Takashi Ishihara¹, Naoki Kobayashi², Kei Enohata², Masayuki Umemura³, and Kenji Shiraishi⁴

Astrophysical Journal 854, 81 Link to Article [DOI: 10.3847/1538-4357/aaa976]
¹Graduate School of Environmental and Life Science, Okayama University, Okayama 700-8530, Japan
²Department of Computational Science and Engineering, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan
³Center for Computational Sciences, University of Tsukuba, Tsukuba 305-8577, Japan
⁴Institute of Materials and Systems for Sustainability, Nagoya University, Nagoya 464-8601, Japan

The coagulation of dust particles is a key process in planetesimal formation. However, the radial drift and bouncing barriers are not completely resolved, especially for silicate dust. Since the collision velocities of dust particles are regulated by turbulence in a protoplanetary disk, turbulent clustering should be properly treated. To that end, direct numerical simulations (DNSs) of the Navier–Stokes equations are requisite. In a series of papers, Pan & Padoan used a DNS with Reynolds number Re ~ 1000. Here, we perform DNSs with up to Re = 16,100, which allow us to track the motion of particles with Stokes numbers of 0.01  St  0.2 in the inertial range. Through the DNSs, we confirm that the rms relative velocity of particle pairs is smaller by more than a factor of two, compared to that by Ormel & Cuzzi. The distributions of the radial relative velocities are highly non-Gaussian. The results are almost consistent with those by Pan & Padoan or Pan et al. at low Re. Also, we find that the sticking rates for equal-sized particles are much higher than those for different-sized particles. Even in the strong-turbulence case with α-viscosity of 10⁻², the sticking rates are as high as 50% and the bouncing probabilities are as low as ~10% for equal-sized particles of St  0.01. Thus, turbulent clustering plays a significant role in the growth of centimeter-sized compact aggregates (pebbles) and also enhances the solid abundance, which may lead to the streaming instability in a disk.

Lev Arzamasskiy¹ and Roman R. Rafikov^2,3

Astrophysical Journal 854, 84 Link to Article [DOI: 10.3847/1538-4357/aaa8e8]
¹Department of Astrophysical Sciences, Princeton University, Ivy Lane, Princeton, NJ 08540, USA
²Centre for Mathematical Sciences, Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Wilberforce Road, Cambridge CB3 0WA, UK
³Institute for Advanced Study, Einstein Drive, Princeton, NJ 08540, USA

Spiral density waves are known to exist in many astrophysical disks, potentially affecting disk structure and evolution. We conduct a numerical study of the effects produced by a density wave, evolving into a shock, on the characteristics of the underlying disk. We measure the deposition of angular momentum in the disk by spiral shocks of different strengths and verify the analytical prediction of Rafikov for the behavior of this quantity, using shock amplitude (which is potentially observable) as the input variable. Good agreement between theory and numerics is found as we vary the shock amplitude (including highly nonlinear shocks), disk aspect ratio, equation of state, radial profiles of the background density and temperature, and pattern speed of the wave. We show that high numerical resolution is required to properly capture shock-driven transport, especially at small wave amplitudes. We also demonstrate that relating the local mass-accretion rate to shock dissipation in rapidly evolving disks requires accounting for the time-dependent contribution to the angular momentum budget caused by the time dependence of the radial pressure support. We provide a simple analytical prescription for the behavior of this contribution and demonstrate its excellent agreement with the simulation results. Using these findings, we formulate a theoretical framework for studying the one-dimensional (in radius) evolution of shock-mediated accretion disks, which can be applied to a variety of astrophysical systems.

Kunihiro Myojo¹, Tetsuya Yokoyama¹, Satoki Okabayashi¹, Shigeyuki Wakaki², Naoji Sugiura³, and Hikaru Iwamori^1,4
Astrophysical Journal 853, 48 Link to Article [DOI: 10.3847/1538-4357/aa9f2e]
¹Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan
²Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology, 200 Monobe Otsu, Nankoku City, Kochi, Japan
³Emeritus professor, Department of Earth and Planetary Science, University of Tokyo, Japan
⁴Japan Agency for Marine-Earth Science and Technology, Japan

