¹Takuji Tsujimoto, ²Tetsuya Yokoyama, ³Kenji Bekki
The Astrophysical Journal Letters 835, L3 Link to Article [http://dx.doi.org/10.3847/2041-8213/835/1/L3]
¹National Astronomical Observatory of Japan, Mitaka-shi, Tokyo 181-8588, Japan
²Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan
³ICRAR, M468, The University of Western Australia, 35 Stirling Highway, Crawley Western Australia 6009, Australia

Meteoritic abundances of r-process elements are analyzed to deduce the history of chemical enrichment by the r-process, from the beginning of disk formation to the present time in the solar vicinity. Our analysis combines the abundance information from short-lived radioactive nuclei such as 244Pu with the abundance information from stable r-process nuclei such as Eu. These two types of nuclei can be associated with one r-process event and an accumulation of events until the formation of the solar system, respectively. With the help of the observed local star formation (SF) history, we deduce the chemical evolution of 244Pu and obtain three main results: (i) the last r-process event occurred 130–140 Myr before the formation of the solar system; (ii) the present-day low 244Pu abundance as measured in deep-sea reservoirs results from the low recent SF rate compared to ~4.5−5 Gyr ago; and (iii) there were ~15 r-process events in the solar vicinity from the formation of the Galaxy to the time of solar system’s formation and ~30 r-process events to the present time. Then, adopting the hypothesis that a neutron star (NS) merger is the r-process production site, we find that the ejected r-process elements are extensively spread out and mixed with interstellar matter, with a mass of $\sim 3.5\times {10}^{6}$ M ⊙, which is about 100 times larger than that for supernova ejecta. In addition, the event frequency of r-process production is estimated to be 1 per ~1400 core-collapse supernovae, which is identical to the frequency of NS mergers estimated from the analysis of stellar abundances.

¹Shigeru Wakita, ^1,2Yuji Matsumoto, ¹Shoichi Oshino, ³Yasuhiro Hasegawa
The Astrophysical Journal 834, 125 Link to Article [http://dx.doi.org/10.3847/1538-4357/834/2/125]
¹Center for Computational Astrophysics, National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan
²Planetary Exploration Research Center, Narashino, Chiba 275-0016, Japan
³Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA

Chondritic meteorites contain unique spherical materials named chondrules: sub-mm sized silicate grains once melted in a high temperature condition in the solar nebula. We numerically explore one of the chondrule forming processes—planetesimal collisions. Previous studies have found that impact jetting via protoplanet–planetesimal collisions can make chondrules with 1% of the impactors’ mass, when the impact velocity exceeds 2.5 km s−1. Based on the mineralogical data of chondrules, undifferentiated planetesimals would be more suitable for chondrule-forming collisions than potentially differentiated protoplanets. We examine planetesimal–planetesimal collisions using a shock physics code and find two things: one is that planetesimal–planetesimal collisions produce nearly the same amount of chondrules as protoplanet–planetesimal collisions (~1%). The other is that the amount of produced chondrules becomes larger as the impact velocity increases when two planetesimals collide with each other. We also find that progenitors of chondrules can originate from deeper regions of large targets (planetesimals or protoplanets) than small impactors (planetesimals). The composition of targets is therefore important, to fully account for the mineralogical data of currently sampled chondrules.

^1,2Cristobal Petrovich, ³Diego J. Muñoz
The Astrophysical Journal 834, 116  Link to Article [http://dx.doi.org/10.3847/1538-4357/834/2/116]
¹Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St George Street, ON M5S 3H8, Canada
²Centre for Planetary Sciences, Department of Physical & Environmental Sciences, University of Toronto at Scarborough, Toronto, Ontario M1C 1A4, Canada
³Cornell Center for Astrophysics and Planetary Science, Department of Astronomy, Cornell University, Ithaca, NY 14853, USA

