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.

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.