Nucleosynthetic isotope anomalies in meteorites are useful for investigating the origin of materials in the protoplanetary disk and dynamical processes of planetary formation. In particular, calcium and aluminum-rich inclusions (CAIs) found in chondrites are key minerals for decoding the initial conditions of the solar system before the accretion of small planetary bodies. In this study, we report isotopic analyses for three Allende CAIs, fluffy type A (FTA), type B, and fine-grained spinel rich (FS) inclusions, with a specific emphasis on the measurements of ⁸⁴Sr/⁸⁶Sr ratios. It was found that the average μ ⁸⁴Sr values (10⁶ relative deviations from a standard material) were 175, 129, and 56 ppm for the samples of FTA, type B, and FS inclusions, respectively. Additionally, the FTA samples exhibited heterogeneous μ ⁸⁴Sr values, while those for the type B and FS inclusions were homogeneous within individual inclusions. The elevated μ ⁸⁴Sr values were most likely explained by the relative enrichment of r-process nuclides in the CAI formation region. The variation of μ ⁸⁴Sr values between the FTA and type B inclusions, as well as within the FTA inclusion, suggests the presence of multiple CAI source reservoirs with distinct isotopic compositions, which is either inherited from isotopic heterogeneity in the molecular cloud or caused by the selective destruction of r-process-enriched supernova grains via nebular thermal processing. On the other hand, the reaction between a refractory precursor of the FS inclusion and a gaseous reservoir enriched in Mg, Si, and ¹⁶O resulted in the lowest μ ⁸⁴Sr values for the FS inclusion.

J. Escobar-Cerezo¹, A. Penttilä², T. Kohout^2,3, O. Muñoz¹, F. Moreno¹, and K. Muinonen^2,4
Astrophysical Journal 853, 71 Link to Article [DOI: 10.3847/1538-4357/aaa24d]
¹Instituto de Astrofísica de Andalucía, CSIC, Glorieta de la Astronomía s/n, E-18008 Granada, Spain
²Department of Physics, P.O. Box 64, FI-00014 University of Helsinki, Finland
³Institute of Geology, The Czech Academy of Sciences, Prague, Czech Republic
⁴National Land Survey of Finland, Finnish Geospatial Research Institute, P.O. Box 84, FI-00521 Helsinki, Finland

Lunar soil spectra differ from pulverized lunar rocks spectra by reddening and darkening effects, and shallower absorption bands. These effects have been described in the past as a consequence of space weathering. In this work, we focus on the effects of nanophase iron (npFe⁰) inclusions on the experimental reflectance spectra of lunar regolith particles. The reflectance spectra are computed using SIRIS3, a code that combines ray optics with radiative-transfer modeling to simulate light scattering by different types of scatterers. The imaginary part of the refractive index as a function of wavelength of immature lunar soil is derived by comparison with the measured spectra of the corresponding material. Furthermore, the effect of adding nanophase iron inclusions on the reflectance spectra is studied. The computed spectra qualitatively reproduce the observed effects of space weathered lunar regolith.

Alan M. Gersch, Lori M. Feaga, and Michael F. A’Hearn¹
Astrophysical Journal 854, 149 Link to Article [DOI: 10.3847/1538-4357/aa9795]
Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
¹Deceased.

We have adapted Coupled Escape Probability, a new exact method of solving radiative transfer problems, for use in asymmetrical spherical situations for use in modeling optically thick cometary comae. Here we present the extension of our model and corresponding results for two additional primary volatile species of interest, H₂O and CO₂, in purely theoretical comets. We also present detailed modeling and results for the specific examples of CO, H₂O, and CO₂ observations of C/2009 P1 Garradd by the Deep Impact flyby spacecraft.

Rainer Schräpler¹, Jürgen Blum¹, Sebastiaan Krijt^2,3, and Jan-Hendrik Raabe¹
Astrophysical Journal 853, 74 Link to Article [DOI: 10.3847/1538-4357/aaa0d2]
¹Institut für Geophysik und extraterrestrische Physik, Technische Universität Braunschweig, Mendelssohnstr. 3, D-38106 Braunschweig, Germany
²Department of the Geophysical Sciences, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, USA
³Hubble Fellow.

In a protoplanetary disk, dust aggregates in the μm to mm size range possess mean collision velocities of 10–60 m s⁻¹ with respect to dm- to m-sized bodies. We performed laboratory collision experiments to explore this parameter regime and found a size- and velocity-dependent threshold between erosion and growth. By using a local Monte Carlo coagulation calculation and along with a simple semi-analytical timescale approach, we show that erosion considerably limits particle growth in protoplanetary disks and leads to a steady-state dust-size distribution from μm- to dm-sized particles.

