Shinya Wanajo^1,2,3
Astrophysical Journal 868, 65 Link to Article [DOI: 10.3847/1538-4357/aae0f2]
¹Max Planck Institute for Gravitational Physics (Albert Einstein Institute), Am Mühlenberg 1, Potsdam-Golm, D-14476, Germany
²Department of Engineering and Applied Sciences, Sophia University, Chiyoda-ku, Tokyo 102-8554, Japan
³iTHEMS Research Group, RIKEN, Wako, Saitama 351-0198, Japan

Radioactive energies from unstable nuclei made in the ejecta of neutron star mergers play principal roles in powering kilonovae. In previous studies, power-law-type heating rates (e.g., ) have frequently been used, which may be inadequate if the ejecta are dominated by nuclei other than the A ~ 130 region. We consider, therefore, two reference abundance distributions that match the r-process residuals to the solar abundances for A ≥ 69 (light trans-iron plus r-process elements) and A ≥ 90 (r-process elements). Nucleosynthetic abundances are obtained by using free-expansion models with three parameters: expansion velocity, entropy, and electron fraction. Radioactive energies are calculated as an ensemble of weighted free-expansion models that reproduce the reference abundance patterns. The results are compared with the bolometric luminosity (> a few days since merger) of the kilonova associated with GW170817. We find that the former case (fitted for A ≥ 69) with an ejecta mass 0.06 M _⊙ reproduces the light curve remarkably well, including its steepening at 7 days, in which the mass of r-process elements is ≈0.01 M _⊙. Two β-decay chains are identified: ⁶⁶Ni ⁶⁶Cu ⁶⁶Zn and ⁷²Zn ⁷²Ga ⁷²Ge with similar halflives of parent isotopes (≈2 days), which leads to an exponential-like evolution of heating rates during 1–15 days. The light curve at late times (>40 days) is consistent with additional contributions from the spontaneous fission of ²⁵⁴Cf and a few Fm isotopes. If this is the case, the GW170817 event is best explained by the production of both light trans-iron and r-process elements that originate from dynamical ejecta and subsequent disk outflows from the neutron star merger.

Guillermo Stenborg, Johnathan R. Stauffer¹, and Russell A. Howard
Astrophysical Journal 868, 34 Link to Article [DOI: 10.3847/1538-4357/aae6cb]
Space Science Division, U.S. Naval Research Laboratory, Washington, DC 20375, USA
¹Current address: Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO 80309, USA.

To test a technique to be used on the white-light imager onboard the recently launched Parker Solar Probemission, we performed a numerical differentiation of the brightness profiles along the photometric axis of the F-corona models that are derived from STEREO Ahead Sun Earth Connection Heliospheric Investigation observations recorded with the HI-1 instrument between 2007 December and 2014 March. We found a consistent pattern in the derivatives that can be observed from any S/C longitude between about 18° and 23° elongation with a maximum at about 21°. These findings indicate the presence of a circumsolar dust density enhancement that peaks at about 23° elongation. A straightforward integration of the excess signal in the derivative space indicates that the brightness increase over the background F-corona is on the order of 1.5%–2.5%, which implies an excess dust density of about 3%–5% at the center of the ring. This study has also revealed (1) a large-scale azimuthal modulation of the inner boundary of the pattern, which is in clear association with Mercury’s orbit; and (2) a localized modulation of the inner boundary that is attributable to the dust trail of Comet 2P/Encke, which occurs near ecliptic longitudes corresponding to the crossing of Encke’s and Mercury’s orbital paths. Moreover, evidence of dust near the S/C in two restricted ranges of ecliptic longitudes has also been revealed by this technique, which is attributable to the dust trails of (1) comet 73P/Schwassmann–Wachmann 3, and (2) 169P/NEAT.

