Mohammadtaher Safarzadeh, Richard Sarmento, and Evan Scannapieco
Astrophysical Journal 876, 28 Link to Article [DOI: 10.3847/1538-4357/ab1341 ]
School of Earth and Space Exploration, Arizona State University, USA

We model the history of Galactic r-process enrichment using high-redshift, high-resolution zoom cosmological simulations of a Milky Way–type halo. We assume that all r-process sources are neutron star mergers (NSMs) with a power-law delay time distribution. We model the time to mix pollutants at subgrid scales, which allows us to better compute the properties of metal-poor (MP) and carbon-enhanced metal-poor (CEMP) stars, along with statistics of their r-process-enhanced subclasses. Our simulations underpredict the cumulative ratios of r-process-enhanced MP and CEMP stars (MP-r, CEMP-r) over MP and CEMP stars by about one order of magnitude, even when the minimum coalescence time of the double neutron stars (DNSs), t _min, is set to 1 Myr. No r-process-enhanced stars form if t _min = 100 Myr. Our results show that even when we adopt the r-process yield estimates observed in GW170817, NSMs by themselves can only explain the observed frequency of r-process-enhanced stars if the birth rate of DNSs per unit mass of stars is boosted to .

Anibal Sierra¹, Susana Lizano¹, Enrique Macías², Carlos Carrasco-González¹, Mayra Osorio³, and Mario Flock⁴
Astrophysical Journal 876, 7 Link to Article [DOI: 10.3847/1538-4357/ab1265 ]
¹Instituto de Radioastronomía y Astrofísica, UNAM, Apartado Postal 3-72, 58089 Morelia Michoacán, México
²Department of Astronomy, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
³Instituto de Astrofísica de Andalucía (CSIC) Glorieta de la Astronomía s/n E-18008 Granada, Spain
⁴Max Planck Institute fűr Astronomy (MPIA), Kőnigsthul 17, D-69117 Heidelberg, Germany

We study dust concentration in axisymmetric gas rings in protoplanetary disks. Given the gas surface density, we derived an analytical total dust surface density by taking into account the differential concentration of all grain sizes. This model allows us to predict the local dust-to-gas mass ratio and the slope of the particle size distribution, as a function of radius. We test this analytical model by comparing it with a 3D magnetohydrodynamical simulation of dust evolution in an accretion disk. The model is also applied to the disk around HD 169142. By fitting the disk continuum observations simultaneously at λ = 0.87, 1.3, and 3.0 mm, we obtain a global dust-to-gas mass ratio and a viscosity coefficient α = 1.35 × 10⁻². This model can be easily implemented in numerical simulations of accretion disks.

Anusha Kalyaan and Steven J. Desch
Astrophysical Journal 875, 43 Link to Article [DOI: 10.3847/1538-4357/ab0e6c ]
School of Earth & Space Exploration, Arizona State University, 550 E Tyler Mall Tempe, AZ 85287, USA

The snow line in a protoplanetary disk demarcates regions with H₂O ice from regions with H₂O vapor. Where a planet forms relative to this location determines how much water and other volatiles it forms with. Giant-planet formation may be triggered at the water–snow line if vapor diffuses outward and is cold-trapped beyond the snow line faster than icy particles can drift inward. In this study, we investigate the distribution of water across the snow line, considering three different radial profiles of the turbulence parameter α(r), corresponding to three different angular momentum transport mechanisms. We consider the radial transport of water vapor and icy particles by diffusion, advection, and drift. We show that even for similar values of α, the gradient of α(r) across the snow line significantly changes the snow line location, the sharpness of the volatile gradient across the snow line, and the final water/rock ratio in planetary bodies. A profile of radially decreasing α, consistent with transport by hydrodynamic instabilities plus magnetic disk winds, appears consistent with the distribution of water in the solar nebula, with monotonically increasing radial water content and a diverse population of asteroids with different water content. We argue that Σ(r) and water abundance are likely a diagnostic of α(r) and thus of the mechanism for angular momentum transport in inner disks.

Saverio Cambioni¹, Erik Asphaug¹, Alexandre Emsenhuber¹, Travis S. J. Gabriel², Roberto Furfaro³, and Stephen R. Schwartz¹
Astrophysical Journal 875, 40 Link to Article [DOI: 10.3847/1538-4357/ab0e8a ]
¹Lunar and Planetary Laboratory, University of Arizona, 1629 E. University Blvd., Tucson, AZ 85721, USA
²School of Earth and Space Exploration, Arizona State University, 781 E. Terrace Mall, Tempe, AZ 85287, USA
³Systems and Industrial Engineering Department, University of Arizona, 1127 E. James E. Rogers Way, Tucson, AZ 85721, USA

