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.

Shing-Chi Leung and Ken’ichi Nomoto
The Astrophysical Journal 861, 143 Link to Article [https://doi.org/10.3847/1538-4357/aac2df]
Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan

We present 2D hydrodynamics simulations of near-Chandrasekhar-mass white dwarf (WD) models for Type Ia supernovae (SNe Ia) using the turbulent deflagration model with a deflagration-to-detonation transition (DDT). We perform a parameter survey for 41 models to study the effects of the initial central density (i.e., WD mass), metallicity, flame shape, DDT criteria, and turbulent flame formula for a much wider parameter space than in earlier studies. The final isotopic abundances of ¹¹C to ⁹¹Tc in these simulations are obtained by post-process nucleosynthesis calculations. The survey includes SN Ia models with the central density from 5 × 10⁸ g cm⁻³ to 5 × 10⁹ g cm⁻³ (WD masses of 1.30–1.38 M _⊙), metallicity from 0 to 5 Z _⊙, C/O mass ratio from 0.3 to 1.0, and ignition kernels, including centered and off-centered ones. We present the yield tables of stable isotopes from ¹²Cl to ⁷⁰Zn, as well as the major radioactive isotopes for 33 models. Observational abundances of ⁵⁵Mn, ⁵⁶Fe, ⁵⁷Fe, and ⁵⁸Ni obtained from the solar-composition, well-observed SN Ia and SN Ia remnants are used to constrain the explosion models and the SN progenitor. The connection between the pure turbulent deflagration model and the subluminous SNe Iax is discussed. We find that dependencies of the nucleosynthesis yields on the metallicity and the central density (WD mass) are large. To fit these observational abundances, and also for the application of galactic chemical evolution modeling, these dependencies on the metallicity and WD mass should be taken into account.

Alexandre Zanchet¹, Octavio Roncero, Marcelino Agúndez, and José Cernicharo
The Astrophysical Journal 862, 38 Link to Article [https://doi.org/10.3847/1538-4357/aaccff]
1 Present address: Facultad de Ciencias Químicas, Universidad Complutense de Madrid, Plaza de las Ciencias, Ciudad Universitaria, E-28040 Madrid, Spain.

The presence of SiS in space seems to be restricted to a few selected types of astronomical environments. It is long known to be present in circumstellar envelopes around evolved stars and it has also been detected in a handful of star-forming regions with evidence of outflows, like Sgr B2, Orion KL, and more recently, L1157-B1. The kinetics of reactions involving SiS is very poorly known and here we revisit the chemistry of SiS in space by studying some potentially important reactions of the formation and destruction of this molecule. We calculated ab initio potential energy surfaces of the SiOS system and computed rate coefficients in the temperature range of 50–2500 K for the reaction of the destruction of SiS in collisions with atomic O, and of its formation, through the reaction between Si and SO. We find that both of the reactions are rapid, with rate coefficients of a few times 10⁻¹⁰ cm³ s⁻¹, almost independent of temperature. In the reaction between Si and SO, SiO production is 5–7 times more efficient than SiS formation. The reaction of SiS with O atoms can play an important role in destroying SiS in envelopes around evolved stars. We built a simple chemical model of a postshock gas to study the chemistry of SiS in protostellar outflows and we found that SiS forms with a lower abundance and later than SiO, that SiS is efficiently destroyed through reaction with O, and that the main SiS-forming reactions are Si + SO and Si + SO₂.

Kazuhiro D. Kanagawa^1,2, Hidekazu Tanaka³, and Ewa Szuszkiewicz¹
The Astrophysical Journal 861, 140 Link to Article [https://doi.org/10.3847/1538-4357/aac8d9]
¹Institute of Physics and CASA*, Faculty of Mathematics and Physics, University of Szezecin, Wielkopolska 15, PL-70-451 Szczecin, Poland
²Research Center for the Early Universe, Graduate School of Science, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
³Astronomical Institute, Tohoku University, Sendai, Miyagi 980-8578, Japan

A large planet orbiting a star in a protoplanetary disk opens a density gap along its orbit due to the strong disk–planet interaction and migrates with the gap in the disk. It is expected that in the ideal case, a gap-opening planet migrates at the viscous drift speed, which is referred to as type II migration. However, recent hydrodynamic simulations have shown that, in general, the gap-opening planet is not locked to the viscous disk evolution. A new physical model is required to explain the migration speed of such a planet. For this reason, we re-examined the migration of a planet in the disk, by carrying out the two-dimensional hydrodynamic simulations in a wide parameter range. We have found that the torque exerted on the gap-opening planet depends on the surface density at the bottom of the gap. The planet migration slows down as the surface density of the bottom of the gap decreases. Using the gap model developed in our previous studies, we have constructed an empirical formula of the migration speed of the gap-opening planets, which is consistent with the results given by the hydrodynamic simulations performed by us and other researchers. Our model easily explains why the migration speed of the gap-opening planets can be faster than the viscous gas drift speed. It can also predict the planet mass at which the type I migration is no longer adequate due to the gap development in the disk, providing a gap formation criterion based on planetary migration.