Chao-Chin Yang (楊朝欽)^1,2, Mordecai-Mark Mac Low^3,4, and Anders Johansen¹
Astrophysical Journal 868, 1 Link to Article [DOI: 10.3847/1538-4357/aae7d1]
¹Lund Observatory, Department of Astronomy and Theoretical Physics, Lund University, Box 43, SE-221 00 Lund, Sweden
²Department of Physics and Astronomy, University of Nevada, Las Vegas, 4505 S. Maryland Parkway, Box 454002, Las Vegas, NV 89154-4002, USA
³Department of Astrophysics, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024-5192, USA
⁴Center for Computational Astrophysics, Flatiron Institute, New York, NY, USA

The streaming instability is a promising mechanism to drive the formation of planetesimals in protoplanetary disks. To trigger this process, it has been argued that sedimentation of solids onto the mid-plane needs to be efficient, and therefore that a quiescent gaseous environment is required. It is often suggested that dead-zone or disk-wind structure created by non-ideal magnetohydrodynamical (MHD) effects meets this requirement. However, simulations have shown that the mid-plane of a dead zone is not completely quiescent. In order to examine the concentration of solids in such an environment, we use the local-shearing-box approximation to simulate a particle-gas system with an Ohmic dead zone including mutual drag force between the gas and the solids. We systematically compare the evolution of the system with ideal or non-ideal MHD, with or without backreaction drag force from particles on gas, and with varying solid abundances. Similar to previous investigations of dead-zone dynamics, we find that particles of dimensionless stopping time do not sediment appreciably more than those in ideal magnetorotational turbulence, resulting in a vertical scale height an order of magnitude larger than in a laminar disk. Contrary to the expectation that this should curb the formation of planetesimals, we nevertheless find that strong clumping of solids still occurs in the dead zone when solid abundances are similar to the critical value for a laminar environment. This can be explained by the weak radial diffusion of particles near the mid-plane. The results imply that the sedimentation of particles to the mid-plane is not a necessary criterion for the formation of planetesimals by the streaming instability.

Kirsten Vincke and Susanne Pfalzner
Astrophysical Journal 868, 1 Link to Article [DOI: 10.3847/1538-4357/aae7d1]
Max Planck Institute for Radio Astronomy, Auf dem Hügel 69, D-53121 Bonn, Germany

Only star clusters that are sufficiently compact and massive survive largely unharmed beyond 10 . However, their compactness means a high stellar density, which can lead to strong gravitational interactions between the stars. As young stars are often initially surrounded by protoplanetary disks and later on potentially by planetary systems, the question arises to what degree these strong gravitational interactions influence planet formation and the properties of planetary systems. Here, we perform simulations of the evolution of compact high-mass clusters like Trumpler 14 and Westerlund 2 from the embedded to the gas-free phase and study the influence of stellar interactions. We concentrate on the development of the mean disk size in these environments. Our simulations show that in high-mass open clusters 80%–90% of all disks/planetary systems should be smaller than 50 just as a result of the strong stellar interactions in these environments. Already in the initial phases, three to four close flybys lead to typical disk sizes within the range of 18–27 . Afterward, the disk sizes are altered only to a small extent. Our findings agree with the recent observation that the disk sizes in the once dense environment of the Upper Scorpio OB association, NGC 2362, and h/χPersei are at least three times smaller in size than, for example, in Taurus. We conclude that the observed planetary systems in high-mass open clusters should also be on average smaller than those found around field stars; in particular, planets on wide orbits are expected to be extremely rare in such environments.

Yanxia Xie¹, Luis C. Ho^1,2, Aigen Li³, and Jinyi Shangguan^1,2
Astrophysical Journal 867, 91 Link to Article [DOI: 10.3847/1538-4357/aa2b0]
¹Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, People’s Republic of China
²Department of Astronomy, School of Physics, Peking University, Beijing 100871, People’s Republic of China
³Department of Physics and Astronomy, University of Missouri, Columbia, MO 65211, USA

