mainly ions. Since they are mostly field-aligned, they have much greater impacts on the lunar near side than on its far side. We present a statistical study of these upstream energetic ions at the lunar orbit using ARTEMIS observations. During the five-year time interval from 2012 to 2016, 496 energetic ion events with time durations ≥30 minutes were identified. The average duration of an event is about 1.93 hr. Most events occurred at the dawn-looking quadrants of Earth, showing an asymmetric dawn–dusk distribution in space. In 490 of the 496 events, the magnetic field lines directly extend to the bow shock. This is very important in order for energetic ions to arrive at the lunar orbit along field lines. The highest energies of these upstream energetic ions range from 101.5 to 658.5 keV based on the energy channels of ARTEMIS. The spatial distribution of the events depends on the highest energy. Events with higher energies tend to occur near the subsolar region and are related to greater AE indices, indicating stronger disturbances of the geomagnetosphere. By taking into account upstream energetic ions, in addition to galactic cosmic rays and solar energetic particles, our results provide a more comprehensive understanding of the energetic particle environment of the lunar near side.

Manuel Moreno-Ibáñez¹, Elizabeth A. Silber², Maria Gritsevich^3,4, and Josep M. Trigo-Rodríguez^1,5
The Astrophysical Journal 863, 174 Link to Article [https://doi.org/10.3847/1538-4357/aad334]
¹Institute of Space Sciences (ICE, CSIC), Meteorites, Minor Bodies and Planetary Science Group, Campus UAB, Carrer de Can Magrans, s/n E-08193 Cerdanyola del Vallés, Barcelona, Catalonia, Spain
²Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI 02912, USA
³Department of Physics, University of Helsinki, Gustaf Hällströmin katu 2a, P.O. Box 64, FI-00014 Helsinki, Finland
⁴Institute of Physics and Technology, Ural Federal University, Mira str. 19. 620002 Ekaterinburg, Russia
⁵Institut d’Estudis Espacials de Catalunya (IEEC), C/ Gran Capitá, 2-4, Ed. Nexus, desp. 201, E-08034 Barcelona, Catalonia, Spain

Infrasound monitoring has proved to be effective in detection of meteor-generated shock waves. When combined with optical observations of meteors, this technique is also reliable for detecting centimeter-sized meteoroids that usually ablate at high altitudes, thus offering relevant clues that open the exploration of the meteoroid flight regimes. Since a shock wave is formed as a result of a passage of the meteoroid through the atmosphere, the knowledge of the physical parameters of the surrounding gas around the meteoroid surface can be used to determine the meteor flow regime. This study analyzes the flow regimes of a data set of 24 centimeter-sized meteoroids for which well-constrained infrasound and photometric information is available. This is the first time that the flow regimes for meteoroids in this size range are validated from observations. From our approach, the Knudsen and Reynolds numbers are calculated, and two different flow regime evaluation approaches are compared in order to validate the theoretical formulation. The results demonstrate that a combination of fluid dynamic dimensionless parameters is needed to allow a better inclusion of the local physical processes of the phenomena.

Tomas Tamfal, Joanna Dra̧żkowska, Lucio Mayer, and Clement Surville
The Astrophysical Journal 863, 97 Link to Article [https://doi.org/10.3847/1538-4357/aad1f4]
Center for Theoretical Astrophysics and Cosmology, Institute for Computational Science, University of Zurich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland

We present the first 2D hydrodynamical finite-volume simulations in which dust is fully coupled with the gas, including its back-reaction onto it, and at the same time the dust size is evolving according to coagulation and fragmentation based on a subgrid model. The aim of this analysis is to present the differences occurring when dust evolution is included relative to simulations with fixed dust size, with and without an embedded Jupiter-mass planet that triggers gap formation. We use the two-fluid polar Godunov-type code RoSSBi developed by Surville et al. combined with a new local subgrid method for dust evolution based on the model by Birnstiel et al. We find striking differences between simulations with variable and fixed dust sizes. The timescales for dust depletion differ significantly and yield a completely different evolution of the dust surface density. In general, sharp features such as pileups of dust in the inner disk and near gap edges, when a massive planet is present, become much weaker. This has important implications for the interpretation of observed substructure in disks, suggesting that the presence of a massive planet does not necessarily cause sharp gaps and rings in the dust component. Also, particles with different dust sizes show a different distribution, pointing to the importance of multiwavelength synthetic observations in order to compare with observations by ALMA and other instruments. We also find that simulations adopting fixed intermediate particle sizes, in the range of 10⁻² to 10⁻¹ cm, best approximate the surface density evolution seen in simulations with dust evolution.

Dominik C. Hezel^1,2 and Eric J. R. Parteli³
The Astrophysical Journal 863, 54 Link to Article [https://doi.org/10.3847/1538-4357/aad041]
¹University of Cologne, Department of Geology and Mineralogy Zülpicher Str. 49b, D-50674 Köln, Germany
²Department of Mineralogy, Natural History Museum, Cromwell Road, London, SW7 5BD, UK
³University of Cologne, Department of Geosciences Pohligstr. 3, D-50969 Köln, Germany

Chondrules are a major component of chondritic meteorites and potentially populated the entire protoplanetary disk before planet formation. Chondrules provide insights into the physical and chemical evolution of the protoplanetary disk. An important constraint for the protoplanetary disk is whether chondrules in individual chondrite groups formed in spatially separate reservoirs and were then transported and mixed throughout the disk, finally accreting in chondrites, or did chondrules in individual chondrite groups form and then accrete in the same reservoir and locality, without large-scale transport and mixing involved. Both scenarios have been proposed. Here we use bulk chondrule compositional data from the recently published ChondriteDB database in combination with a mixing model we developed to test whether the compositional distributions of chondrule populations in individual chondrites (1) are the result of mixing chondrules from multiple parental reservoirs or (2) originated from single parental reservoirs. We thereby provide a fundamental framework that each mixing model needs to obey. Although one mixing model is principally possible, this particular model is unlikely, and it therefore appears more reasonable that chondrules in individual chondrites originated from single, although different, parental reservoirs. Significant disk-wide transport or mixing of chondrules seems unlikely, while chondrule-forming models that produce chondrules from single reservoirs seem more likely. Anomalous minor element and nucleosynthetic isotope chondrule compositions are possibly best explained by admixing tiny nuggets such as refractory or presolar grains with distinct elemental or isotopic compositions into chondrules.