O. Mousis¹ et al. (10)
Astrophysical Journal Letters 865, L11 Link to Article [DOI: 10.3847/2041-8213/aadf89]
¹Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France

The origin of cometary volatiles remains a major open question in planetary science. Comets may have either agglomerated from crystalline ices condensed in the protosolar nebula (PSN) or from amorphous ice originating from the molecular cloud and interstellar medium. Here, based on the recent argon, krypton, and xenon measurements performed by the ROSINA mass spectrometer on board the European Space Agency’s Rosetta spacecraft in the coma of 67P/Churyumov–Gerasimenko, we show that these noble gas relative abundances can be explained if the comet’s building blocks formed from a mixture of gas and H₂O grains resulting from the annealing of pristine amorphous ice (i.e., originating from the presolar cloud) in the PSN. In this scenario, the different volatiles released during the amorphous-to-crystalline ice phase transition would have been subsequently trapped at lower temperatures in stoichiometric hydrate or clathrate hydrate forms by the crystalline water ice generated by the transition. Once crystalline water was completely consumed by clathration in the ~25–80 K temperature range, the volatile species remaining in the gas phase would have formed pure condensates at lower temperatures. The formation of clathrates hydrates and pure condensates to explain the noble gas relative abundances is consistent with a proposed interstellar origin of molecular oxygen detected in 67P/Churyumov–Gerasimenko, and with the measured molecular nitrogen depletion in comets.

Ozge Ozgurel¹, Olivier Mousis², Françoise Pauzat¹, Yves Ellinger¹, Alexis Markovits¹, Steven Vance³, and François Leblanc⁴
Astrophysical Journal Letters 865, L16 Link to Article [DOI: 10.3847/2041-8213/aae091]
¹Sorbonne Université, CNRS, Laboratoire de Chimie Théorique, LCT, F-75005 Paris, France
²Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
³Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
⁴Sorbonne Université, UVSQ, CNRS, LATMOS/IPSL, Paris F-75005, France

Sodium and potassium are known to be present as neutral elements in the exosphere of Europa. The question of the origin of these alkalis—endogenous or exogenous—remains open. They have been ascribed to exogenous contamination due to volcanism from nearby Io, or the accretion of meteorites and dust. However, these mechanisms fail to fit the observed sodium-to-potassium ratio. Sodium and potassium have also been considered to originate from Europa’s putative subsurface ocean, generated by past rock-water leaching. The latter scenario implies a journey of the ions and atoms throughout the ice covering Europa. This raises questions about their stability into the bulk as well as on top of ice. These questions are addressed with first principle periodic solid-state density functional theory simulations describing the relative propensities of sodium, potassium, and calcium for being trapped in the bulk. The evolution of the ionic character of these atoms is followed by means of a topological analysis as they come up to the surface of the ice crust. We find that the metals, almost totally ionized in the ice bulk (net charge ~+0.8) where they are stabilized by ~1 eV or more, recover a quasi-neutrality (net charge ~+0.2) when weakly adsorbed at the surface by ~0.15 eV. Our results are consistent with the assumption that sodium and potassium observed in Europa exosphere come from the sputtering of materials issued from the underlying ocean and exposed by resurfacing events. They also suggest that calcium should be searched for by future missions.

Benjamin A. Sargent^1,2
Astrophysical Journal Letters 866, L1 Link to Article [DOI: 10.3847/2041-8213/aae085]
¹Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
²Center for Imaging Science and Laboratory for Multiwavelength Astrophysics, Rochester Institute of Technology, 54 Lomb Memorial Drive, Rochester NY 14623, USA

Many emission features remain unidentified in the infrared spectra of asymptotic giant branch (AGB) stars. In particular, features at ~11, 20, 28, and 32 μm have been noted in mid-infrared spectra of oxygen-rich AGB stars. Here, I present models of dust excess emission in 36 spectra of 24 AGB stars from the Short Wavelength Spectrometer on board the Infrared Space Observatory and the Infrared Spectrograph on the Spitzer Space Telescope. The models include opacities of grains composed of mixtures of various polymorphs of alumina obtained by preparing bayerite and boehmite at high temperatures, and these dust components provide satisfactory fits to the 11, 20, 28, and 32 μm features. Though not a direct conclusion from this study, the presence of grains of the various polymorphs of aluminas in circumstellar dust shells around AGB stars suggests that corundum may have a role in giving rise to the 13 μm feature.

