Alexis Bouquet^1,2, Christopher R. Glein¹, and J. Hunter Waite Jr.^1,3
Astrophysical Journal 873, 28 Link to Article [DOI: 10.3847/1538-4357/ab0100 ]
¹Space Science and Engineering Division, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX, 78238, USA
²Aix Marseille Université, CNRS, CNES, LAM, Marseille, France
³Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, TX, USA

We study the effect of adsorption of volatile organic compounds (VOCs) in Enceladus’ geysers, both onto the ice grains ejected in the plumes, and onto the ice walls of the cracks connecting Enceladus’ internal ocean to its surface. We use a model of adsorption/desorption based on the Polanyi–Wiegner equation and experimental values of binding energies (energy of desorption E _des) of the adsorbed compounds to water ice. We find that under conditions expected at Enceladus, the process of adsorption tends to ensure that the VOCs with the highest binding energy are over-represented on the ice surface, even if their abundance is comparatively lower than those of other compounds. We find that VOCs with E _des ≤ 0.5 eV are insignificantly affected by adsorption while compounds with E _des ≥ 0.7 eV are readily retained on the surface and compete to occupy most of the adsorption sites. We also deduce that ice grains falling back onto the surface are likely to retain most of the molecules adsorbed on their surface. The implication is that remote observation or sampling of the ice in the cracks or of the surface around it would show a mixture of VOCs that would not be representative of the gas phase of the plumes, with the high E _des VOCs dominating the adsorbed phase.

Wei Zhu (祝伟)
Astrophysical Journal 873, 8 Link to Article [DOI: 10.3847/1538-4357/ab0205 ]
Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George Street, Toronto, ON M5S 3H8, Canada

We study the influence of stellar metallicity on the fraction of stars with planets (i.e., the occurrence rate of planetary systems) and the average number of planets per star (i.e., the occurrence rate of planets). The former directly reveals the planet formation efficiency, whereas the latter reveals the final product of formation and evolution. We show that these two occurrence rates have different dependences on stellar metallicity. Specifically, the fraction of stars with planets rises gradually with metallicity, from ~25% to ~36% for 0.4 dex of [Fe/H] for all Kepler-like planets (period P < 400 days and radius ). The average number of planets per star reaches a plateau (or possibly starts declining) at [Fe/H]  0.1. This is plausibly caused by the emergence of distant giant planets at high metallicities, given that the close-in small planets and the distant giants preferentially coexist in the same system.

¹Allan H. Treiman,²Michael J. Kulis,³Allen F. Glazner
American Mineralogist 104, 370-384 Link to Article [https://doi.org/10.2138/am-2019-6652]
¹Lunar and Planetary Institute, Universities Space Research Association, 3600 Bay Area Boulevard, Houston, Texas 77058, U.S.A. Orcid 0000-0002-8073-2839
²Extreme Environments Laboratory, NASA Glenn Research Center, 2100 Brookpark Road, Cleveland, Ohio 44135, U.S.A.
³Department of Geological Sciences, University of North Carolina, Chapel Hill, North Carolina 27599, U.S.A. Orcid 0000-0002-3111-5885
Copyright: The Mineralogical Society of America

Magnesium aluminate spinel, (Mg,Fe)Al₂O₄, is uncommon in lunar rocks but petrologically significant. Recent near-infrared spectra of the Moon have delineated regions where spinel is the only ferromagnesian mineral; the rock is inferred to be spinel anorthosite. One hypothesis is that significant pressure is required for spinel formation; another is that spinel-bearing rocks form by low-pressure assimilation of highlands anorthosite into olivine-rich basaltic (i.e., picritic) magmas. Here, we evaluate the heat (i.e., enthalpy) required for this assimilation process. Magma compositions are the picritic Apollo 14 B green glass and an estimation of the magma parental to Mg-suite cumulate rocks. From calculated enthalpy-composition phase diagrams, assimilation of anorthite into either magma cannot produce spinel anorthosite unless the anorthite is already hotter than ~1300 °C. For cooler anortho-site, assimilation will produce olivine- and/or pyroxene-bearing rocks. Such hot anorthite could be produced by the nearby passage of large volumes of magma, but this is not obviously consistent with occurrences of spinel far from outcrops of basaltic rocks. Hot anorthosite could also be produced by global tidal flexure; that mechanism could have only been efficient early in lunar history when a solid anorthosite crust floated above an evolved magma ocean, and it is not clear how picritic magma could pass through the magma ocean to interact with anorthite in the crust. On the other hand, spinel-bearing anorthosite can form directly upon cooling of superliquidus melts of anorthite-rich composition. Such superliquidus melts can be generated by impact events; this mechanism seems likely, given the Moon’s ubiquity of impact craters, abundance of impact-metamorphosed lunar rocks, and common presence in lunar regolith of impact glasses (quenched superliquidus impact melts) of appropriate compositions. High pressure does stabilize spinel in basaltic and peridotitic systems, but available models do not permit quantitative evaluation of the effects of pressure on the enthalpy required for assimilation. Near the lunar surface, the most likely process of spinel formation is rapid crystallization of impact melts of anorthosite + picrite or peridotite compositions. The presence of spinel anorthosite on the walls and central peaks of impact craters results from rapid cooling and partial crystallization of superliquidus melts produced in the impacts, and not from uplift of deep material to the Moon’s surface.

