15:30 〜 15:45
[PCG20-07] rプロセス核種の起源解明に向けた隕石中の核合成同位体異常の応用に関する最近の進展
★招待講演
キーワード：rプロセス、隕石、同位体異常
Nearly half the amounts of trans-Fe elements in the Solar System were produced by a rapid neutron capture reaction of the r-process. Core-collapse supernovae (ccSNe), historically known as a promising site of r-process, now became clear to synthesize relatively light r-nuclides of mass number (A) below 130 [1] with exceptions that exhibit unusual physical conditions such as very high entropy or strong magnetic field [2]. In contrast, several lines of evidence from astrophysical modelling and observations, including recent spectral identification of a bright kilonova associated with the observation of gravitational waves from a neutron star merger (NSM), suggested the production of not only abundant heavy r-nuclides with A > 130, but also Sr (A = ~88) in merging compact binaries [3,4]. More recently, Tsujimoto [5] found that the relation between [r-process/Fe] and the age of Sun-like stars in the inner Galactic region was incompatible with the hypothesis of a sole site for r-process production, suggesting a hybrid origin with NSM and ccSNe for r-process nuclides.
On the other hand, precise isotopic analyses of trans-Fe elements in meteorites offer another opportunity for investigating the origin of r-nuclides in the Solar System. Recent innovation in mass spectrometry techniques enabled detection of subtle isotopic differences in planetary materials for some trans-iron elements. More importantly, carbonaceous chondrites and some iron meteorites (CC-meteorites) are found to be enriched in r-process components compared to the other meteorites (NC-meteorites)[6], suggesting the presence of two isotopically distinct reservoirs in the asteroid belt [7]. In this study, we analyzed literature data for the isotopic compositions of trans-Fe elements in bulk meteorites with a specific emphasis on the anomalies of r-process nuclides as a function of isotopic mass and the 50% condensation temperature (T50%) of each element. To quantitatively analyze the extent of r-process excesses in individual meteorites, we determined the r-process enrichment factor for each element. We found that the r-excess in CC-meteorites was prominent at Sr-Zr-Mo while it decreased gradually toward Ru. No r-excesses were observed for elements heavier than Te. The observation indicates that the extent of r-excess in CC-meteorites was controlled mainly by the mass of nuclides rather than the T50% of individual elements. We propose that the materials originated from a stellar site that synthesized r-process nuclides lighter than Te are responsible for the r-excesses observed in CC-meteorites. The other r-process nuclides including heavy nuclides, synthesized by other stellar sites, were homogeneously distributed in the early Solar System. Importantly, the lifetime of dust grains in the Galaxy is estimated to be up to 108 years [8]. It follows that carrier phases for r-nuclides produced more than 108 years before the start of the Solar System have been all destroyed and the r-nuclides were homogeneously distributed in the molecular cloud. On the contrary, the materials for the r-excesses in CC meteorites were synthesized within 108 years before the onset of the Solar System as they were heterogeneously distributed in the solar nebula at the time of planetesimal formation.
References
[1] S. Wanajo. Astrophys. J. 770, L22 (2013). [2] N. Nishimura, et al. Astrophys. J. 810, 109 (2015). [3] E. Pian et al. Nature 551, 67 (2017). [4] Watson et al. Nature 574, 497-500 (2019). [5] T. Tsujimoto. Astrophys. J. 920, L32 (2021). [6] T. S. Kruijer, et al. Proc. Natl. Acad. Sci. U.S.A. 114, 6712-6716 (2017). [7] P. H. Warren. Earth Planet. Sci. Lett. 311, 93-100 (2011). [8] A. Jones, J. Nuth. Astronomy & Astrophysics 530, A44 (2011).
On the other hand, precise isotopic analyses of trans-Fe elements in meteorites offer another opportunity for investigating the origin of r-nuclides in the Solar System. Recent innovation in mass spectrometry techniques enabled detection of subtle isotopic differences in planetary materials for some trans-iron elements. More importantly, carbonaceous chondrites and some iron meteorites (CC-meteorites) are found to be enriched in r-process components compared to the other meteorites (NC-meteorites)[6], suggesting the presence of two isotopically distinct reservoirs in the asteroid belt [7]. In this study, we analyzed literature data for the isotopic compositions of trans-Fe elements in bulk meteorites with a specific emphasis on the anomalies of r-process nuclides as a function of isotopic mass and the 50% condensation temperature (T50%) of each element. To quantitatively analyze the extent of r-process excesses in individual meteorites, we determined the r-process enrichment factor for each element. We found that the r-excess in CC-meteorites was prominent at Sr-Zr-Mo while it decreased gradually toward Ru. No r-excesses were observed for elements heavier than Te. The observation indicates that the extent of r-excess in CC-meteorites was controlled mainly by the mass of nuclides rather than the T50% of individual elements. We propose that the materials originated from a stellar site that synthesized r-process nuclides lighter than Te are responsible for the r-excesses observed in CC-meteorites. The other r-process nuclides including heavy nuclides, synthesized by other stellar sites, were homogeneously distributed in the early Solar System. Importantly, the lifetime of dust grains in the Galaxy is estimated to be up to 108 years [8]. It follows that carrier phases for r-nuclides produced more than 108 years before the start of the Solar System have been all destroyed and the r-nuclides were homogeneously distributed in the molecular cloud. On the contrary, the materials for the r-excesses in CC meteorites were synthesized within 108 years before the onset of the Solar System as they were heterogeneously distributed in the solar nebula at the time of planetesimal formation.
References
[1] S. Wanajo. Astrophys. J. 770, L22 (2013). [2] N. Nishimura, et al. Astrophys. J. 810, 109 (2015). [3] E. Pian et al. Nature 551, 67 (2017). [4] Watson et al. Nature 574, 497-500 (2019). [5] T. Tsujimoto. Astrophys. J. 920, L32 (2021). [6] T. S. Kruijer, et al. Proc. Natl. Acad. Sci. U.S.A. 114, 6712-6716 (2017). [7] P. H. Warren. Earth Planet. Sci. Lett. 311, 93-100 (2011). [8] A. Jones, J. Nuth. Astronomy & Astrophysics 530, A44 (2011).