The presence of a planetary system can shield a planetesimal disk from the secular gravitational perturbations due to distant outer massive objects (planets or stellar companions). As the host star evolves off the main sequence to become a white dwarf, these planets can be engulfed during the giant phase, triggering secular instabilities and leading to the tidal disruptions of small rocky bodies. These disrupted bodies can feed the white dwarfs with rocky material and possibly explain the high-metallicity material in their atmospheres. We illustrate how this mechanism can operate when the gravitational perturbations are due to the KL mechanism from a stellar binary companion, a process that is activated only after the planet has been removed/engulfed. We show that this mechanism can explain the observed accretion rates if: (1) the planetary engulfment happens rapidly compared to the secular timescale, which is generally the case for wide binaries ($\gt 100$ au) and planetary engulfment during the asymptotic giant branch; (2) the planetesimal disk has a total mass of $\sim {10}^{-4}-{10}^{-2}{M}_{\oplus }$. We show that this new mechanism can provide a steady supply of material throughout the entire life of the white dwarfs for all cooling ages and can account for a large fraction (up to nearly half) of the observed polluted white dwarfs.

¹G. Micca Longo, ²S. Longo
International Journal of Astrobiology (in Press) Link to Article [DOI: https://doi.org/10.1017/S1473550416000495]
¹Department of Chemistry, University of Bari, via Orabona 4, 70126 Bari, Italy
²CNR-Nanotec, via Amendola 122/D, 70126 Bari, Italy

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¹Carl Melis, ²P. Dufour
The Astrophysical journal (in Press) Link to Article [http://dx.doi.org/10.3847/1538-4357/834/1/1]
¹Center for Astrophysics and Space Sciences, University of California, San Diego, CA 92093-0424, USA
²Institut de Recherche sur les Exoplanètes (iREx), Université de Montréal, Montréal, QC H3C 3J7, Canada

We present spectroscopic observations of the dust- and gas-enshrouded, polluted, single white dwarf star SDSS J104341.53+085558.2 (hereafter SDSS J1043+0855). Hubble Space Telescope Cosmic Origins Spectrograph far-ultraviolet spectra combined with deep Keck HIRES optical spectroscopy reveal the elements C, O, Mg, Al, Si, P, S, Ca, Fe, and Ni and enable useful limits for Sc, Ti, V, Cr, and Mn in the photosphere of SDSS J1043+0855. From this suite of elements we determine that the parent body being accreted by SDSS J1043+0855 is similar to the silicate Moon or the outer layers of Earth in that it is rocky and iron-poor. Combining this with comparison to other heavily polluted white dwarf stars, we are able to identify the material being accreted by SDSS J1043+0855 as likely to have come from the outermost layers of a differentiated object. Furthermore, we present evidence that some polluted white dwarfs (including SDSS J1043+0855) allow us to examine the structure of differentiated extrasolar rocky bodies. Enhanced levels of carbon in the body polluting SDSS J1043+0855 relative to the Earth–Moon system can be explained with a model where a significant amount of the accreted rocky minerals took the form of carbonates; specifically, through this model the accreted material could be up to 9% calcium-carbonate by mass.

¹A.Kereszturi, ²I. Gyollai, ³S. Jozsa, ⁴Z. Kanuchova
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12802]
¹Konkoly Astronomical Institute, Research Centre for Astronomy and Earth Sciences, Budapest, Hungary
²Institute for Geological and Geochemical Research, Research Centre for Astronomy and Earth Sciences, Budapest, Hungary
³Faculty of Science, Department of Petrology and Geochemistry, Hungarian Academy of Sciences, Eotvos Lorand University of Sciences, Budapest, Hungary
⁴Astronomical Institute of Slovak Academy of Sciences, T. Lomnica, Slovakia
Published by arrangement with John Wiley & Sons

