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.

Susanne Pfalzner¹, Asmita Bhandare^1,2, Kirsten Vincke¹, and Pedro Lacerda³
The Astrophysical Journal 863, 45 Link to Article [https://doi.org/10.3847/1538-4357/aad23c]
¹Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
²Max-Planck-Institut für Astronomy, Königstuhl 17 D-69117, Heidelberg, Germany
³Astrophysics Research Centre, Queen’s University, Belfast, UK

The planets of our solar system formed from a gas-dust disk. However, there are some properties of the solar system that are peculiar in this context. First, the cumulative mass of all objects beyond Neptune (trans-Neptunian objects [TNOs]) is only a fraction of what one would expect. Second, unlike the planets themselves, the TNOs do not orbit on coplanar, circular orbits around the Sun, but move mostly on inclined, eccentric orbits and are distributed in a complex way. This implies that some process restructured the outer solar system after its formation. However, some of the TNOs, referred to as Sednoids, move outside the zone of influence of the planets. Thus, external forces must have played an important part in the restructuring of the outer solar system. The study presented here shows that a close fly-by of a neighboring star can simultaneously lead to the observed lower mass density outside 30 au and excite the TNOs onto eccentric, inclined orbits, including the family of Sednoids. In the past it was estimated that such close fly-bys are rare during the relevant development stage. However, our numerical simulations show that such a scenario is much more likely than previously anticipated. A fly-by also naturally explains the puzzling fact that Neptune has a higher mass than Uranus. Our simulations suggest that many additional Sednoids at high inclinations still await discovery, perhaps including bodies like the postulated planet X.

M. Shadmehri¹, F. Khajenabi¹, and M. E. Pessah²
The Astrophysical Journal 863, 33 Link to Article [https://doi.org/10.3847/1538-4357/aad047]
¹Department of Physics, Faculty of Science, Golestan University, Gorgan 49138-15739, Iran
²Niels Bohr International Academy, Niels Bohr Institute, Blegdamsvej 17, DK-2100 Copenhagen Ø, Denmark

We present an analytical model to investigate the production of pebbles and their radial transport through a protoplanetary disk (PPD) with magnetically driven winds. While most of the previous analytical studies in this context assumed that the radial turbulent coefficient is equal to the vertical dust diffusion coefficient, in the light of the results of recent numerical simulations, we relax this assumption by adopting effective parameterizations of the turbulent coefficients involved, in terms of the strength of the magnetic fields driving the wind. Theoretical studies have already pointed out that even in the absence of winds, these coefficients are not necessarily equal, though how this absence affects pebble production has not been explored. In this paper, we investigate the evolution of the pebble production line, the radial mass flux of the pebbles, and their corresponding surface density as a function of the plasma parameter at the disk midplane. Our analysis explicitly demonstrates that the presence of magnetically driven winds in a PPD leads to considerable reduction of the rate and duration of the pebble delivery. We show that when the wind is strong, the core growth in mass due to the pebble accretion is so slow that it is unlikely that a core could reach a pebble isolation mass during a PPD lifetime. When the mass of a core reaches this critical value, pebble accretion is halted due to core-driven perturbations in the gas. With decreasing wind strength, however, pebble accretion may, in a shorter time, increase the mass of a core to the pebble isolation mass.

Petr Pokorný^1,2, Menelaos Sarantos², and Diego Janches²
The Astrophysical Journal 863, 31 Link to Article [https://doi.org/10.3847/1538-4357/aad051]
¹Department of Physics, The Catholic University of America, Washington, DC 20064, USA
²Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA

To characterize the meteoroid environment around Mercury and its contribution to the planet’s exosphere, we combined four distinctive sources of meteoroids in the solar system: main-belt asteroids, Jupiter-family comets, Halley-type comets, and Oort Cloud comets. All meteoroid populations are described by currently available dynamical models. We used a recent calibration of the meteoroid influx onto Earth as a constraint for the combined population model on Mercury. We predict vastly different distributions of orbital elements, impact velocities, and directions of arrival for all four meteoroid populations at Mercury. We demonstrate that the most likely model of Mercury’s meteoroid environment—in the sense of agreement with Earth—provides good agreement with previously reported observations of Mercury’s exosphere by the MESSENGER spacecraft and is not highly sensitive to variations of uncertain parameters such as the ratio of these populations at Earth, the size–frequency distribution, and the collisional lifetime of meteoroids. Finally, we provide a fully calibrated model consisting of high-resolution maps of mass influx and surface vaporization rates for different values of Mercury’s true anomaly angle.