Planet formation simulations are capable of directly integrating the evolution of hundreds to thousands of planetary embryos and planetesimals as they accrete pairwise to become planets. In principle, these investigations allow us to better understand the final configuration and geochemistry of the terrestrial planets, and also to place our solar system in the context of other exosolar systems. While these simulations classically prescribe collisions to result in perfect mergers, recent computational advances have begun to allow for more complex outcomes to be implemented. Here we apply machine learning to a large but sparse database of giant impact studies, which allows us to streamline the simulations into a classifier of collision outcomes and a regressor of accretion efficiency. The classifier maps a four-dimensional (4D) parameter space (target mass, projectile-to-target mass ratio, impact velocity, impact angle) into the four major collision types: merger, graze-and-merge, hit-and-run, and disruption. The definition of the four regimes and their boundary is fully data-driven. The results do not suffer from any model assumption in the fitting. The classifier maps the structure of the parameter space and it provides insights into the outcome regimes. The regressor is a neural network that is trained to closely mimic the functional relationship between the 4D space of collision parameters, and a real-variable outcome, the mass of the largest remnant. This work is a prototype of a more complete surrogate model, that will be based on extended sets of simulations (big data), that will quickly and reliably predict specific collision outcomes for use in realistic N-body dynamical studies of planetary formation.

Benoit Côté^1,2,3,17et al. (>10)
Astrophysical Journal 875, 106 Link to Article [DOI: 10.3847/1538-4357/ab10db ]
¹Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, Konkoly Thege Miklos ut 15-17, H-1121 Budapest, Hungary

Probing the origin of r-process elements in the universe represents a multidisciplinary challenge. We review the observational evidence that probes the properties of r-process sites, and address them using galactic chemical evolution simulations, binary population synthesis models, and nucleosynthesis calculations. Our motivation is to define which astrophysical sites have significantly contributed to the total mass of r-process elements present in our Galaxy. We found discrepancies with the neutron star (NS–NS) merger scenario. When we assume that they are the only site, the decreasing trend of [Eu/Fe] at [Fe/H] > −1 in the disk of the Milky Way cannot be reproduced while accounting for the delay-time distribution (DTD) of coalescence times (∝t ⁻¹) derived from short gamma-ray bursts (GRBs) and population synthesis models. Steeper DTD functions (∝t ^−1.5) or power laws combined with a strong burst of mergers before the onset of supernovae (SNe) Ia can reproduce the [Eu/Fe] trend, but this scenario is inconsistent with the similar fraction of short GRBs and SNe Ia occurring in early-type galaxies, and it reduces the probability of detecting GW170817 in an early-type galaxy. One solution is to assume an additional production site of Eu that would be active in the early universe, but would fade away with increasing metallicity. If this is correct, this additional site could be responsible for roughly 50% of the Eu production in the early universe before the onset of SNe Ia. Rare classes of supernovae could be this additional r-process source, but hydrodynamic simulations still need to ensure the conditions for a robust r-process pattern.

Marc Neveu^1,2 and Pierre Vernazza³
Astrophysical Journal 875, 30 Link to Article [DOI: 10.3847/1538-4357/ab0d87 ]
¹University of Maryland, 4296 Stadium Dr., College Park, MD 20742, USA
²NASA Goddard Space Flight Center, 8800 Greenbelt Rd., Greenbelt, MD 20770, USA
³Aix-Marseille Université, CNRS, Laboratoire d’Astrophysique de Marseille, 38 Rue Frederic Joliot Curie, F-13013 Marseille, France

The parent bodies of ordinary chondrites, carbonaceous CM chondrites, and interplanetary dust particles (IDPs) represent most of the mass of the solar system’s small (D ≤ 250 km) bodies. The times of formation of the ordinary and carbonaceous CM chondrite parent bodies have previously been pinpointed, respectively, to ≈2 and 3–4 million years after calcium–aluminum-rich inclusions (CAIs). However, the timing of the formation of IDP parent bodies such as P- and D-type main-belt asteroids and Jupiter Trojans has not been tightly constrained. Here, we show that they formed later than 5–6 million years after CAIs. We use models of their thermal and structural evolution to show that their anhydrous surface composition would otherwise have been lost due to melting and ice-rock differentiation driven by heating from the short-lived radionuclide ²⁶Al. This suggests that IDP-like volatile-rich small bodies may have formed after the gas of the protoplanetary disk dissipated and thus later than the massive cores of the giant planets. It also confirms an intuitive increase in formation times with increased heliocentric distance, and suggests that there may have been a gap in time between the formation of carbonaceous chondrite (chondrule-rich) and IDP (chondrule-poor) parent bodies.

