J. Escobar-Cerezo¹, A. Penttilä², T. Kohout^2,3, O. Muñoz¹, F. Moreno¹, and K. Muinonen^2,4
Astrophysical Journal 853, 71 Link to Article [DOI: 10.3847/1538-4357/aaa24d]
¹Instituto de Astrofísica de Andalucía, CSIC, Glorieta de la Astronomía s/n, E-18008 Granada, Spain
²Department of Physics, P.O. Box 64, FI-00014 University of Helsinki, Finland
³Institute of Geology, The Czech Academy of Sciences, Prague, Czech Republic
⁴National Land Survey of Finland, Finnish Geospatial Research Institute, P.O. Box 84, FI-00521 Helsinki, Finland

Lunar soil spectra differ from pulverized lunar rocks spectra by reddening and darkening effects, and shallower absorption bands. These effects have been described in the past as a consequence of space weathering. In this work, we focus on the effects of nanophase iron (npFe⁰) inclusions on the experimental reflectance spectra of lunar regolith particles. The reflectance spectra are computed using SIRIS3, a code that combines ray optics with radiative-transfer modeling to simulate light scattering by different types of scatterers. The imaginary part of the refractive index as a function of wavelength of immature lunar soil is derived by comparison with the measured spectra of the corresponding material. Furthermore, the effect of adding nanophase iron inclusions on the reflectance spectra is studied. The computed spectra qualitatively reproduce the observed effects of space weathered lunar regolith.

Alan M. Gersch, Lori M. Feaga, and Michael F. A’Hearn¹
Astrophysical Journal 854, 149 Link to Article [DOI: 10.3847/1538-4357/aa9795]
Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
¹Deceased.

We have adapted Coupled Escape Probability, a new exact method of solving radiative transfer problems, for use in asymmetrical spherical situations for use in modeling optically thick cometary comae. Here we present the extension of our model and corresponding results for two additional primary volatile species of interest, H₂O and CO₂, in purely theoretical comets. We also present detailed modeling and results for the specific examples of CO, H₂O, and CO₂ observations of C/2009 P1 Garradd by the Deep Impact flyby spacecraft.

Rainer Schräpler¹, Jürgen Blum¹, Sebastiaan Krijt^2,3, and Jan-Hendrik Raabe¹
Astrophysical Journal 853, 74 Link to Article [DOI: 10.3847/1538-4357/aaa0d2]
¹Institut für Geophysik und extraterrestrische Physik, Technische Universität Braunschweig, Mendelssohnstr. 3, D-38106 Braunschweig, Germany
²Department of the Geophysical Sciences, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, USA
³Hubble Fellow.

In a protoplanetary disk, dust aggregates in the μm to mm size range possess mean collision velocities of 10–60 m s⁻¹ with respect to dm- to m-sized bodies. We performed laboratory collision experiments to explore this parameter regime and found a size- and velocity-dependent threshold between erosion and growth. By using a local Monte Carlo coagulation calculation and along with a simple semi-analytical timescale approach, we show that erosion considerably limits particle growth in protoplanetary disks and leads to a steady-state dust-size distribution from μm- to dm-sized particles.

Erik Weaver, Andrea Isella, and Yann Boehler
Astrophysical Journal 853, 113 Link to Article [DOI: 10.3847/1538-4357/aaa481]
Department of Physics and Astronomy, Rice University, 6100 Main Street, MS-108, Houston, TX 77005 USA

The accurate measurement of temperature in protoplanetary disks is critical to understanding many key features of disk evolution and planet formation, from disk chemistry and dynamics, to planetesimal formation. This paper explores the techniques available to determine temperatures from observations of single, optically thick molecular emission lines. Specific attention is given to issues such as the inclusion of optically thin emission, problems resulting from continuum subtraction, and complications of real observations. Effort is also made to detail the exact nature and morphology of the region emitting a given line. To properly study and quantify these effects, this paper considers a range of disk models, from simple pedagogical models to very detailed models including full radiative transfer. Finally, we show how the use of the wrong methods can lead to potentially severe misinterpretations of data, leading to incorrect measurements of disk temperature profiles. We show that the best way to estimate the temperature of emitting gas is to analyze the line peak emission map without subtracting continuum emission. Continuum subtraction, which is commonly applied to observations of line emission, systematically leads to underestimation of the gas temperature. We further show that once observational effects such as beam dilution and noise are accounted for, the line brightness temperature derived from the peak emission is reliably within 10%–15% of the physical temperature of the emitting region, assuming optically thick emission. The methodology described in this paper will be applied in future works to constrain the temperature, and related physical quantities, in protoplanetary disks observed with ALMA.

