Daiki Yamamoto¹, Minami Kuroda¹, Shogo Tachibana^1,2, Naoya Sakamoto³, and Hisayoshi Yurimoto^1,4
Astrophysical Journal 865, 98 Link to Article [DOI: 10.3847/1538-4357/aadcee]
¹Department of Natural History Sciences, Hokkaido University, Sapporo, Hokkaido 060-0810, Japan
²UTokyo Organization for Planetary Space Science, The University of Tokyo, Hongo, Tokyo 113-0033, Japan
³Isotopic Imaging Laboratory, Hokkaido University, Sapporo, 001-0021, Japan
⁴Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa, 252-210, Japan

Meteoritic evidence suggests that oxygen isotopic exchange between ¹⁶O-rich amorphous silicate dust and ¹⁶O-poor water vapor occurred in the early solar system. In this study, we experimentally investigated the kinetics of oxygen isotopic exchange between submicron-sized amorphous forsterite grains and water vapor at protoplanetary disk-like low pressures of water vapor. The isotopic exchange reaction rate is controlled either by diffusive isotopic exchange in the amorphous structure or by the supply of water molecules from the vapor phase. The diffusive oxygen isotopic exchange occurred with a rate constant D (m² s⁻¹) = (1.5 ± 1.0) × 10⁻¹⁹ exp[−(161.5 ± 14.1 (kJ mol⁻¹))R⁻¹(1/T−1/1200)] at temperatures below ~800–900 K, and the supply of water molecules from the vapor phase could determine the rate of oxygen isotopic exchange at higher temperatures in the protosolar disk. On the other hand, the oxygen isotopic exchange rate dramatically decreases if the crystallization of amorphous forsterite precedes the oxygen isotopic exchange reaction with amorphous forsterite. According to the kinetics for oxygen isotopic exchange in protoplanetary disks, original isotopic compositions of amorphous forsterite dust could be preserved only if the dust was kept at temperatures below 500–600 K in the early solar system. The ¹⁶O-poor signatures for the most pristine silicate dust observed in cometary materials implies that the cometary silicate dust experienced oxygen isotopic exchange with ¹⁶O-poor water vapor through thermal annealing at temperatures higher than 500–600 K prior to their accretion into comets in the solar system.

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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