Motohiko Kusakabe^1,2, Myung-Ki Cheoun^1,2,3, K. S. Kim⁴, Masa-aki Hashimoto⁵, Masaomi Ono⁶, Ken’ichi Nomoto⁷, Toshio Suzuki^2,8, Toshitaka Kajino^1,2,9, and Grant J. Mathews^2,10
Astrophysical Journal 872, 164 Link to Article [DOI: 10.3847/1538-4357/aafc35 ]
¹School of Physics, and International Research Center for Big-Bang Cosmology and Element Genesis, Beihang University, 37, Xueyuan Rd., Haidian-qu, Beijing 100083, People’s Republic of China
²National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
³Department of Physics and Origin of Matter and Evolution of Galaxy (OMEG) Institute, Soongsil University, Seoul 156-743, Republic of Korea
⁴School of Liberal Arts and Science, Korea Aerospace University, Goyang 412-791, Republic of Korea
⁵Kyushu University, Hakozaki, Fukuoka 812-8581, Japan
⁶RIKEN, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan
⁷Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8583, Japan
⁸Department of Physics, College of Humanities and Science, Nihon University, Sakurajosui 3-25-40, Setagaya-ku, Tokyo 156-8550, Japan
⁹Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
¹⁰Center for Astrophysics, Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA

We reinvestigate effects of neutrino oscillations on the production of ⁷Li and ¹¹B in core-collapse supernovae (SNe). During the propagation of neutrinos from the proto–neutron star, their flavors change, and the neutrino reaction rates for spallation of ¹²C and ⁴He are affected. In this work, corrected neutrino spallation cross sections for ⁴He and ¹²C are adopted. Initial abundances involving heavy s-nuclei and other physical conditions are derived in a new calculation of the SN 1987A progenitor in which the effects of the progenitor metallicity are included. A dependence of the SN nucleosynthesis and final yields of ⁷Li and ¹¹B on the neutrino mass hierarchy are shown in several stellar locations. In the normal hierarchy case, the charged-current (CC) reaction rates of are enhanced, and yields of proton-rich nuclei, along with ⁷Be and ¹¹C, are increased. In the inverted hierarchy case, the CC reaction rates of are enhanced, and yields of neutron-rich nuclei, along with ⁷Li and ¹¹B, are increased. We find that variation of the metallicity modifies the yields of ⁷Li, ⁷Be, ¹¹B, and ¹¹C. This effect is caused by changes in the neutron abundance during SN nucleosynthesis. Therefore, accurate calculations of Li and B production in SNe should take into account the metallicity of progenitor stars.

Yudong Luo^1,2,3, Toshitaka Kajino^1,2,3, Motohiko Kusakabe^1,3, and Grant J. Mathews^1,4
Astrophysical Journal 872, 172 Link to Article [DOI: 10.3847/1538-4357/ab0088 ]
¹National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
²Department of Astronomy, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
³School of Physics and Nuclear Energy Engineering, and International Research Center for Big-Bang Cosmology and Element Genesis, Beihang University 37, Xueyuan Road, Haidian-qu, Beijing 100083, People’s Republic of China
⁴Center for Astrophysics, Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA

We investigate the effect on the Big Bang nucleosynthesis (BBN) from the presence of a stochastic primordial magnetic field (PMF) whose strength is spatially inhomogeneous. We assume a uniform total energy density and a Gaussian distribution of field strength. In this case, domains of different temperatures exist in the BBN epoch due to variations in the local PMF. We show that in such a case, the effective distribution function of particle velocities averaged over domains of different temperatures deviates from the Maxwell–Boltzmann distribution. This deviation is related to the scale invariant strength of the PMF energy density ρ _Bc and the fluctuation parameter σ _B. We perform BBN network calculations taking into account the PMF strength distribution and deduce the element abundances as functions of the baryon-to-photon ratio η, ρ _Bc, and σ _B. We find that the fluctuations of the PMF reduce the ⁷Be production and enhance D production. We analyze the averaged thermonuclear reaction rates compared with those of a single temperature and find that the averaged charged-particle reaction rates are very different. Finally, we constrain the parameters ρ _Bcand σ _B from observed abundances of ⁴He and D and find that the ⁷Li abundance is significantly reduced. We also find that if the η value during BBN was larger than the present-day value due to a dissipation of the PMF or a radiative decay of exotic particles after BBN or if the stellar depletion of ⁷Li occurred, abundances of all light elements can be consistent with observational constraints.

¹Takashi Yoshizaki, ¹Daisuke Nakashima, ¹Tomoki Nakamura,²Changkun Park, ³Naoya Sakamoto, ¹Hatsumi Ishida, ⁴Shoichi Itoh
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.02.034]
¹Department of Earth Science, Graduate School of Science, Tohoku University, Aoba, Sendai, Miyagi 980-8578, Japan
²Division of Earth-System Sciences, Korea Polar Research Institute, Incheon 21990, South Korea
³Creative Research Institution Sousei, Hokkaido University, Kita, Sapporo 001-0021, Japan
⁴Department of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Sakyo, Kyoto 606-8502, Japan
Copyright Elsevier

Ultrarefractory (UR) phases in calcium-aluminum-rich inclusions (CAIs) could have formed at higher temperature compared to common CAI minerals and thus they potentially provide constraints on very high temperature processes in the solar nebula. We report a detailed characterization of the mineralogy, petrology and oxygen isotopic composition of an UR phase davisite (CaScAlSiO₆) bearing CAI from a reduced type CV chondrite. The CAI is an irregular-shaped, compound inclusion that consists of five units that are composed of melilite + spinel + Al,Ti-rich pyroxene ±perovskite with various modal abundances of minerals and lithologies, and surrounded by the Wark-Lovering (WL) rim. Davisite occurs only in one lithological unit that consists of three chemically and isotopically distinct parts: i) 16O-poor (-20‰⩽δ18O ⩽0‰) regions with reversely-zoned melilite and davisite; ii) 16O-rich (-50‰⩽δ18O ⩽-40‰) regions consisting of unzoned, gehlenitic melilite, Al,Ti-rich diopside and spinel; and iii) spinel framboids composed of 16O-rich spinel and 16O-poor melilite. Absence of secondary iron- and/or alkali-rich phases, occurrence of low-iron, manganese-enriched (LIME) olivine, and random distribution of the oxygen isotopic heterogeneity indicate that primitive chemical and isotopic compositions are preserved in the inclusion. The occurrence of chemical and isotopic heterogeneities with sharp boundaries in the CAI indicates formation of the inclusion by an aggregation of mineral assemblages formed and processed separately at different time and/or space in the solar nebula. Although isotope exchange between 16O-rich solids and 16O-poor gases prior to the final agglomeration of the CAI cannot be ruled out, we suggest that modification of chemical and isotopic composition of porous CAI precursors or aggregation of isotopically distinct mineral assemblages are alternative scenarios for the origin of oxygen isotopic heterogeneity in CAIs. In either case, coexistence of spatially and/or temporally distinct 16O-rich and 16O-poor gaseous reservoirs at the earliest stage of the solar system formation is required. The grain-scale oxygen isotopic disequilibrium in the CAI indicate that post-formation heating of the inclusion (i.e., the WL rim formation event) was short (e.g., ≲103hours at 1400 K; ≲105hours at 1100 K), which can be achieved by rapid outward transport of the CAI. High Ti³⁺/Ti^tot ratios of pyroxene from CAI interior and the rim and LIME composition of the olivine rim document that the entire CAI formation process took place under highly reducing conditions.

Liisa-Ida Sorsa¹, Mika Takala¹, Patrick Bambach², Jakob Deller², Esa Vilenius², and Sampsa Pursiainen¹
Astrophysical Journal 872, 44 Link to Article [DOI: 10.3847/1538-4357/aafba2 ]
¹Laboratory of Mathematics, Tampere University, P.O. Box 553, FI-33101 Tampere, Finland
²Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, D-37077 Göttingen, Germany

In this paper, we investigate full-wave computed radar tomography (CRT) using a rubble-pile asteroid model in which a realistic shape (Itokawa) is coupled with a synthetic material composition and structure model. The aim is to show that sparse bistatic radar measurements can distinguish details inside a complex-structured rubble-pile asteroid. The results obtained suggest that distinct local permittivity distribution changes such as surface layers, voids, low-permittivity anomalies, high-permittivity boulders, and cracks can be detected with bistatic CRT, when the total noise level in the data is around −10 dB with respect to the signal amplitude. Moreover, the bistatic measurement setup improves the robustness of the inversion compared to the monostatic case. Reconstructing the smooth Gaussian background distribution was found to be difficult with the present approach, suggesting that complementary techniques, such as gravimetry, might be needed to improve the reliability of the inference in practice.