N. G. Rudraswami¹, D. Fernandes¹, A. K. Naik¹, M. Shyam Prasad¹, J. D. Carrillo-Sánchez², J. M. C. Plane², W. Feng^2,3, and S. Taylor⁴
Astrophysical Journal 853, 38 Link to Article [DOI: 10.3847/1538-4357/aaa5f7]
¹National Institute of Oceanography (Council of Scientific and Industrial Research), Dona Paula, Goa 403004, India
²School of Chemistry, University of Leeds, Leeds LS2 9JT, UK
³NCAS, School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK
⁴Cold Regions Research and Engineering Laboratory, 72 Lyme Road, Hanover, NH 03755–1290, USA

All known extraterrestrial dust (micrometeoroids) entering the Earth’s atmosphere is anticipated to have a significant contribution from ordinary chondritic precursors, as seen in meteorites, but this is an apparent contradiction that needs to be addressed. Ordinary chondrites represent a minor contribution to the overall meteor influx compared to carbonaceous chondrites, which are largely dominated by CI and/or CM chondrites. However, the near-Earth asteroid population presents a scenario with sufficient scope for generation of dust-sized debris from ordinary chondritic sources. The bulk chemical composition of 3255 micrometeorites (MMs) collected from Antarctica and deep-sea sediments has shown Mg/Si largely dominated by carbonaceous chondrites, and less than 10% having ordinary chondritic precursors. The chemical ablation model is combined with different initial chondritic compositions (CI, CV, L, LL, H), and the results clearly indicate that high-density (≥2.8 g cm⁻³) precursors, such as CV and ordinary chondrites in the size range 100–700 μm and zenith angle 0°–70°, ablate at much faster rates and lose their identity even before reaching the Earth’s surface and hence are under-represented in our collections. Moreover, their ability to survive as MMs remains grim for high-velocity micrometeoroids (>16 km s⁻¹). The elemental ratio for CV and ordinary chondrites are also similar to each other irrespective of the difference in the initial chemical composition. In conclusion, MMs belonging to ordinary chondritic precursors’ concentrations may not be insignificant in thermosphere, as they are found on Earth’s surface.

C. Travaglio^1,2, T. Rauscher^3,4,11, A. Heger^5,6,7,8,12, M. Pignatari^8,9, and C. West^7,8,10
Astrophysical Journal 854, 18 Link to Article [DOI: 10.3847/1538-4357/aaa4f7]
¹INFN—Istituto Nazionale Fisica Nucleare, Turin, Italy
²B2FH Association—Turin, Italy
³Department of Physics, University of Basel, Switzerland
⁴Centre for Astrophysics Research, University of Hertfordshire, UK
⁵Monash Centre for Astrophysics, Monash University, Melbourne, Victoria, 3800, Australia
⁶Astronomy Department, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China
⁷School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA
⁸Joint Institute for Nuclear Astrophysics—Center for the Evolution of the Elements, USA
⁹E.A. Milne Centre for Astrophysics, University of Hull, HU6 7RX, UK
¹⁰Center for Academic Excellence, Metropolitan State University, St. Paul, MN, 55106, USA
¹¹UK Network for Bridging Disciplines of Galactic Chemical Evolution (BRIDGCE), https://www.bridgce.net.
¹²The NuGrid Collaboration, http://www.nugridstars.org.

