Takuya Ojima¹, Yuhri Ishimaru¹, Shinya Wanajo^2,3, Nikos Prantzos⁴, and Patrik François^5,6
Astrophysical Journal 865, 87 Link to Article [DOI: 10.3847/1538-4357/aada11]
¹Department of Material Science, International Christian University, 3-10-2 Osawa, Mitaka, Tokyo 181-8585, Japan
²Department of Engineering and Applied Sciences, Sophia University, Chiyoda-ku, Tokyo 102-8554, Japan
³iTHEMS Research Group, RIKEN, Wako, Saitama 351-0198, Japan
⁴Institut d’Astrophysique de Paris, UMR7095 CNRS, Univ. P. & M. Curie, 98bis Bd. Arago, F-75104 Paris, France
⁵GEPI, Observatoire de Paris, PSL Research University, CNRS, 61 Avenue de l’Observatoire, F-75014 Paris, France
⁶Université de Picardie Jules Verne, 33 rue St Leu, Amiens, France

Mergers of compact binaries (of a neutron star and another neutron star or a black hole, NSMs) are suggested to be the promising astrophysical site of the r-process. While the average coalescence timescale of NSMs appears to be , most of previous chemical evolution models indicate that the observed early appearance and large dispersion of in Galactic halo stars at favors shorter coalescence times of 1–10 Myr. We argue that this is not the case for the models assuming the formation of the Galactic halo from clustering of subhalos with different star formation histories as suggested by Ishimaru et al. We present a stochastic chemical evolution model of the subhalos, in which the site of the r-process is assumed to be mainly NSMs with a coalescence timescale of . In view of the scarcity of NSMs, their occurrence in each subhalo is computed with a Monte Carlo method. Our results show that the less massive subhalos evolve at lower metallicities and generate highly r-process-enhanced stars. An assembly of these subhalos leaves behind the large star-to-star scatters of in the Galactic halo as observed. However, the observed scatters of [Sr/Ba] at low metallicities indicate the presence of an additional site that partially contributes to the enrichment of light neutron-capture elements such as Sr. The high enhancements of at low metallicities found in our low-mass subhalo models also qualitatively reproduce the abundance signatures of the stars in the recently discovered ultra-faint dwarf galaxy Reticulum II. Therefore, our results suggest NSMs as the dominant sources of r-process elements in the Galactic halo.

¹Francis Nimmo²Katherine Kretke,³Shigeru Ida,⁴Soko Matsumura,⁵Thorsten Kleine
Space Science Reviews 214, 101 Link to Article [DOI
https://doi.org/10.1007/s11214-018-0533-2]
¹Dept. Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, USA
²South-west Research Institute, Boulder, USA
³Earth-Life Science Institute, Tokyo Institute of Technology, Tokyo, Japan
⁴Dept. Physics, Dundee University, Dundee, UK
⁵Institut fur Planetologie, Universitat Muenster Münster, Germany

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^1,2,3L. Suárez-Andrés, ^1,2G. Israelian, ^1,2J. I. González Hernández, ⁴V. Zh. Adibekyan, ⁴E. Delgado Mena, ^4,5N. C. Santos, ^4,5S. G. Sousa
Astronomy & Astrophysics 614, A84 Link to Article [https://doi.org/10.1051/0004-6361/201730743]
¹Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
²Departmento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
³Isaac Newton Group of Telescopes, Apartado de Correos 321, 38700 Santa Cruz de la Palma, Spain
⁴Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁵Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, 4169-007 Porto, Portugal

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¹James M.D.May
American Mineralogist 103, 11 Link to Article [https://doi.org/10.2138/am-2018-6368]
¹Scripps Institution of Oceanography, University of California San Diego, La Jolla, California 92093-0244, U.S.A.
Copyright: The Mineralogical Society of America