Shoji Mori¹, Xue-Ning Bai², and Satoshi Okuzumi¹
Astrophysical Journal 872, 98 Link to Article [DOI: 10.3847/1538-4357/ab0022 ]
¹Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro-ku, Tokyo, 152-8551, Japan
²Institute for Advanced Study and Tsinghua Center for Astrophysics, Tsinghua University, Beijing 100084, People’s Republic of China

The gas temperature in protoplanetary disks (PPDs) is determined by a combination of irradiation heating and accretion heating, with the latter conventionally attributed to turbulent dissipation. However, recent studies have suggested that the inner disk (a few au) is largely laminar, with accretion primarily driven by magnetized disk winds, as a result of nonideal magnetohydrodynamic (MHD) effects from weakly ionized gas, suggesting an alternative heating mechanism by Joule dissipation. We perform local stratified MHD simulations including all three nonideal MHD effects (ohmic, Hall, and ambipolar diffusion) and investigate the role of Joule heating and the resulting disk vertical temperature profiles. We find that in the inner disk, as ohmic and ambipolar diffusion strongly suppress electrical current around the midplane, Joule heating primarily occurs at several scale heights above the midplane, making the midplane temperature much lower than that with the conventional viscous heating model. Including the Hall effect, Joule heating is enhanced/reduced when the magnetic fields threading the disks are aligned/anti-aligned with the disk rotation, but it is overall ineffective. Our results further suggest that the midplane temperature in the inner PPDs is almost entirely determined by irradiation heating, unless viscous heating can trigger thermal ionization in the disk innermost region to self-sustain magnetorotational instability turbulence.

¹C.J.Bierson,¹F.Nimmo
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.01.027]
¹Department of Earth and Planetary Sciences, UC Santa Cruz, Santa Cruz, CA 95064, USA
Copyright Elsevier

Telescopic observations of Kuiper Belt objects have enabled bulk density determinations for 17 objects. These densities vary systematically with size, perhaps suggesting systematic variations in bulk composition. We find this trend can be explained instead by variations in porosity arising from the higher pressures and warmer temperatures in larger objects. We are able to match the density of 14 of 17 KBOs within their 2σ errors with a constant rock mass fraction of 70%, suggesting a compositionally homogeneous, rock-rich reservoir. Because early 26Al would have removed too much porosity in small (∼ 100 km) KBOs we find the minimum formation time to be 4 Myr after solar system formation. This suggests that coagulation, and not gravitational collapse, was the dominant mechanism for KBO formation, or the gas disk lingered in the outer solar system. We also use this model to make predictions for the density of Makemake, 2007 OR10, and MU69

Pengfei Tang and Liping Jin
Astrophysical Journal 871, 222 Link to Article [DOI: 10.3847/1538-4357/aafb6f ]
College of Physics, Jilin University Changchun, Jilin 130012, People’s Republic of China

We construct an analytical model of gravitationally unstable protoplanetary disks consisting of three regions: the inner region where the internal dissipation dominates the heating, the intermediate region where the central protostar irradiation dominates, and the outer region where background irradiation dominates. We use this analytical model and an evolutionary numerical model of protoplanetary disks to calculate the cooling time and find out the location of the isothermal region. We investigate the effects of the isothermal region on the disk instability model for giant planet formation. We find that the fragmentation region found in previous studies is contained in the isothermal region of a disk. In this case, the cooling time criterion is not applicable for fragmentation. Therefore, the constraint on the disk instability model caused by the cooling time criterion should be relieved. The viability of the disk instability model is improved. When the isothermal region is considered, the inner boundary of the fragmentation region is extended inward to ~20 au. We also show that if the contribution of the protostar irradiation to the disk surface temperature can be included in the cooling rate, the fragmentation region defined by the cooling time criterion can be extended inward to ~26 au. We find that a disk tends to be isothermal in the region where the cooling time criterion is satisfied. We also find that at the later stage of disk instability, the inner boundary of the fragmentation region is determined by the inner boundary of the gravitationally unstable region.