Rogerio Deienno, Kevin J. Walsh, Katherine A. Kretke, and Harold F. Levison
Astrophysical Journal 876, 103 Link to Article [DOI: 10.3847/1538-4357/ab16e1]
Department of Space Studies, Southwest Research Institute, 1050 Walnut St., Boulder, CO 80302, USA

It is often asserted that more accurate treatment of large collisions in planet formation simulations will lead to vastly different results—in particular a lower final angular momentum deficit (AMD—commonly used to measure orbital excitement). As nearly all simulations to date consider perfect merging (100% energy dissipation) during embryo–embryo collisions, and typically end up with an overexcited final terrestrial planetary system, it has been suggested that a better treatment of energy dissipation during large collisions could decrease the final dynamical excitation (or AMD). Although some work related to energy dissipation has been done (mostly during the runaway growth phase when planetesimals grow into protoplanets), this had never been fully tested in the post-runaway phase, where protoplanets (embryos) grow chaotically into planets via large collisions among themselves. In this work, we test varying amounts of energy dissipation within embryo–embryo collisions, by assuming a given coefficient of restitution for collisions. Our results show that varying the level of energy dissipated within embryo–embryo collisions do not play any important role in the final terrestrial planetary system. We have found a strong linear correlation in our results related to the final number of planets formed and the final AMD. Additionally, reproducing the current radial mass concentration of the terrestrial planets, even when starting from an annulus of material, is challenging when modeling growth from planetesimals to planets.

Sarah Millholland^1,3 and Konstantin Batygin²
Astrophysical Journal 876, 119 Link to Article [DOI: 10.3847/1538-4357/ab19be]
¹Department of Astronomy, Yale University, New Haven, CT 06511, USA
²Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
³NSF Graduate Research Fellow.

The tilt of a planet’s spin axis off its orbital axis (“obliquity”) is a basic physical characteristic that plays a central role in determining the planet’s global circulation and energy redistribution. Moreover, recent studies have also highlighted the importance of obliquities in sculpting not only the physical features of exoplanets but also their orbital architectures. It is therefore of key importance to identify and characterize the dominant processes of excitation of nonzero axial tilts. Here we highlight a simple mechanism that operates early on and is likely fundamental for many extrasolar planets and perhaps even solar system planets. While planets are still forming in the protoplanetary disk, the gravitational potential of the disk induces nodal recession of the orbits. The frequency of this recession decreases as the disk dissipates, and when it crosses the frequency of a planet’s spin axis precession, large planetary obliquities may be excited through capture into a secular spin–orbit resonance. We study the conditions for encountering this resonance and calculate the resulting obliquity excitation over a wide range of parameter space. Planets with semimajor axes in the range 0.3 au  a  2 au are the most readily affected, but large-a planets can also be impacted. We present a case study of Uranus and Neptune, and show that this mechanism likely cannot help explain their high obliquities. While it could have played a role if finely tuned and envisioned to operate in isolation, large-scale obliquity excitation was likely inhibited by gravitational planet–planet perturbations.