Low oxygen fugacity (fO2) in the lunar interior (one log unit below the iron-wüstite buffer [IW-1]) offers the possibility that stable Fe-metal and sulfide phases exist as restites within lunar mare basalt source regions. Metal and sulfide phases have high metal-melt and sulfide-melt partition coefficients for chalcophile, siderophile (>100), and highly siderophile elements (>>100 000; HSE: Os, Ir, Ru, Rh, Pt, Pd, Re, Au). If these phases are residual after mare basalt extraction, they would be expected to retain significant quantities of these elements, likely generating non-chondritic HSE inter-element ratios, including Re/Os in the silicate magma. If such phases were present, then the estimated HSE abundances of the bulk silicate moon (BSM) would be proportionally higher than current estimates (0.00023 ± 2 × CI chondrite), and perhaps closer to the bulk silicate earth (BSE) estimate (0.009 ± 2 × CI chondrite). Here I show that relationships between elements of similar incompatibility but with siderophile (W), chalcophile (Cu), and lithophile tendencies (Th, U, Yb) do not deviate from expected trends generated by magmatic differentiation during cooling and crystallization of mare basalts. These results, combined with chondrite-relative HSE abundances and near-chondritic measured 187Os/188Os compositions of primitive high-MgO mare basalts, imply that lunar mantle melts were generated from residual metal- and sulfide-free sources, or experienced complete exhaustion of metal and sulfides during partial melt extraction. Evidence for the loss of moderately volatile elements during lunar formation and early differentiation indicates that the BSM is >4 to 10 times more depleted in S than BSE. Because of an S-depleted BSM, mare basalt melts are unlikely to have reached S saturation, even if sulfide concentration at sulfide saturation (SCSS) was lowered relative to terrestrial values due to low lunar fO2. In the absence of residual sulfide or metal, resultant partial melt models indicate that a lunar mantle source with 25 to 75 μg/g S and high sulfide-melt partition coefficients can account for the chondritic-relative abundances of the HSE in mare basalts from a BSM that experienced <0.02% by mass of late accretion.

^1,3A.Drouard et al. (>10)
Astronomy & Astrophysics 613, A54 Link to Article [https://doi.org/10.1051/0004-6361/201732225]
¹Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
²Aix-Marseille Université, CNRS, IRD, Coll France, CEREGE UM34, 13545 Aix en Provence, France

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¹K.Demyk et al. (>10)
Astronomy & Astrophysics 600, A123 Link to Article [https://doi.org/10.1051/0004-6361/201629711]
¹ IRAP, Université de Toulouse, CNRS, UPS IRAP, 9 avenue Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France

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¹R. de Nutte et al. (>10)
Astronomy & Astrophysics 600, A71 Link to Article [https://doi.org/10.1051/0004-6361/201629195]
¹Institute of Astronomy, KU Leuven, Celestijnenlaan 200D B2401, 3001 Leuven, Belgium
e-mail: rutger.denutte@ster.kuleuven.be

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¹Martin Ferus et al. (>10)
Astronomy & Astrophysics 610, A73 Link to Article [https://doi.org/10.1051/0004-6361/201629950]
¹J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejškova 3, 18223 Prague 8, Czech Republic

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^1,2Quirico, E., ^1,2Bonal, L.
Advances in Astrobiology and Biogeophysics (Book Chapter) Link to Article [DOI: 10.1007/978-3-319-96175-0_2]
¹Université Grenoble Alpes, Grenoble, France
²Institut de Planétologie et d’Astrophysique de Grenoble, Grenoble, France

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¹Erokhin, Y.V., ¹Koroteev, V.A., ¹Khiller, V.V., ¹Ivanov, K.S., ²Kleimenov, D.A.
Doklady Earth Sciences 482, 1189-1192 Link to Article [DOI: 10.1134/S1028334X18090118]
¹Zavaritsky Institute of Geology and Geochemistry, Ural Branch, Russian Academy of Sciences, Yekaterinburg, 620016, Russian Federation
²Ural State Mining University, Yekaterinburg, 620144, Russian Federation

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