John B. Biersteker¹, Benjamin P. Weiss¹, Philip Heinisch², David Herčik², Karl-Heinz Glassmeier², and Hans-Ulrich Auster²
Astrophysical Journal 875, 39 Link to Article [DOI: 10.3847/1538-4357/ab0f2a ]
¹Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139-4307, USA
²Technische Universität Braunschweig, Mendelssohnstrasse 3, D-38106 Braunschweig, Germany

The remanent magnetization of solar system bodies reflects their accretion mechanism, the space environment in which they formed, and their subsequent geological evolution. In particular, it has been suggested that some primitive bodies may have formed large regions of coherent remanent magnetization as a consequence of their accretion in a background magnetic field. Measurements acquired by the Rosetta Magnetometer and Plasma Monitor have shown that comet 67P/Churyumov–Gerasimenko (67P) has a surface magnetic field of less than 0.9 nT. To constrain the spatial scale and intensity of remanent magnetization in 67P, we modeled its magnetic field assuming various characteristic spatial scales of uniform magnetization. We find that for regions of coherent magnetization with ≥10 cm radius, the specific magnetic moment is  5 × 10⁻⁶ . If 67P formed during the lifetime of the solar nebula and has not undergone significant subsequent collisional or aqueous alteration, this very low specific magnetization is inconsistent with its formation from the gentle gravitational collapse of a cloud of millimeter-sized pebbles in a background magnetic field 3 μT. Given the evidence from other Rosetta instruments that 67P formed by pebble-pile processes, this would indicate that the nebular magnetic field was 3 μT at 15–45 au from the young Sun. This constraint is consistent with theories of magnetically driven evolution of protoplanetary disks.

Ryan Miranda¹ and Roman R. Rafikov^1,2
Astrophysical Journal 875, 37 Link to Article [DOI: 10.3847/1538-4357/ab0f9e ]
¹Institute for Advanced Study, Einstein Drive, Princeton, NJ 08540, USA
²Centre for Mathematical Sciences, Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Wilberforce Road, Cambridge CB3 0WA, UK

Recent observations of protoplanetary disks, as well as simulations of planet–disk interaction, have suggested that a single planet may excite multiple spiral arms in the disk, in contrast to the previous expectations based on linear theory (predicting a one-armed density wave). We reassess the origin of multiple arms in the framework of linear theory by solving for the global two-dimensional response of a nonbarotropic disk to an orbiting planet. We show that the formation of a secondary arm in the inner disk, at about half of the orbital radius of the planet, is a robust prediction of linear theory. This arm becomes stronger than the primary spiral at several tenths of the orbital radius of the planet. Several additional, weaker spiral arms may also form in the inner disk. On the contrary, a secondary spiral arm is unlikely to form in the outer disk. Our linear calculations, fully accounting for the global behavior of both the phases and amplitudes of perturbations, generally support the recently proposed WKB phase argument for the secondary arm origin (as caused by the intricacy of constructive interference of the azimuthal harmonics of the perturbation at different radii). We provide analytical arguments showing that the process of a single spiral wake splitting up into multiple arms is a generic linear outcome of wave propagation in differentially rotating disks. It is not unique to planet-driven waves and also occurs in linear calculations of spiral wakes freely propagating with no external torques. These results are relevant for understanding formation of multiple rings and gaps in protoplanetary disks.

Olivier Mousis¹, Thomas Ronnet^1,2, and Jonathan I. Lunine³
Astrophysical Journal 875, 9 Link to Article [DOI: 10.3847/1538-4357/ab0a72 ]
¹Aix Marseille Université, CNRS, CNES, LAM, Marseille, France
²Lund Observatory, Department of Astronomy and Theoretical Physics, Lund University, Box 43, SE-221 00 Lund, Sweden
³Department of Astronomy, Cornell University, Ithaca, NY 14853, USA

Argon, krypton, xenon, carbon, nitrogen, sulfur, and phosphorus have all been measured and found to be enriched by a quasi uniform factor in the 2–4 range, compared to their protosolar values, in the atmosphere of Jupiter. To elucidate the origin of these volatile enrichments, we investigate the possibility of an inward drift of particles made of amorphous ice and adsorbed volatiles, and their ability to enrich in heavy elements the gas phase of the protosolar nebula, once they cross the amorphous-to-crystalline ice transition zone, following the original idea formulated by Monga & Desch. To do so, we use a simple accretion disk model coupled to modules depicting the radial evolution of icy particles and vapors, assuming growth, fragmentation, and crystallization of amorphous grains. We show that it is possible to accrete supersolar gas from the nebula onto proto-Jupiter’s core to form its envelope, and allowing it to match the observed volatile enrichments. Our calculations suggest that nebular gas, with a metallicity similar to that measured in Jupiter, can be accreted by its envelope if the planet is formed in the ~0.5–2 Myr time range and in the 0.5–20 au distance range from the Sun, depending on the adopted viscosity parameter of the disk. These values match a wide range of Jupiter’s formation scenarios, including in situ formation and migration/formation models.

¹Chunlai Li et al. (>10)
Nature 569, 378-382 Link to Article [https://doi.org/10.1038/s41586-019-1189-0]
¹Key Laboratory of Lunar and Deep Space Exploration, National Astronomical Observatories, Chinese Academy of Sciences, Beijing, China

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