Maude Gull¹ et al. (>10)
The Astrophysical Journal 862, 174 Link to Article [https://doi.org/10.3847/1538-4357/aacbc3]
¹Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

We present a high-resolution (R ~ 35,000), high signal-to-noise ratio (S/N > 200) Magellan/MIKE spectrum of the star RAVE J094921.8−161722, a bright (V = 11.3) metal-poor red giant star with [Fe/H] = −2.2, identified as a carbon-enhanced metal-poor (CEMP) star from the RAVE survey. We report its detailed chemical abundance signature of light fusion elements and heavy neutron-capture elements. We find J0949−1617 to be a CEMP star with s-process enhancement that must have formed from gas enriched by a prior r-process event. Light neutron-capture elements follow a low-metallicity s-process pattern, while the heavier neutron-capture elements above Eu follow an r-process pattern. The Pb abundance is high, in line with an s-process origin. Thorium is also detected, as expected from an r-process origin, as Th is not produced in the s-process. We employ nucleosynthesis model predictions that take an initial r-process enhancement into account, and then determine the mass transfer of carbon and s-process material from a putative more massive companion onto the observed star. The resulting abundances agree well with the observed pattern. We conclude that J0949−1617 is the first bonafide CEMP-r + s star identified. This class of objects has previously been suggested to explain stars with neutron-capture element patterns that originate from neither the r– nor the s-process alone. We speculate that J0949−1617 formed in an environment similar to those of ultra-faint dwarf galaxies like Tucana III and Reticulum II, which were enriched in r-process elements by one or multiple neutron star mergers at the earliest times.

Sierra L. Grant¹, Catherine C. Espaillat¹, S. Thomas Megeath², Nuria Calvet³, William J. Fischer⁴, Christopher J. Miller³, Kyoung Hee Kim⁵, Amelia M. Stutz^6,7, Álvaro Ribas¹, and Connor E. Robinson¹
The Astrophysical Journal 863, 13 Link to Article [https://doi.org/10.3847/1538-4357/aacda7]
¹Department of Astronomy, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
²Ritter Astrophysical Research Center, Department of Physics and Astronomy, University of Toledo, Toledo, OH 43606, USA
³Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA
⁴Space Telescope Science Institute, Baltimore, MD 21218, USA
⁵Department of Earth Science Education, Kongju National University, 56 Gongjudaehak-ro, Gongju-si, Chungcheongnam-do 32588, Republic of Korea
⁶Departmento de Astronomía, Universidad de Concepción, Casilla 160-C, Concepción, Chile
⁷Max-Planck-Institute for Astronomy, Königstuhl 17, D-69117 Heidelberg, Germany

We analyze Herschel Space Observatory observations of 104 young stellar objects with protoplanetary disks in the ~1.5 Myr star-forming region Lynds 1641 (L1641) within the Orion A Molecular Cloud. We present spectral energy distributions from the optical to the far-infrared including new photometry from the Herschel Photodetector Array Camera and Spectrometer at 70 μm. Our sample, taken as part of the Herschel Orion Protostar Survey, contains 24 transitional disks, 8 of which we identify for the first time in this work. We analyze the full disks (FDs) with irradiated accretion disk models to infer dust settling properties. Using forward modeling to reproduce the observed index for the FD sample, we find the observed disk indices are consistent with models that have depletion of dust in the upper layers of the disk relative to the midplane, indicating significant dust settling. We perform the same analysis on FDs in Taurus with Herschel data and find that Taurus is slightly more evolved, although both samples show signs of dust settling. These results add to the growing literature that significant dust evolution can occur in disks by ~1.5 Myr.

Robert I. Citron¹, Hagai B. Perets², and Oded Aharonson³
The Astrophysical Journal 862, 5 Link to Article [https://doi.org/10.3847/1538-4357/aaca2d]
¹Department of Earth and Planetary Science, University of California, Berkeley, CA, USA
²Department of Astrophysics, Israel Institute of Technology, Haifa, Israel
³Department of Earth and Planetary Science, Weizmann Institute of Science, Rehovot, Israel