Chin-Fei Lee^1,2, Claudio Codella^3,4, Zhi-Yun Li⁵, and Sheng-Yuan Liu¹
Astrophysical Journal 876, 63 Link to Article [DOI: 10.3847/1538-4357/ab15db ]
¹Academia Sinica Institute of Astronomy and Astrophysics, P.O. Box 23-141, Taipei 106, Taiwan
²Graduate Institute of Astronomy and Astrophysics, National Taiwan University, No. 1, Section 4, Roosevelt Road, Taipei 10617, Taiwan
³INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy
⁴Univ. Grenoble Alpes, CNRS, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), F-38000 Grenoble, France
⁵Astronomy Department, University of Virginia, Charlottesville, VA 22904, USA

HH 212 is one of the well-studied protostellar systems, showing the first vertically resolved disk with a warm atmosphere around the central protostar. Here we report a detection of nine organic molecules (including newly detected ketene, formic acid, deuterated acetonitrile, methyl formate, and ethanol) in the disk atmosphere, confirming that the disk atmosphere is, for HH 212, the chemically rich component, identified before at a lower resolution as a “hot corino.” More importantly, we report the first systematic survey and abundance measurement of organic molecules in the disk atmosphere within ~40 au of the central protostar. The relative abundances of these molecules are similar to those in the hot corinos around other protostars and in Comet Lovejoy. These molecules can be either (i) originally formed on icy grains and then desorbed into gas phase or (ii) quickly formed in the gas phase using simpler species ejected from the dust mantles. The abundances and spatial distributions of the molecules provide strong constraints on models of their formation and transport in star formation. These molecules are expected to form even more complex organic molecules needed for life and deeper observations are needed to find them.

D. Estrella-Trujillo¹, L. Hernández-Martínez¹, P. F. Velázquez¹, A. Esquivel^1,2, and A. C. Raga¹
Astrophysical Journal 876, 29 Link to Article [DOI: 10.3847/1538-4357/ab12e1 ]
¹Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México. Apartado Postal 70-543, 04510 Ciudad de México, México
²Instituto de Astronomía Teórica y Experimental, Universidad Nacional de Córdoba, X500BGR Córdoba, Argentina

We have carried out 3D hydrodynamic simulations of a precessing jet/counterjet ejection. We have included the photoionization from the central source, considering three different ionizing photon rates (, 10⁴⁶, and 10⁴⁷ phots s⁻¹), in order to determine its effect on the morphology and kinematics of the protoplanetary nebula. We have considered a time-dependent ejection density that generates dense knot structures in the jet, which are then partially photoionized by the ionizing photon field from the central source. We also explore the role of the medium in which the jet is propagated, under these conditions. The photoionization results in a larger Hα emission of the knots, and in an acceleration of the knots as a result of the so-called “rocket effect.” We find that for larger values of the ionizing photon rate, a clear outwards acceleration of the knots is produced. These models are appropriate for explaining protoplanetary nebulae in which such outwards accelerations are observed.

Bernd Helber¹, Bruno Dias^1,2, Federico Bariselli^1,3,4, Luiza F. Zavalan¹, Lidia Pittarello⁵, Steven Goderis⁶, Bastien Soens⁶, Seann J. McKibbin^6,7,8, Philippe Claeys⁶, and Thierry E. Magin¹
Astrophysical Journal 876, 120 Link to Article [DOI: 10.3847/1538-4357/ab16f0 ]
¹Aeronautics and Aerospace Department, von Karman Institute for Fluid Dynamics, Rhode-Saint-Genèse, Belgium
²Institute of Mechanics, Materials and Civil Engineering, Université catholique de Louvain, Louvain-la-Neuve, Belgium
³Research Group Electrochemical and Surface Engineering, Vrije Universiteit Brussel, Brussels, Belgium
⁴Dipartimento di Scienze e Tecnologie Aerospaziali, Politecnico di Milano, Milano, Italy
⁵Department of Lithospheric Research, University of Vienna, Vienna, Austria
⁶Analytical, Environmental, and Geo-Chemistry, Vrije Universiteit Brussel, Brussels, Belgium
⁷Institute of Earth and Environmental Science, University of Potsdam, Potsdam-Golm, Germany
⁸Geowissenschaftliches Zentrum, Georg-August-Universität Göttingen, Göttingen, Germany

