Charli M. Sakari¹ (>10)
Astrophysical Journal Letters 854, L20 Link to Article [DOI: 10.3847/2041-8213/aaa9b4]
¹Department of Astronomy, University of Washington, Seattle, WA 98195-1580, USA

A high-resolution spectroscopic analysis is presented for a new highly r-process-enhanced ([Eu/Fe] = 1.27, [Ba/Eu] = −0.65), very metal-poor ([Fe/H] = −2.09), retrograde halo star, RAVE J153830.9–180424, discovered as part of the R-Process Alliance survey. At V = 10.86, this is the brightest and most metal-rich r-II star known in the Milky Way halo. Its brightness enables high-S/N detections of a wide variety of chemical species that are mostly created by the r-process, including some infrequently detected lines from elements like Ru, Pd, Ag, Tm, Yb, Lu, Hf, and Th, with upper limits on Pb and U. This is the most complete r-process census in a very metal-poor r-II star. J1538–1804 shows no signs of s-process contamination, based on its low [Ba/Eu] and [Pb/Fe]. As with many other r-process-enhanced stars, J1538–1804’s r-process pattern matches that of the Sun for elements between the first, second, and third peaks, and does not exhibit an actinide boost. Cosmo-chronometric age-dating reveals the r-process material to be quite old. This robust main r-process pattern is a necessary constraint for r-process formation scenarios (of particular interest in light of the recent neutron star merger, GW170817), and has important consequences for the origins of r-II stars. Additional r-I and r-II stars will be reported by the R-Process Alliance in the near future.

Siteng Fan and Konstantin Batygin
Astrophysical Journal Letters 851, L37 Link to Article [DOI: 10.3847/2041-8213/aa9f0b]
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA

Over the course of the last decade, the Nice model has dramatically changed our view of the solar system’s formation and early evolution. Within the context of this model, a transient period of planet–planet scattering is triggered by gravitational interactions between the giant planets and a massive primordial planetesimal disk, leading to a successful reproduction of the solar system’s present-day architecture. In typical realizations of the Nice model, self-gravity of the planetesimal disk is routinely neglected, as it poses a computational bottleneck to the calculations. Recent analyses have shown, however, that a self-gravitating disk can exhibit behavior that is dynamically distinct, and this disparity may have significant implications for the solar system’s evolutionary path. In this work, we explore this discrepancy utilizing a large suite of Nice model simulations with and without a self-gravitating planetesimal disk, taking advantage of the inherently parallel nature of graphic processing units. Our simulations demonstrate that self-consistent modeling of particle interactions does not lead to significantly different final planetary orbits from those obtained within conventional simulations. Moreover, self-gravitating calculations show similar planetesimal evolution to non-self-gravitating numerical experiments after dynamical instability is triggered, suggesting that the orbital clustering observed in the distant Kuiper Belt is unlikely to have a self-gravitational origin.

Pablo Benítez-Llambay and Martin E. Pessah
Astrophysical Journal Letters 855, L28 Link to Article [DOI: 10.3847/2041-8213/aab2ae]
Niels Bohr International Academy, Niels Bohr Institute, Blegdamsvej 17, DK-2100 Copenhagen Ø, Denmark

Fast inward migration of planetary cores is a common problem in the current planet formation paradigm. Even though dust is ubiquitous in protoplanetary disks, its dynamical role in the migration history of planetary embryos has not been assessed. In this Letter, we show that the scattered pebble-flow induced by a low-mass planetary embryo leads to an asymmetric dust-density distribution that is able to exert a net torque. By analyzing a large suite of multifluid hydrodynamical simulations addressing the interaction between the disk and a low-mass planet on a fixed circular orbit, and neglecting dust feedback onto the gas, we identify two different regimes, gas- and gravity-dominated, where the scattered pebble-flow results in almost all cases in positive torques. We collect our measurements in a first torque map for dusty disks, which will enable the incorporation of the effect of dust dynamics on migration into population synthesis models. Depending on the dust drift speed, the dust-to-gas mass ratio/distribution, and the embryo mass, the dust-induced torque has the potential to halt inward migration or even induce fast outward migration of planetary cores. We thus anticipate that dust-driven migration could play a dominant role during the formation history of planets. Because dust torques scale with disk metallicity, we propose that dust-driven outward migration may enhance the occurrence of distant giant planets in higher-metallicity systems.