Erik Weaver, Andrea Isella, and Yann Boehler
Astrophysical Journal 853, 113 Link to Article [DOI: 10.3847/1538-4357/aaa481]
Department of Physics and Astronomy, Rice University, 6100 Main Street, MS-108, Houston, TX 77005 USA

The accurate measurement of temperature in protoplanetary disks is critical to understanding many key features of disk evolution and planet formation, from disk chemistry and dynamics, to planetesimal formation. This paper explores the techniques available to determine temperatures from observations of single, optically thick molecular emission lines. Specific attention is given to issues such as the inclusion of optically thin emission, problems resulting from continuum subtraction, and complications of real observations. Effort is also made to detail the exact nature and morphology of the region emitting a given line. To properly study and quantify these effects, this paper considers a range of disk models, from simple pedagogical models to very detailed models including full radiative transfer. Finally, we show how the use of the wrong methods can lead to potentially severe misinterpretations of data, leading to incorrect measurements of disk temperature profiles. We show that the best way to estimate the temperature of emitting gas is to analyze the line peak emission map without subtracting continuum emission. Continuum subtraction, which is commonly applied to observations of line emission, systematically leads to underestimation of the gas temperature. We further show that once observational effects such as beam dilution and noise are accounted for, the line brightness temperature derived from the peak emission is reliably within 10%–15% of the physical temperature of the emitting region, assuming optically thick emission. The methodology described in this paper will be applied in future works to constrain the temperature, and related physical quantities, in protoplanetary disks observed with ALMA.

N. G. Rudraswami¹, D. Fernandes¹, A. K. Naik¹, M. Shyam Prasad¹, J. D. Carrillo-Sánchez², J. M. C. Plane², W. Feng^2,3, and S. Taylor⁴
Astrophysical Journal 853, 38 Link to Article [DOI: 10.3847/1538-4357/aaa5f7]
¹National Institute of Oceanography (Council of Scientific and Industrial Research), Dona Paula, Goa 403004, India
²School of Chemistry, University of Leeds, Leeds LS2 9JT, UK
³NCAS, School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK
⁴Cold Regions Research and Engineering Laboratory, 72 Lyme Road, Hanover, NH 03755–1290, USA

All known extraterrestrial dust (micrometeoroids) entering the Earth’s atmosphere is anticipated to have a significant contribution from ordinary chondritic precursors, as seen in meteorites, but this is an apparent contradiction that needs to be addressed. Ordinary chondrites represent a minor contribution to the overall meteor influx compared to carbonaceous chondrites, which are largely dominated by CI and/or CM chondrites. However, the near-Earth asteroid population presents a scenario with sufficient scope for generation of dust-sized debris from ordinary chondritic sources. The bulk chemical composition of 3255 micrometeorites (MMs) collected from Antarctica and deep-sea sediments has shown Mg/Si largely dominated by carbonaceous chondrites, and less than 10% having ordinary chondritic precursors. The chemical ablation model is combined with different initial chondritic compositions (CI, CV, L, LL, H), and the results clearly indicate that high-density (≥2.8 g cm⁻³) precursors, such as CV and ordinary chondrites in the size range 100–700 μm and zenith angle 0°–70°, ablate at much faster rates and lose their identity even before reaching the Earth’s surface and hence are under-represented in our collections. Moreover, their ability to survive as MMs remains grim for high-velocity micrometeoroids (>16 km s⁻¹). The elemental ratio for CV and ordinary chondrites are also similar to each other irrespective of the difference in the initial chemical composition. In conclusion, MMs belonging to ordinary chondritic precursors’ concentrations may not be insignificant in thermosphere, as they are found on Earth’s surface.

C. Travaglio^1,2, T. Rauscher^3,4,11, A. Heger^5,6,7,8,12, M. Pignatari^8,9, and C. West^7,8,10
Astrophysical Journal 854, 18 Link to Article [DOI: 10.3847/1538-4357/aaa4f7]
¹INFN—Istituto Nazionale Fisica Nucleare, Turin, Italy
²B2FH Association—Turin, Italy
³Department of Physics, University of Basel, Switzerland
⁴Centre for Astrophysics Research, University of Hertfordshire, UK
⁵Monash Centre for Astrophysics, Monash University, Melbourne, Victoria, 3800, Australia
⁶Astronomy Department, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China
⁷School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA
⁸Joint Institute for Nuclear Astrophysics—Center for the Evolution of the Elements, USA
⁹E.A. Milne Centre for Astrophysics, University of Hull, HU6 7RX, UK
¹⁰Center for Academic Excellence, Metropolitan State University, St. Paul, MN, 55106, USA
¹¹UK Network for Bridging Disciplines of Galactic Chemical Evolution (BRIDGCE), https://www.bridgce.net.
¹²The NuGrid Collaboration, http://www.nugridstars.org.