Meteoroids largely disintegrate during their entry into the atmosphere, contributing significantly to the input of cosmic material to Earth. Yet, their atmospheric entry is not well understood. Experimental studies on meteoroid material degradation in high-enthalpy facilities are scarce and when the material is recovered after testing, it rarely provides sufficient quantitative data for the validation of simulation tools. In this work, we investigate the thermo-chemical degradation mechanism of a meteorite in a high-enthalpy ground facility able to reproduce atmospheric entry conditions. A testing methodology involving measurement techniques previously used for the characterization of thermal protection systems for spacecraft is adapted for the investigation of ablation of alkali basalt (employed here as meteorite analog) and ordinary chondrite samples. Both materials are exposed to a cold-wall stagnation point heat flux of 1.2 MW m⁻². Numerous local pockets that formed on the surface of the samples by the emergence of gas bubbles reveal the frothing phenomenon characteristic of material degradation. Time-resolved optical emission spectroscopy data of ablated species allow us to identify the main radiating atoms and ions of potassium, calcium, magnesium, and iron. Surface temperature measurements provide maximum values of 2280 K for the basalt and 2360 K for the chondrite samples. We also develop a material response model by solving the heat conduction equation and accounting for evaporation and oxidation reaction processes in a 1D Cartesian domain. The simulation results are in good agreement with the data collected during the experiments, highlighting the importance of iron oxidation to the material degradation.

Mohammadtaher Safarzadeh, Richard Sarmento, and Evan Scannapieco
Astrophysical Journal 876, 28 Link to Article [DOI: 10.3847/1538-4357/ab1341 ]
School of Earth and Space Exploration, Arizona State University, USA

We model the history of Galactic r-process enrichment using high-redshift, high-resolution zoom cosmological simulations of a Milky Way–type halo. We assume that all r-process sources are neutron star mergers (NSMs) with a power-law delay time distribution. We model the time to mix pollutants at subgrid scales, which allows us to better compute the properties of metal-poor (MP) and carbon-enhanced metal-poor (CEMP) stars, along with statistics of their r-process-enhanced subclasses. Our simulations underpredict the cumulative ratios of r-process-enhanced MP and CEMP stars (MP-r, CEMP-r) over MP and CEMP stars by about one order of magnitude, even when the minimum coalescence time of the double neutron stars (DNSs), t _min, is set to 1 Myr. No r-process-enhanced stars form if t _min = 100 Myr. Our results show that even when we adopt the r-process yield estimates observed in GW170817, NSMs by themselves can only explain the observed frequency of r-process-enhanced stars if the birth rate of DNSs per unit mass of stars is boosted to .

Anibal Sierra¹, Susana Lizano¹, Enrique Macías², Carlos Carrasco-González¹, Mayra Osorio³, and Mario Flock⁴
Astrophysical Journal 876, 7 Link to Article [DOI: 10.3847/1538-4357/ab1265 ]
¹Instituto de Radioastronomía y Astrofísica, UNAM, Apartado Postal 3-72, 58089 Morelia Michoacán, México
²Department of Astronomy, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
³Instituto de Astrofísica de Andalucía (CSIC) Glorieta de la Astronomía s/n E-18008 Granada, Spain
⁴Max Planck Institute fűr Astronomy (MPIA), Kőnigsthul 17, D-69117 Heidelberg, Germany

We study dust concentration in axisymmetric gas rings in protoplanetary disks. Given the gas surface density, we derived an analytical total dust surface density by taking into account the differential concentration of all grain sizes. This model allows us to predict the local dust-to-gas mass ratio and the slope of the particle size distribution, as a function of radius. We test this analytical model by comparing it with a 3D magnetohydrodynamical simulation of dust evolution in an accretion disk. The model is also applied to the disk around HD 169142. By fitting the disk continuum observations simultaneously at λ = 0.87, 1.3, and 3.0 mm, we obtain a global dust-to-gas mass ratio and a viscosity coefficient α = 1.35 × 10⁻². This model can be easily implemented in numerical simulations of accretion disks.