The Earth–Moon system is suggested to have formed through a single giant collision, in which the Moon accreted from the impact-generated debris disk. However, such giant impacts are rare, and during its evolution, the Earth experienced many more smaller impacts, producing smaller satellites that potentially coevolved. In the multiple-impact hypothesis of lunar formation, the current Moon was produced from the mergers of several smaller satellites (moonlets), each formed from debris disks produced by successive large impacts. In the Myr between impacts, a pre-existing moonlet tidally evolves outward until a subsequent impact forms a new moonlet, at which point both moonlets will tidally evolve until a merger or system disruption. In this work, we examine the likelihood that pre-existing moonlets survive subsequent impact events, and explore the dynamics of Earth–moonlet systems that contain two moonlets generated Myr apart. We demonstrate that pre-existing moonlets can tidally migrate outward, remain stable during subsequent impacts, and later merge with newly created moonlets (or re-collide with the Earth). Formation of the Moon from the mergers of several moonlets could therefore be a natural byproduct of the Earth’s growth through multiple impacts. More generally, we examine the likelihood and consequences of Earth having prior moons, and find that the stability of moonlets against disruption by subsequent impacts implies that several large impacts could post-date Moon formation.

Hiroshi Kobayashi¹ and Hidekazu Tanaka²
The Astrophysical Journal 862, 127 Link to Article [https://doi.org/10.3847/1538-4357/aacdf5]
¹Department of Physics, Nagoya University, Nagoya, Aichi 464-8602, Japan
²Astronomical Institute, Tohoku University, Aramaki, Aoba-ku, Sendai 980-8578, Japan

In the core accretion scenario, gas giant planets are formed form solid cores with several Earth masses via gas accretion. We investigate the formation of such cores via collisional growth from kilometer-sized planetesimals in turbulent disks. The stirring by forming cores induces collisional fragmentation, and surrounding planetesimals are ground down until radial drift. The core growth is therefore stalled by the depletion of surrounding planetesimals due to collisional fragmentation and radial drift. The collisional strength of planetesimals determines the planetesimal-depletion timescale, which is prolonged for large planetesimals. The size of planetesimals around growing cores is determined by the planetesimal size distribution at the onset of runaway growth. Strong turbulence delays the onset of runaway growth, resulting in large planetesimals. Therefore, the core mass evolution depends on the turbulent parameter α; the formation of cores massive enough without significant depletion of surrounding planetesimals needs a strong turbulence of α 10⁻³. However, strong turbulence with α 10⁻³ leads to a significant delay of the onset of runaway growth and prevents the formation of massive cores within the disk lifetime. The formation of cores massive enough within several million years therefore requires that solid surface densities are several times higher, which is achieved in the inner disk 10 au due to pile-up of drifting dust aggregates. In addition, the collisional strength even for kilometer-sized or smaller bodies affects the growth of cores; for bodies 1 km is likely for this gas giant formation.

Jan Render¹, Gregory A. Brennecka¹, Shui-Jiong Wang², Laura E. Wasylenki², and Thorsten Kleine¹
The Astrophysical Journal 862, 26 Link to Article [https://doi.org/10.3847/1538-4357/aacb7e]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Straße 10, D-48149 Münster, Germany
²Department of Earth and Atmospheric Sciences, Indiana University Bloomington, 1001 East 10th Street, Bloomington IN 47405, USA

As the earliest dated solids, calcium–aluminum-rich inclusions (CAIs) provide a unique window into the early solar system. However, for many elements, CAIs have been shown to exhibit a very different nucleosynthetic isotope signature from that of later-formed bulk meteorites. To explore this critical difference between solar system materials, we investigate a broad set of CAI samples for both mass-dependent and non-mass-dependent (nucleosynthetic) isotope variations in the siderophile element nickel (Ni). We find that fine-grained CAIs show little if any mass-dependent Ni isotopic fractionation, whereas coarse-grained inclusions exhibit a broad range of isotopically heavy signatures. Because mass-dependent variations appear to be coupled with nucleosynthetic anomalies in CAIs, a part of this Ni isotope variability could be due to thermal processing that acted on these samples. Nucleosynthetic Ni isotopic signatures show that CAIs share a genetic heritage with carbonaceous meteorites and provide a clear distinction from the isotopic reservoirs occupied by terrestrial Ni and non-carbonaceous meteorites. However, whereas nucleosynthetic Ni isotope heterogeneity in previously investigated bulk meteorites was ascribed to variation in the neutron-poor isotope ⁵⁸Ni, we here find that CAI signatures require variability in other, more neutron-rich Ni isotopes. Taken in aggregate with previous work, this highlights a change in the nucleosynthetic character from CAIs to later-formed solids that cannot be explained by variable admixture of a single presolar phase or material from a specific supernova shell. Instead, these data reveal the complex evolution of the solar system, including blending and reprocessing of matter from several generations and types of stars.