Tom Seccull¹, Wesley C. Fraser¹, Thomas H. Puzia², Michael E. Brown³, and Frederik Schönebeck⁴
Astrophysical Journal Letters 851, L12 Link to Article [DOI: 10.3847/2041-8213/aab3dc]
¹Astrophysics Research Centre, Queen’s University Belfast, Belfast BT7 1NN, UK
²Institute of Astrophysics, Pontificia Universidad Católica de Chile, Av. Vincuña Mackenna 4860, 7820436, Santiago, Chile
³Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
⁴Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstraße 12-14, D-69120 Heidelberg, Germany

Models of the Solar System’s dynamical evolution predict the dispersal of primitive planetesimals from their formative regions among the gas-giant planets due to the early phases of planetary migration. Consequently, carbonaceous objects were scattered both into the outer asteroid belt and out to the Kuiper Belt. These models predict that the Kuiper Belt should contain a small fraction of objects with carbonaceous surfaces, though to date, all reported visible reflectance spectra of small Kuiper Belt Objects (KBOs) are linear and featureless. We report the unusual reflectance spectrum of a small KBO, (120216) 2004 EW₉₅, exhibiting a large drop in its near-UV reflectance and a broad shallow optical absorption feature centered at ~700 nm, which is detected at greater than 4σsignificance. These features, confirmed through multiple epochs of spectral photometry and spectroscopy, have respectively been associated with ferric oxides and phyllosilicates. The spectrum bears striking resemblance to those of some C-type asteroids, suggesting that 2004 EW₉₅ may share a common origin with those objects. 2004 EW₉₅ orbits the Sun in a stable mean motion resonance with Neptune, at relatively high eccentricity and inclination, suggesting it may have been emplaced there by some past dynamical instability. These results appear consistent with the aforementioned model predictions and are the first to show a reliably confirmed detection of silicate material on a small KBO.

J. Martin Laming¹ et al. (>10)
Astrophysical Journal Letters 851, L12 Link to Article [DOI: 10.3847/2041-8213/aa9bf0]
¹Space Science Division, Naval Research Laboratory, Code 7684, Washington, DC 20375, USA

We compare element and isotopic fractionations measured in bulk solar wind samples collected by NASA’s Genesis mission with those predicted from models incorporating both the ponderomotive force in the chromosphere and conservation of the first adiabatic invariant in the low corona. Generally good agreement is found, suggesting that these factors are consistent with the process of solar wind fractionation. Based on bulk wind measurements, we also consider in more detail the isotopic and elemental abundances of O. We find mild support for an O abundance in the range 8.75–8.83, with a value as low as 8.69 disfavored. A stronger conclusion must await solar wind regime-specific measurements from the Genesis samples.

Quan-Zhi Ye (叶泉志)^1,2, Qicheng Zhang³, Michael S. P. Kelley⁴, and Peter G. Brown^5,6
Astrophysical Journal Letters 851, L5 Link to Article [DOI: 10.3847/2041-8213/aa9a34]
¹Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
²Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125, USA
³Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
⁴Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
⁵Department of Physics and Astronomy, The University of Western Ontario, London, ON N6A 3K7, Canada
⁶Centre for Planetary Science and Exploration, The University of Western Ontario, London, ON N6A 5B8, Canada

1I/2017 U1 (‘Oumuamua), a recently discovered asteroid in a hyperbolic orbit, is likely the first macroscopic object of extrasolar origin identified in the solar system. Here, we present imaging and spectroscopic observations of ‘Oumuamua using the Palomar Hale Telescope as well as a search of meteor activity potentially linked to this object using the Canadian Meteor Orbit Radar. We find that ‘Oumuamua exhibits a moderate spectral gradient of , a value significantly lower than that of outer solar system bodies, indicative of a formation and/or previous residence in a warmer environment. Imaging observation and spectral line analysis show no evidence that ‘Oumuamua is presently active. Negative meteor observation is as expected, since ejection driven by sublimation of commonly known cometary species such as CO requires an extreme ejection speed of ~40 m s⁻¹ at ~100 au in order to reach the Earth. No obvious candidate stars are proposed as the point of origin for ‘Oumuamua. Given a mean free path of ~10⁹ ly in the solar neighborhood, ‘Oumuamua has likely spent a very long time in interstellar space before encountering the solar system.