Interstellar dust spans a wide range in size distribution, ranging from ultrasmall grains of a few Ångströms to micrometer-size grains. While the presence of nanometer-size dust grains in the Galactic interstellar medium was speculated six decades ago and was previously suggested based on early infrared observations, systematic and direct analysis of their properties over a wide range of environments has been lacking. Here we report the detection of nanometer-size dust grains that appear to be universally present in a wide variety of astronomical environments, from Galactic high-latitude clouds to nearby star-forming galaxies and galaxies with low levels of nuclear activity. The prevalence of such a grain population is revealed conclusively as prominent mid-infrared continuum emission at λ  10 μm seen in the Spitzer/Infrared Spectrograph data, characterized by temperatures of ~300–400 K that are significantly higher than the equilibrium temperatures of common, submicron-size grains in typical galactic environments. We propose that the optimal carriers of this pervasive, featureless hot dust component are very small carbonaceous (e.g., graphite) grains of nanometer size that are transiently heated by single-photon absorption. This grain population accounts for ~1.4% of the total infrared emission at ~5–3000 μm and ~0.4% of the total interstellar dust mass.

Stephanie C. Werner
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13263]
Centre for Earth Evolution and Dynamics, Department of Geosciences, University of Oslo, , 0315 Oslo, Norway
Published by arrangement with John Wiley & Sons

Crater densities on planetary surfaces allow assessing relative ages but so far firm calibration of so‐called cratering‐chronology models is available only for the Moon and limited to the past 4.1 billion years. Most planetary geological time scales are still model‐dependent, and essentially constrained by meteorite ages or by comparison to (dynamical) solar system evolution models. Here we describe in situ calibration of the Martian cratering chronology using cosmogenic and radiogenic isotope ages obtained by the NASA Curiosity rover. We determined the cratering‐rate ratio between Moon and Mars for recent times, and extended the calibration of cratering rates to earlier times than those based exclusively on lunar data. Our preferred interpretation supports monotonic flux decay since at least 4.24 Ga and likely since about 4.45 Ga, implying orbital migration of the giant planets, and its direct, transient, dynamical effect on the planetesimal populations was initiated early. But only Martian Sample Return will provide strongly needed capability for distinction of the different models currently available.

Andreas T. Hertwig^a, Kimura Makoto^b, Takayuki Ushikubo^c, Céline Defouilloy, ^aNoriko T.Kita^a
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.02.020]
^aWiscSIMS, Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706, USA
^bNational Institute of Polar Research, Meteorite Research Center, Midoricho 10-3, Tachikawa, Tokyo 190-8518, Japan
^cKochi Institute for Core Sample Research, JAMSTEC, 200 Monobe-otsu, Nankoku, Kochi 783-8502 Japan
Copyright Elsevier

The ²⁶Al-²⁶Mg ages of FeO-rich (type II) chondrules from Acfer 094, one of the least thermally metamorphosed carbonaceous chondrites, were determined by SIMS analysis of plagioclase and olivine/pyroxene using a radio frequency (RF) plasma oxygen ion source. In combination with preexisting ²⁶Al-²⁶Mg ages of FeO-poor (type I) chondrules, the maximum range of formation ages recorded in chondrules from a single meteorite is determined to help provide constraints on models of material transport in the proto-planetary disk. We also report new SIMS oxygen three-isotope analyses of type II chondrules in Acfer 094. All but one of the plagioclase analyses show resolvable excesses in ²⁶Mg and isochron regressions yield initial ²⁶Al/²⁷Al ratios of type II chondrules that range from (3.62 ± 0.86) × 10^–6 to (9.3 ± 1.1) × 10^–6_, which translates to formation ages between 2.71 ^–0.22/_+0.28 Ma and 1.75 ^–0.11/_+0.12 Ma after CAI. This overall range is indistinguishable from that determined for type I chondrules in Acfer 094. The initial ²⁶Al/²⁷Al ratio of the oldest type II chondrule is resolved from that of all other type II chondrules in Acfer 094. Importantly, the oldest type I chondrule and the oldest type II chondrule in Acfer 094 possess within analytical error indistinguishable initial ²⁶Al/²⁷Al ratios and Δ¹⁷O values of ∼0‰. Ages and oxygen isotope ratios clearly set these two chondrules apart from all other chondrules in Acfer 094. It is therefore conceivable that the formation region of these two chondrules differs from that of other chondrules and in turn suggests that Acfer 094 contains two distinct chondrule generations.