The NWA 5491 CV3 meteorite is a CVoxA subtype, and composed of two substantially different units (titled “upper” and “lower” units) in the cm size range with original accreted material and also subsequent alteration produced features. Based on the large chondrules in the “upper” unit and the small chondrules plus CAIs in the “lower” unit, they possibly accreted material from different parts of the solar nebula and/or at different times, whereas substantial changes happened in the nebula’s composition. Differences are observed in the level of early fragmentation too, which was stronger in the upper units. During later alteration oxidizing fluids possibly circulated only in the upper unit, mechanical fragmentation and resorption were also stronger there. In the last phase of the geological history these two rock units came into physical contact, but impact-driven shock effects were not observed. The characteristics of this meteorite provide evidence that the same parent body might accrete substantially different material and also the later processes could differ spatially in the parent body.

¹W. M. Farrell, ²D. M. Hurley, ³V. J. Esposito, ⁴J. L. McLain, ²M. I. Zimmerman
Journal of Geophysical Research (Planets) Link to Article [DOI: 10.1002/2016JE005168]
¹NASA/Goddard Space Flight Center, Greenbelt, MD, USA
²Johns Hopkins University/Applied Physics Laboratory, Laurel, MD, USA
³NASA Goddard Summer Intern Program, NASA/Goddard Space Flight Center, Greenbelt, MD, USA
⁴University of Maryland, College Park, MD, USA
Published by arrangement with John Wiley & Sons

We present a new formalism to describe the outgassing of hydrogen initially implanted by the solar wind protons into exposed soils on airless bodies. The formalism applies a statistical mechanics approach similar to that applied recently to molecular adsorption onto activated surfaces. The key element enabling this formalism is the recognition that the inter-atomic potential between the implanted H and regolith-residing oxides is not of singular value, but possess a distribution of trapped energy values at a given temperature, F(U, T). All subsequent derivations of the outward diffusion and H retention rely on the specific properties of this distribution. We find that solar wind hydrogen can be retained if there are sites in the implantation layer with activation energy values exceeding 0.5 eV. We especially examine the dependence of H retention applying characteristic energy values found previously for irradiated silica and mature lunar samples. We also apply the formalism to two cases that differ from the typical solar wind implantation at the Moon. First, we test for a case of implantation in magnetic anomaly regions where significantly lower energy ions of solar wind origin are expected to be incident with the surface. In magnetic anomalies, H retention is found to be reduced due to the reduced ion flux and shallower depth of implantation. Second, we also apply the model to Phobos where the surface temperature range is not as extreme as the Moon. We find the H atom retention in this second case is higher than the lunar case due to the reduced thermal extremes (that reduces outgassing).

^1,4Chloë E. Bonamici ¹William S. Kinman, ²John H. Fournelle, ^1,5Mindy M. Zimmer, ¹Anthony D. Pollington, ³Kirk D. Rector
Contributions to Mineralogy and Petrolology 172, 2 Link to Article [doi:10.1007/s00410-016-1320-2]
¹Nuclear and Radiochemistry Group, Chemistry Division, Los Alamos National Laboratory, Los Alamos, USA
²Department of Geoscience, University of Wisconsin-Madison, Madison, USA
³Physical Chemistry and Applied Spectroscopy Group, Chemistry Division, Los Alamos National Laboratory, Los Alamos, USA
⁴Department of Earth and Environmental Science, New Mexico Tech, Socorro, USA
⁵Pacific Northwest National Laboratory, Richland, USA

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¹Yanhao Lin, ¹Elodie J. Tronche, ¹Edgar S. Steenstra, ¹Wim van Westrenen
Nature Geoscience 10, 14-18 Link to Article [doi:10.1038/ngeo2845]
¹Faculty of Earth and Life Sciences, VU Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands

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^1,2Olivier Namur, ¹Bernard Charlier
Nature Geoscience 10, 9-13 Link to Article [doi:10.1038/ngeo2860]
¹Leibniz University Hannover, Institute of Mineralogy, 30167 Hannover, Germany
²University of Liège, Department of Geology, 4000 Sart-Tilman, Belgium

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