Possible records of space weathering on Vesta: Case study in a brecciated eucrite Northwest Africa 1109

Shu-Zhou WANG1, Ai-Cheng ZHANG1,2, 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.

Physical Conditions for the r-process. I. Radioactive Energy Sources of Kilonovae

Shinya Wanajo1,2,3
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., $\propto {t}^{-1.3}$) 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 gsim7 days, in which the mass of r-process elements is ≈0.01 M . Two β-decay chains are identified: 66Ni $\,\to \,$ 66Cu $\,\to \,$ 66Zn and 72Zn $\,\to \,$72Ga $\,\to \,$ 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.

Evidence for a Circumsolar Dust Ring Near Mercury’s Orbit

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.

Diffusion and Concentration of Solids in the Dead Zone of a Protoplanetary Disk

Chao-Chin Yang (楊朝欽)1,2, Mordecai-Mark Mac Low3,4, 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 ${\tau }_{s}=0.1$ 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.

How Do Disks and Planetary Systems in High-mass Open Clusters Differ from Those around Field Stars?

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.

The Widespread Presence of Nanometer-size Dust Grains in the Interstellar Medium of Galaxies

Yanxia Xie1, Luis C. Ho1,2, Aigen Li3, and Jinyi Shangguan1,2
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 lesssim 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.

In situ calibration of the Martian cratering chronology

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.