The production of the heavy stable proton-rich isotopes between ⁷⁴Se and ¹⁹⁶Hg—the p nuclides—is due to the contribution from different nucleosynthesis processes, activated in different types of stars. Whereas these processes have been subject to various studies, their relative contributions to Galactic chemical evolution (GCE) are still a matter of debate. Here we investigate for the first time the nucleosynthesis of p nuclides in GCE by including metallicity and progenitor mass-dependent yields of core-collapse supernovae (ccSNe) into a chemical evolution model. We used a grid of metallicities and progenitor masses from two different sets of stellar yields and followed the contribution of ccSNe to the Galactic abundances as a function of time. In combination with previous studies on p-nucleus production in thermonuclear supernovae (SNIa), and using the same GCE description, this allows us to compare the respective roles of SNeIa and ccSNe in the production of p-nuclei in the Galaxy. The γprocess in ccSN is very efficient for a wide range of progenitor masses (13 M _⊙–25 M _⊙) at solar metallicity. Since it is a secondary process with its efficiency depending on the initial abundance of heavy elements, its contribution is strongly reduced below solar metallicity. This makes it challenging to explain the inventory of the p nuclides in the solar system by the contribution from ccSNe alone. In particular, we find that ccSNe contribute less than 10% of the solar p nuclide abundances, with only a few exceptions. Due to the uncertain contribution from other nucleosynthesis sites in ccSNe, such as neutrino winds or α-rich freeze out, we conclude that the light p-nuclides ⁷⁴Se, ⁷⁸Kr, ⁸⁴Sr, and ⁹²Mo may either still be completely or only partially produced in ccSNe. The γ-process accounts for up to twice the relative solar abundances for ⁷⁴Se in one set of stellar models and ¹⁹⁶Hg in the other set. The solar abundance of the heaviest p nucleus ¹⁹⁶Hg is reproduced within uncertainties in one set of our models due to photodisintegration of the Pb isotopes ^208,207,206Pb. For all other p nuclides, abundances as low as 2% of the solar level were obtained.

Ryuki Hyodo^1,2, Pascal Rosenblatt^3,3, Hidenori Genda¹, and Sébastien Charnoz²
Astrophysical Journal 851, 122 Link to Article [DOI: 10.3847/1538-4357/aa9984]
¹Earth-Life Science Institute/Tokyo Institute of Technology, 2-12-1 Tokyo, Japan
²Institut de Physique du Globe/Université Paris Diderot/CNRS, F-75005 Paris, France
³Royal Observatory of Belgium, B-1180 Brussels, Belgium

Phobos and Deimos are the two small Martian moons, orbiting almost on the equatorial plane of Mars. Recent works have shown that they can accrete within an impact-generated inner dense and outer light disk, and that the same impact potentially forms the Borealis basin, a large northern hemisphere basin on the current Mars. However, there is no a priori reason for the impact to take place close to the north pole (Borealis present location), nor to generate a debris disk in the equatorial plane of Mars (in which Phobos and Deimos orbit). In this paper, we investigate these remaining issues on the giant impact origin of the Martian moons. First, we show that the mass deficit created by the Borealis impact basin induces a global reorientation of the planet to realign its main moment of inertia with the rotation pole (True Polar Wander). This moves the location of the Borealis basin toward its current location. Next, using analytical arguments, we investigate the detailed dynamical evolution of the eccentric inclined disk from the equatorial plane of Mars that is formed by the Martian-moon-forming impact. We find that, as a result of precession of disk particles due to the Martian dynamical flattening J ₂ term of its gravity field and particle–particle inelastic collisions, eccentricity and inclination are damped and an inner dense and outer light equatorial circular disk is eventually formed. Our results strengthen the giant impact origin of Phobos and Deimos that can finally be tested by a future sample return mission such as JAXA’s Martian Moons eXploration mission.