Conel M. O’D.Alexander
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.02.008]
Dept. Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington DC 20015, USA
Copyright Elsevier

A quantitative understanding of the elemental and isotopic fractionations recorded in the compositions of the chondritic meteorites would provide fundamental constraints for astrophysical models of early Solar System evolution. Here it is shown through least squares fitting that almost all features of the bulk elemental and isotopic compositions of the main carbonaceous chondrite (CC) groups, as well as the ungrouped Tagish Lake (C2) meteorite, can be reproduced using mixtures of the same four components. The fractionations amongst the non-CCs (ordinary, Rumuruti and enstatite chondrites) are distinctly different to those in the CCs and are the subject of a separate study (Alexander, 2019). The four CC components are: (1) a ‘chondrule’ (or chondrule precursor) component that partially lost Fe,Ni metal and volatiles, but is otherwise CI-like, (2) the CC-RI component that has a refractory inclusion-like bulk composition and is largely responsible for the refractory element enrichments and nucleosynthetic isotope anomalies in the bulk CCs, (3) anhydrous and reduced but otherwise CI-like matrix that accounts for almost all of the most volatile element (e.g., Zn, Se and C) contents of the CCs, and (4) water with relatively high Δ¹⁷O and δ¹⁸O values. Comparison of the inferred component compositions to additional meteoritic constraints produces some notable results. The ε⁴⁸Ca≈8 and ε⁵⁰Ti≈8 values for the CC-RI component are consistent with the average value for refractory inclusions. On the other hand, the ε⁵⁴Cr≈-10 is not, but is required by the negative correlation between ε⁵⁰Ti and ε⁵⁴Cr amongst the bulk CCs. The CC-RI component may be comprised of a more CAI-like sub-component that carries the ε⁴⁸Ca and ε⁵⁰Ti anomalies, and a more ferromagnesian sub-component that carries the negative ε⁵⁴Cr anomalies. The compositions of the volatile and metal subcomponents lost from the ‘chondrule’ component are consistent with condensation models, suggesting that the fractionations predated chondrule formation. The isotopic compositions of chondrules from the more CC-RI-rich CC groups (e.g., CV, CO and CM) seem to require the addition of some of the CC-RI component to their precursors. The assumption that matrix is CI-like is inconsistent with chondrule-matrix complementarity, but is justified by the success of the fits and the relatively uniform and CI-like abundances of organics and presolar grains in the matrices of the most primitive CCs. The inferred Δ¹⁷O=3.5 ‰ for the water component is consistent with most constraints from secondary phases in the CCs. The large O isotopic mass fractionation (δ¹⁸O≈18-21 ‰) of the water is consistent with ∼89-95 % condensation of ice from a vapor under Rayleigh conditions at 150-170 K. The water was entirely accreted with the matrix with fairly constant (0.32±0.06 by wt.) and CI-like (∼0.38 by wt.) water/matrix ratios. These water/matrix ratios are much less than the water/rock ratio of one that is often cited for a nebula of solar composition, but can be explained if much of the C in the CC formation regions was present as CO and CO₂, and the abundance of CH₄ was low.