The production of the heavy stable proton-rich isotopes between ⁷⁴Se and ¹⁹⁶Hg—the p nuclides—is due to the contribution from different nucleosynthesis processes, activated in different types of stars. Whereas these processes have been subject to various studies, their relative contributions to Galactic chemical evolution (GCE) are still a matter of debate. Here we investigate for the first time the nucleosynthesis of p nuclides in GCE by including metallicity and progenitor mass-dependent yields of core-collapse supernovae (ccSNe) into a chemical evolution model. We used a grid of metallicities and progenitor masses from two different sets of stellar yields and followed the contribution of ccSNe to the Galactic abundances as a function of time. In combination with previous studies on p-nucleus production in thermonuclear supernovae (SNIa), and using the same GCE description, this allows us to compare the respective roles of SNeIa and ccSNe in the production of p-nuclei in the Galaxy. The γprocess in ccSN is very efficient for a wide range of progenitor masses (13 M _⊙–25 M _⊙) at solar metallicity. Since it is a secondary process with its efficiency depending on the initial abundance of heavy elements, its contribution is strongly reduced below solar metallicity. This makes it challenging to explain the inventory of the p nuclides in the solar system by the contribution from ccSNe alone. In particular, we find that ccSNe contribute less than 10% of the solar p nuclide abundances, with only a few exceptions. Due to the uncertain contribution from other nucleosynthesis sites in ccSNe, such as neutrino winds or α-rich freeze out, we conclude that the light p-nuclides ⁷⁴Se, ⁷⁸Kr, ⁸⁴Sr, and ⁹²Mo may either still be completely or only partially produced in ccSNe. The γ-process accounts for up to twice the relative solar abundances for ⁷⁴Se in one set of stellar models and ¹⁹⁶Hg in the other set. The solar abundance of the heaviest p nucleus ¹⁹⁶Hg is reproduced within uncertainties in one set of our models due to photodisintegration of the Pb isotopes ^208,207,206Pb. For all other p nuclides, abundances as low as 2% of the solar level were obtained.

Ryuki Hyodo^1,2, Pascal Rosenblatt^3,3, Hidenori Genda¹, and Sébastien Charnoz²
Astrophysical Journal 851, 122 Link to Article [DOI: 10.3847/1538-4357/aa9984]
¹Earth-Life Science Institute/Tokyo Institute of Technology, 2-12-1 Tokyo, Japan
²Institut de Physique du Globe/Université Paris Diderot/CNRS, F-75005 Paris, France
³Royal Observatory of Belgium, B-1180 Brussels, Belgium

Phobos and Deimos are the two small Martian moons, orbiting almost on the equatorial plane of Mars. Recent works have shown that they can accrete within an impact-generated inner dense and outer light disk, and that the same impact potentially forms the Borealis basin, a large northern hemisphere basin on the current Mars. However, there is no a priori reason for the impact to take place close to the north pole (Borealis present location), nor to generate a debris disk in the equatorial plane of Mars (in which Phobos and Deimos orbit). In this paper, we investigate these remaining issues on the giant impact origin of the Martian moons. First, we show that the mass deficit created by the Borealis impact basin induces a global reorientation of the planet to realign its main moment of inertia with the rotation pole (True Polar Wander). This moves the location of the Borealis basin toward its current location. Next, using analytical arguments, we investigate the detailed dynamical evolution of the eccentric inclined disk from the equatorial plane of Mars that is formed by the Martian-moon-forming impact. We find that, as a result of precession of disk particles due to the Martian dynamical flattening J ₂ term of its gravity field and particle–particle inelastic collisions, eccentricity and inclination are damped and an inner dense and outer light equatorial circular disk is eventually formed. Our results strengthen the giant impact origin of Phobos and Deimos that can finally be tested by a future sample return mission such as JAXA’s Martian Moons eXploration mission.