Arshad ALI^1,2, Iffat JABEEN², Neil R. BANERJEE², Gordon R. OSINSKI^2,3, Ian NICKLIN⁴, David GREGORY⁵, and Patrick HERRMANN⁶
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13072]
¹Earth Sciences Research Centre (ESRC), Sultan Qaboos University, Al-Khoudh, Muscat 123, Sultanate of Oman
²Department of Earth Sciences/Centre for Planetary Science and Exploration (CPSX), University of Western Ontario, London,Ontario, Canada
³Department of Physics and Astronomy, University of Western Ontario, London, Ontario, Canada
⁴Department of Natural History, Royal Ontario Museum, Toronto, Ontario N5R 4P5, Canada
⁵230 First Ave., Suite 108, St. Thomas, Ontario, Canada
⁶Pallasite.ca, Toronto, Ontario, Canada
Published by arrangement with John Wiley & Sons

Oxygen isotope measurements of olivine in main group (MG) pallasites by traditional laser fluorination method are associated with some uncertainties including terrestrial weathering, incomplete olivine reaction, and sample state. We improved our laser fluorination approach by pretreating olivine grains with acid to remove terrestrial weathering products and by modifying the sample holder for an efficient and complete laser reaction. Our experiments on Brahin olivine demonstrate that acid‐washing successfully removes the terrestrial weathering with <0.1‰ variation in δ¹⁸O value and, at the same time, improving the ∆¹⁷O value significantly. We also achieved a complete olivine fluorination by employing a custom‐designed sample holder with “V”‐shaped profile having rounded bottom because incomplete/partial reaction of olivine gives comparatively lighter δ¹⁸O values. Using these new techniques, we present precise triple oxygen isotope data (N = 72) of 25 olivine samples separated from main group pallasites. The data are, on average, ~0.5‰ heavier in δ¹⁸O relative to the values published in the literature for the same samples. Critically, the ∆¹⁷O values of MG pallasites and to some extent their Fo‐contents suggest that there are at least two populations of olivine. Based on our improved data set, we propose that MG pallasites potentially have high‐∆¹⁷O‐ and low‐∆¹⁷O‐bearing subgroups that are statistically distinct. The subgroups present average ∆¹⁷O values of −0.166 ± 0.003 (2SE; N = 16) and −0.220 ± 0.003 (2SE; N = 9), respectively. Furthermore, the high‐∆¹⁷O‐bearing subgroup samples trend toward lower Fo‐contents compared to the other subgroup. Taken together, our data provide evidence that argues against a single parent body origin for MG pallasites.

Christian Iliadis^1,2, Lori N. Downen^1,2, Jordi José^3,4, Larry R. Nittler⁵, and Sumner Starrfield⁶
Astrophysical Journal 855, 76 Link to Article [DOI: 10.3847/1538-4357/aaabb6]
¹Department of Physics & Astronomy, University of North Carolina, Chapel Hill, NC 27599-3255, USA
²Triangle Universities Nuclear Laboratory, Durham, NC 27708-0308, USA
³Departament de Física, EEBE, Universitat Politècnica de Catalunya, c/Eduard Maristany 10, E-08930 Barcelona, Spain
⁴Institut d’Estudis Espacials de Catalunya, c/Gran Capità 2-4, Ed. Nexus-201, E-08034 Barcelona, Spain
⁵Department of Terrestrial Magnetism, Carnegie Institution for Science, Washington, DC 20015, USA
⁶Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA

About 30%–40% of classical novae produce dust 20–100 days after the outburst, but no presolar stardust grains from classical novae have been unambiguously identified yet. Although several studies claimed a nova paternity for certain grains, the measured and simulated isotopic ratios could only be reconciled, assuming that the grains condensed after the nova ejecta mixed with a much larger amount of close-to-solar matter. However, the source and mechanism of this potential post-explosion dilution of the ejecta remains a mystery. A major problem with previous studies is the small number of simulations performed and the implied poor exploration of the large nova parameter space. We report the results of a different strategy, based on a Monte Carlo technique, that involves the random sampling over the most important nova model parameters: the white dwarf composition; the mixing of the outer white dwarf layers with the accreted material before the explosion; the peak temperature and density; the explosion timescales; and the possible dilution of the ejecta after the outburst. We discuss and take into account the systematic uncertainties for both the presolar grain measurements and the simulation results. Only those simulations that are consistent with all measured isotopic ratios of a given grain are accepted for further analysis. We also present the numerical results of the model parameters. We identify 18 presolar grains with measured isotopic signatures consistent with a CO nova origin, without assuming any dilution of the ejecta. Among these, the grains G270_2, M11-334-2, G278, M11-347-4, M11-151-4, and Ag2_6 have the highest probability of a CO nova paternity.

