M. R. Mumpower¹, T. Kawano¹, T. M. Sprouse², N. Vassh², E. M. Holmbeck², R. Surman², and P. Möller¹
Astrophysical Journal 869, 14 Link to Article [DOI: 10.3847/1538-4357/aaeaca]
¹Theoretical Division, Los Alamos National Laboratory Los Alamos, NM 87545, USA
²Department of Physics, University of Notre Dame Notre Dame, IN 46556, USA

We present β-delayed neutron emission and β-delayed fission (βdf) calculations for heavy, neutron-rich nuclei using the coupled Quasi-Particle Random Phase Approximation plus Hauser-Feshbach (QRPA+HF) approach. From the initial population of a compound nucleus after β-decay, we follow the statistical decay, taking into account competition between neutrons, γ-rays, and fission. We find a region of the chart of nuclides where the probability of βdf is ~100%, which likely prevents the production of superheavy elements in nature. For a subset of nuclei near the neutron dripline, neutron multiplicity and the probability of fission are both large, leading to the intriguing possibility of multi-chance βdf, a decay mode for extremely neutron-rich heavy nuclei. In this decay mode, β-decay can be followed by multiple neutron emission, leading to subsequent daughter generations that each have a probability to fission. We explore the impact of βdf in rapid neutron-capture process (r-process) nucleosynthesis in the tidal ejecta of a neutron star–neutron star merger and show that it is a key fission channel that shapes the final abundances near the second r-process peak.

Sakari¹ et al. (>10)
Astrophysical Journal 868, 110 Link to Article [DOI: 10.3847/1538-4357/aae9df]
Space Science Division, U.S. Naval Research Laboratory, Washington, DC 20375, USA
¹ Department of Astronomy, University of Washington, Seattle, WA 98195-1580, USA

This paper presents the detailed abundances and r-process classifications of 126 newly identified metal-poor stars as part of an ongoing collaboration, the R-Process Alliance. The stars were identified as metal-poor candidates from the RAdial Velocity Experiment (RAVE) and were followed up at high spectral resolution (R ~ 31,500) with the 3.5 m telescope at Apache Point Observatory. The atmospheric parameters were determined spectroscopically from Fe i lines, taking into account non-LTE corrections and using differential abundances with respect to a set of standards. Of the 126 new stars, 124 have [Fe/H] < −1.5, 105 have [Fe/H] < −2.0, and 4 have [Fe/H] < −3.0. Nine new carbon-enhanced metal-poor stars have been discovered, three of which are enhanced in r-process elements. Abundances of neutron-capture elements reveal 60 new r-I stars (with +0.3 ≤ [Eu/Fe] ≤ +1.0 and [Ba/Eu] < 0) and 4 new r-II stars (with [Eu/Fe] > +1.0). Nineteen stars are found to exhibit a “limited-r” signature ([Sr/Ba] > +0.5, [Ba/Eu] < 0). For the r-II stars, the second- and third-peak main r-process patterns are consistent with the r-process signature in other metal-poor stars and the Sun. The abundances of the light, α, and Fe-peak elements match those of typical Milky Way (MW) halo stars, except for one r-I star that has high Na and low Mg, characteristic of globular cluster stars. Parallaxes and proper motions from the second Gaia data release yield UVW space velocities for these stars that are consistent with membership in the MW halo. Intriguingly, all r-II and the majority of r-I stars have retrograde orbits, which may indicate an accretion origin.

János Kodolányi¹, Christian Vollmer², Peter Hoppe¹, and Maren Müller³
Astrophysical Journal 868, 34 Link to Article [DOI: 10.3847/1538-4357/aae482]
¹Max Planck Institute for Chemistry, Hahn-Meitner-Weg 1, D-55128 Mainz, Germany
²University of Münster, Institute for Mineralogy, Corrensstrasse 24, D-48149 Münster, Germany
³Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany

We analyzed seven presolar SiC grains of supernova origin (average diameter: 1–2 μm) with transmission electron microscopy. Five grains are polycrystalline, whereas two grains are single crystals. Individual crystal domains of polycrystalline grains are in epitaxial relationship, with two grains consisting almost entirely of twinned crystal domains. Most grains are free of inclusions (only one TiC inclusion and one iron- and nickel-rich inclusion were found in two separate grains). Almost all crystals have cubic symmetry (3C polytype), but we found hexagonal SiC (6H polytype) in two grains. The large range of crystal domain sizes (average diameter: 50–970 nm), as well as the larger fraction of noncubic SiC polytypes in supernova grains relative to SiC grains that crystallized in the winds of asymptotic giant branch (AGB) stars, suggest that SiC condensation in supernova ejecta occurs at a larger range of chemical and physical conditions, including supersaturation, than in the winds of AGB stars. Modeling condensation of SiC struggles to produce SiC grains as large as, or bigger than, observed here, if condensation of large (i.e., several μm in diameter) graphite grains is to precede that of SiC, which is suggested by the presolar grain record and published equilibrium condensation models. We propose that future models of graphite and SiC condensation in SN ejecta explore higher ejecta densities than before, as well as gas compositions that are more silicon- and carbon-rich. Furthermore, we infer that some supernova SiC grains may have formed without prior condensation of graphite from their parent gas.

Shu-Zhou WANG¹, Ai-Cheng ZHANG^1,2, Run-Lian PANG¹, Yang LI³, and Jia-Ni CHEN¹
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13254]
¹State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University,Nanjing 210046, China
²Lunar and Planetary Science Institute, Nanjing University, Nanjing 210046, China
³Center for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China
Published by arrangement with John Wiley & Sons

Records of space weathering are important for understanding the formation and evolution of surface regolith on airless celestial bodies. Current understanding of space weathering processes on asteroids including asteroid‐4 Vesta, the source of the howardite–eucrite–diogenite (HED) meteorites, lags behind what is known for the Moon. In this study, we studied agglutinates, a vesicular glass‐coating lithic clast, and a fine‐grained sulfide replacement texture in the polymict breccia Northwest Africa (NWA) 1109 with electron microscopy. In agglutinates, nanophase grains of FeNi and FeS were observed, whereas npFe⁰ was absent. We suggested that the agglutinates in NWA 1109 formed from fine‐grained surface materials of Vesta during meteorite/micrometeorite bombardment. The fine‐grained sulfide replacement texture (troilite + hedenbergite + silica) should be a result of reaction between S‐rich vapors and pyroxferroite. The unique Fe/Mn values of relict pyroxferroite indicate a different source from normal HED pyroxenes, arguing that the reaction took place on or near the surface of Vesta. The fine‐grained sulfide replacement texture could be a product of nontypical space weathering on airless celestial bodies. We should pay attention to this texture in future returned samples by asteroid exploration missions.

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.