David H. Weinberg
Astrophysical Journal 851, 25 Link to Article [DOI: 10.3847/1538-4357/aa96b2]
Department of Astronomy and Center for Cosmology and AstroParticle Physics, The Ohio State University, Columbus, OH 43210, USA

Observational studies show that the global deuterium-to-hydrogen ratio in the local interstellar medium (ISM) is about 90% of the primordial ratio predicted by Big Bang nucleosynthesis. The high implies that only a small fraction of interstellar gas has been processed through stars, which destroy any deuterium they are born with. Using analytic arguments for one-zone chemical evolution models that include accretion and outflow, I show that the deuterium abundance is tightly coupled to the abundance of core collapse supernova (CCSN) elements, such as oxygen. These models predict that the ratio of the ISM deuterium abundance to the primordial abundance is , where r is the recycling fraction, is the ISM oxygen mass fraction, and is the population-averaged CCSN yield of oxygen. Using values r = 0.4 and appropriate to a Kroupa initial mass function and recent CCSN yield calculations, solar oxygen abundance corresponds to , consistent with the observations. This approximation is accurate for a wide range of parameter values, and physical arguments and numerical tests suggest that it should remain accurate for more complex chemical evolution models. The good agreement with the upper range of observed values supports the long-standing suggestion that sightline-to-sightline variations of deuterium are a consequence of dust depletion, rather than a low global enhanced by localized accretion of primordial composition gas. This agreement limits deviations from conventional yield and recycling values, including models in which most high-mass stars collapse to form black holes without expelling their oxygen in supernovae, and it implies that Galactic outflows eject ISM hydrogen as efficiently as they eject CCSN metals.

Hiroaki Katsuragi^1,2 and Jürgen Blum¹
Astrophysical Journal 851, 23 Link to Article [DOI: 10.3847/1538-4357/aa970d]
¹Institut für Geophysik und extraterrestrische Physik, Technische Universität zu Braunschweig, Mendelssohnstr. 3, D-38106 Braunschweig, Germany
²Department of Earth and Environmental Sciences, Nagoya University, Furocho, Chikusa, Nagoya, Aichi 464-8601, Japan

Dynamic characterization of mechanical properties of dust aggregates has been one of the most important problems to quantitatively discuss the dust growth in protoplanetary disks. We experimentally investigate the dynamic properties of dust aggregates by low-speed (3.2 m s⁻¹) impacts of solid projectiles. Spherical impactors made of glass, steel, or lead are dropped onto a dust aggregate with a packing fraction of  = 0.35 under vacuum conditions. The impact results in cratering or fragmentation of the dust aggregate, depending on the impact energy. The crater shape can be approximated by a spherical segment and no ejecta are observed. To understand the underlying physics of impacts into dust aggregates, the motion of the solid projectile is acquired by a high-speed camera. Using the obtained position data of the impactor, we analyze the drag-force law and dynamic pressure induced by the impact. We find that there are two characteristic strengths. One is defined by the ratio between impact energy and crater volume and is 120 kPa. The other strength indicates the fragmentation threshold of dynamic pressure and is 10 kPa. The former characterizes the apparent plastic deformation and is consistent with the drag force responsible for impactor deceleration. The latter corresponds to the dynamic tensile strength to create cracks. Using these results, a simple model for the compaction and fragmentation threshold of dust aggregates is proposed. In addition, the comparison of drag-force laws for dust aggregates and loose granular matter reveals the similarities and differences between the two materials.

Althea V. Moorhead

Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13066]
NASA Meteoroid Environment Office, Marshall Space Flight Center, Huntsville, Alabama, USA
Published by arrangement with John Wiley & Sons

Debiasing the velocity distribution of meteors observed by the Canadian Meteor Orbit Radar (CMOR) yields a distribution with large numbers of slow meteors. The distribution also contains significant numbers of hyperbolic meteors, in conflict with the expectation that interstellar meteors should be rare. In Moorhead et al. (2017a), we noted that measurement uncertainties were possibly smoothing the speed distribution and redistributing meteors to the extreme ends of the speed distribution. In this report, we use techniques analogous to image sharpening to remove the blurring caused by measurement uncertainties. The deconvolved speed distribution appears to have no meteors slower than 14 km s⁻¹ and none faster than 74 km s⁻¹. The result is to substantially raise the characteristic velocity of incoming meteoroids from 12.9 to 20.0 km s⁻¹.