¹David Baratoux et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13253]
¹Geosciences Environnement Toulouse, UMR5563, CNRS, Universite de Toulouse & IRD, 14, Avenue Edouard Belin, 31400Toulouse, France
Published by arrangement with John Wiley & Sons

The about 10.5 km diameter Bosumtwi impact crater is one of the youngest large impact structures on Earth. The crater rim is readily noticed on topographic maps or in satellite imagery. It defines a circular basin filled by water (Lake Bosumtwi) and lacustrine sediments. The morphology of this impact structure is also characterized by a circular plateau extending beyond the rim and up to 9–10 km from the center of the crater (about 2 crater radii). This feature comprises a shallow ring depression, also described as an annular moat, and a subdued circular ridge at its outer edge. The origin of this outermost feature could so far not be elucidated based on remote sensing data only. Our approach combines detailed topographic analysis, including roughness mapping, with airborne radiometric surveys (mapping near‐surface K, Th, U concentrations) and field observations. This provides evidence that the moat and outer ring are features inherited from the impact event and represent the partially eroded ejecta layer of the Bosumtwi impact structure. The characteristics of the outer ridge indicate that ejecta emplacement was not purely ballistic but requires ejecta fluidization and surface flow. The setting of Bosumtwi ejecta can therefore be considered as a terrestrial analog for rampart craters, which are common on Mars and Venus, and also found on icy bodies of the outer solar system (e.g., Ganymede, Europa, Dione, Tethys, and Charon). Future studies at Bosumtwi may therefore help to elucidate the mechanism of formation of rampart craters.

¹Andreas Morlok,²Stephan Klemme,¹Iris Weber,¹Aleksandra Stojic,³Martin Sohn, ¹Harald Hiesinger,⁴Joern Helbert
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.02.010]
¹Institut für Planetologie, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
²Institut für Mineralogie, Corrensstraße 24, 48149 Münster, Germany
³Hochschule Emden/Leer, Constantiaplatz 4, 26723 Emden, Germany
⁴Institute for Planetary Research, DLR, Rutherfordstrasse 2, 12489 Berlin, Germany
Copyright Elsevier

The MERTIS (MErcury Radiometer and Thermal Infrared Spectrometer) instrument onboard the ESA/JAXA BepiColombo mission will provide mid-infrared data, which will be crucial to characterize the surface mineralogy of Mercury. In order to interpret the results, we are creating a database of mid-infrared spectra. As part of a study of synthetic glasses which are to serve as analog materials for the interpretation of remote sensing and modeling data, we present mid-infrared data for analog materials of Mercury regolith, surface and mantle compositions. In addition, we provide data for similar analogs of Earth, Moon, Venus, and Mars rocks for a coherent picture.
The analog samples have been first characterized by optical microscopy, Raman spectroscopy and EMPA. Powdered size fractions (0–25 μm, 25–63 μm, 63–125 μm, and 125–250 μm) were studied in reflectance in the mid-infrared range from 2.5 to 18 μm (550 to 2000 cm−1), additional micro-FTIR analyses were also obtained.
Results for the size fractions of the surface and regolith analogs for Mercury show typical features for amorphous material with Christiansen Features (CF) at 8–8.1 μm, Reststrahlen Bands (RB) at 9.8–9.9 μm, and the Transparency Feature (TF) at 12 μm. The six bulk silicate Mercury analogs have varying CF positions from 8.1 to 9 μm, with RB crystalline features of various olivines dominating in most samples. Similarly, bulk silicate analogs of the other planetary bodies show glassy features for the surface analogs with CF from 7.9 μm (Earth Continental Crust) to 8.3 μm (Lunar Mare), strong RB from 9.5 μm (Earth Continental Crust) to 10.6 μm (Lunar Mare and Highlands). TF are usually very weak for the glassy analogs.
Bulk silicate analogs for the other planetary bodies are again dominated by crystalline olivine features. Trends between SCFM (SiO2/(SiO2 + CaO + FeO + MgO)) index reflecting polymerization, CF and RB positions, and the SiO2 contents in previous studies are basically confirmed, but there is indication that several samples (Moon Mare and Highlands) do not follow the trend observed for low SiO2 samples and the RB position in earlier studies. Comparison with a ground based mid-IR spectrum of Mercury demonstrates general similarity in band positions with glassy Mercury surface and regolith material, and a chondrite based model of the planet. However, no mix so far can explain all spectral features.