¹Karin Maierhofer,^1,2,3Christian Koeberl,³Julie Brigham‐Grette
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13243]
¹Department of Lithospheric Research, University of Vienna, , A‐1090 Vienna, Austria
² Natural History Museum, , A‐1010 Vienna, Austria
³Department of Geosciences, University of Massachusetts, Amherst, Massachusetts, 01003 USA
Published by arrangement with John Wiley & Sons

The 3.6 Ma El’gygytgyn impact structure, located in northeast Chukotka in Arctic Russia, was largely formed in acidic volcanic rocks. The 18 km diameter circular depression is today filled with Lake El’gygytgyn (diameter of 12 km) that contains a continuous record of lacustrine sediments of the Arctic from the past 3.6 Myr. In 2009, El’gygytgyn became the focus of the International Continental Scientific Drilling Program (ICDP) in which a total of 642.4 m of drill core was recovered. Lithostratigraphically, the drill cores comprise lacustrine sediment sequences, impact breccias, and deformed target rocks. The impactite core was recovered from 316.08 to 517.30 meters below lake floor (mblf). Because of the rare, outstanding recovery, the transition zone, ranging from 311.47 to 317.38 m, between the postimpact lacustrine sediments and the impactite sequences, was studied petrographically and geochemically. The transition layer comprises a mixture of about 6 m of loose sedimentary and volcanic material containing isolated clasts of minerals and melt. Shock metamorphic effects, such as planar fractures (PFs) and planar deformation features (PDFs), were observed in a few quartz grains. The discoveries of silica diaplectic glass hosting coesite, kinked micas and amphibole, lechatelierite, numerous impact melt shards and clasts, and spherules are associated with the impact event. The occurrence of spherules, impact melt clasts, silica diaplectic glass, and lechatelierite, about 1 m below the onset of the transition, marks the beginning of the more coherent impact ejecta layer. The results of siderophile interelement ratios of the transition layer spherules give indications of the relative contribution of the meteoritical component.

Alan P. Boss
Astrophysical Journal 870, 3 Link to Article [DOI: 10.3847/1538-4357/aaf005 ]
Department of Terrestrial Magnetism, Carnegie Institution for Science, 5241 Broad Branch Road NW, Washington, DC 20015-1305, USA

Cosmochemical evaluations of the initial meteoritical abundance of the short-lived radioisotope (SLRI) ²⁶Al have remained fairly constant since 1976, while estimates for the initial abundance of the SLRI ⁶⁰Fe have varied widely recently. At the high end of this range, ⁶⁰Fe initial abundances have seemed to require ⁶⁰Fe nucleosynthesis in a core-collapse supernova, followed by incorporation into primitive meteoritical components within ~1 Myr. This paper continues the detailed exploration of this classical scenario, using models of the self-gravitational collapse of molecular cloud cores that have been struck by suitable shock fronts, leading to the injection of shock front gas into the collapsing cloud through Rayleigh–Taylor fingers formed at the shock–cloud interface. As before, these models are calculated using the FLASH three-dimensional, adaptive mesh refinement, gravitational hydrodynamical code. While the previous models used FLASH 2.5, the new models employ FLASH 4.3, which allows sink particles to be introduced to represent the newly formed protostellar object. Sink particles permit the models to be pushed forward farther in time to the phase where a ~1 M _⊙ protostar has formed, orbited by a rotating protoplanetary disk. These models are thus able to define what type of target cloud core is necessary for the supernova triggering scenario to produce a plausible scheme for the injection of SLRIs into the presolar cloud core: a ~3 M_⊙ cloud core rotating at a rate of ~3 × 10⁻¹⁴ rad s⁻¹ or higher.

Erika M. Holmbeck^1,2, Trevor M. Sprouse¹, Matthew R. Mumpower^2,3, Nicole Vassh¹, Rebecca Surman^1,2, Timothy C. Beers^1,2, and Toshihiko Kawano³
Astrophysical Journal 870, 23 Link to Article [DOI: 10.3847/1538-4357/aaefef ]
¹Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA
²JINA Center for the Evolution of the Elements, USA
³Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA

The rapid neutron-capture (“r-“) process is responsible for synthesizing many of the heavy elements observed in both the solar system and Galactic metal-poor halo stars. Simulations of r-process nucleosynthesis can reproduce abundances derived from observations with varying success, but so far they fail to account for the observed overenhancement of actinides, present in about 30% of r-process-enhanced stars. In this work, we investigate actinide production in the dynamical ejecta of a neutron star merger (NSM) and explore whether varying levels of neutron-richness can reproduce the actinide boost. We also investigate the sensitivity of actinide production on nuclear physics properties: fission distribution, β-decay, and mass model. For most cases, the actinides are overproduced in our models if the initial conditions are sufficiently neutron-rich for fission cycling. We find that actinide production can be so robust in the dynamical ejecta that an additional lanthanide-rich, actinide-poor component is necessary in order to match observations of actinide-boost stars. We present a simple actinide-dilution model that folds in estimated contributions from two nucleosynthetic sites within a merger event. Our study suggests that while the dynamical ejecta of an NSM are likely production sites for the formation of actinides, a significant contribution from another site or sites (e.g., the NSM accretion disk wind) is required to explain abundances of r-process-enhanced, metal-poor stars.

Ruobing Dong (董若冰)¹, Sheng-Yuan Liu (呂聖元)², and Jeffrey Fung (馮澤之)^3,4
Astrophysical Journal 870, 72 Link to Article [DOI: 10.3847/1538-4357/aaf38e ]
¹Department of Physics & Astronomy, University of Victoria, Victoria, BC, V8P 1A1, Canada
²Institute of Astronomy and Astrophysics, Academia Sinica, 11F of ASMAB, AS/NTU No.1, Sec. 4, Roosevelt Road, Taipei 10617, Republic of China
³Department of Astronomy, University of California at Berkeley, Campbell Hall, Berkeley, CA 94720-3411, USA
⁴NASA Sagan Fellow.

Protoplanets can produce structures in protoplanetary disks via gravitational disk–planet interactions. Once detected, such structures serve as signposts of planet formation. Here we investigate the kinematic signatures in disks produced by multi-Jupiter mass (M _J) planets using 3D hydrodynamics and radiative transfer simulations. Such a planet opens a deep gap, and drives transonic vertical motions inside. Such motions include both a bulk motion of the entire half-disk column, and turbulence on scales comparable to and smaller than the scale height. They significantly broaden molecular lines from the gap, producing double-peaked line profiles at certain locations, and a kinematic velocity dispersion comparable to thermal after azimuthal averaging. The same planet does not drive fast vertical motions outside the gap, except at the inner spiral arms and the disk surface. Searching for line broadening induced by multi-M _J planets inside gaps requires an angular resolution comparable to the gap width, an assessment of the gap gas temperature to within a factor of 2, and a high sensitivity needed to detect line emission from the gap.