Yuri Aikawa¹, Kenji Furuya², Ugo Hincelin³, and Eric Herbst³
Astrophysical Journal 855, 119 Link to Article [DOI: 10.3847/1538-4357/aaad6c]
¹Department of Astronomy, The University of Tokyo, Japan
²Center for Computational Sciences, University of Tsukuba, Japan
³Department of Chemistry, The University of Virginia, Charlottesville, VA, USA

We investigate deuterium chemistry coupled with the nuclear spin-state chemistry of H₂ and in protoplanetary disks. Multiple paths of deuterium fractionation are found; exchange reactions with D atoms, such as HCO⁺ + D, are effective in addition to those with HD. In a disk model with grain sizes appropriate for dark clouds, the freeze-out of molecules is severe in the outer midplane, while the disk surface is shielded from UV radiation. Gaseous molecules, including DCO⁺, thus become abundant at the disk surface, which tends to make their column density distribution relatively flat. If the dust grains have grown to millimeter size, the freeze-out rate of neutral species is reduced and the abundances of gaseous molecules, including DCO⁺ and N₂D⁺, are enhanced in the cold midplane. Turbulent diffusion transports D atoms and radicals at the disk surface to the midplane, and stable ice species in the midplane to the disk surface. The effects of turbulence on chemistry are thus multifold; while DCO⁺ and N₂D⁺ abundances increase or decrease depending on the regions, HCN and DCN in the gas and ice are greatly reduced at the innermost radii, compared to the model without turbulence. When cosmic rays penetrate the disk, the ortho-to-para ratio (OPR) of H₂ is found to be thermal in the disk, except in the cold (10 K) midplane. We also analyze the OPR of and H₂D⁺, as well as the main reactions of H₂D⁺, DCO⁺, and N₂D⁺, in order to analytically derive their abundances in the cold midplane.

David V. BEKAERT¹, Guillaume AVICE^1,2, and Bernard MARTY¹
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13069]
¹Centre de Recherches Petrographiques et Geochimiques, UMR 7358 CNRS—Universite de Lorraine, 15 rue Notre Dame desPauvres, BP 20, 54501 Vandoeuvre-les-Nancy, France
²Present address: Division of Geology and Planetary Sciences, California Institute of Technology, 1200 E. California Blvd,Pasadena, California 91125, USA
Published by arrangement with John Wiley & Sons

Lunar basalt 15016 (~3.3 Ga) is among the most vesicular (50% by volume) basalts recovered by the Apollo missions. We investigated the possible occurrence of indigenous lunar nitrogen and noble gases trapped in vesicles within basalt 15016, by crushing several cm‐sized chips. Matrix/mineral gases were also extracted from crush residues by fusion with a CO₂ laser. No magmatic/primordial component could be identified; all isotope compositions, including those of vesicles, pointed to a cosmogenic origin. We found that vesicles contained ~0.2%, ~0.02%, ~0.002%, and ~0.02% of the total amount of cosmogenic ²¹Ne, ³⁸Ar, ⁸³Kr, and ¹²⁶Xe, respectively, produced over the basalt’s 300 Myr of exposure. Diffusion/recoil of cosmogenic isotopes from the basaltic matrix/minerals to intergrain joints and vesicles is discussed. The enhanced proportion of cosmogenic Xe isotopes relative to Kr detected in vesicles could be the result of kinetic fractionation, through which preferential retention of Xe isotopes over Kr within vesicles might have occurred during diffusion from the vesicle volume to the outer space through microleaks. This study suggests that cosmogenic loss, known to be significant for ³He and ²¹Ne, and to a lesser extent for ³⁶Ar (Signer et al. 1977), also occurs to a negligible extent for the heaviest noble gases Kr and Xe.