Thorium in Kilonova Spectra: Exploring the Heaviest Detectable Element
Kilonova spectra provide us with direct information about rapid neutron-capture nucleosynthesis ( r -process) in neutron star (NS) mergers. In this paper, we study the signatures of elements beyond the third r -process peak expected to be produced in neutron-rich ejecta in the photospheric spectra o...
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2024-01-01
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| Series: | The Astrophysical Journal |
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| Online Access: | https://doi.org/10.3847/1538-4357/ad96b3 |
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| author | Nanae Domoto Shinya Wanajo Masaomi Tanaka Daiji Kato Kenta Hotokezaka |
| author_facet | Nanae Domoto Shinya Wanajo Masaomi Tanaka Daiji Kato Kenta Hotokezaka |
| author_sort | Nanae Domoto |
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| description | Kilonova spectra provide us with direct information about rapid neutron-capture nucleosynthesis ( r -process) in neutron star (NS) mergers. In this paper, we study the signatures of elements beyond the third r -process peak expected to be produced in neutron-rich ejecta in the photospheric spectra of kilonova. We select Ra ii , Ac iii , and Th iii as our candidates because they have a small number of valence electrons and low-lying energy levels, which tend to result in strong absorption features. We systematically calculate the strength of bound–bound transitions of these candidates by constructing a line list based on the available atomic database. We find that Th iii is the most promising species, showing strong transitions at near-infrared wavelengths. By performing radiative transfer simulations, we find that Th iii produces broad absorption features at ∼18000 Å in the spectra when the mass ratio of actinides to lanthanides is larger than the solar r -process ratio and the mass fraction of lanthanides is ≲6 × 10 ^−4 . Our models demonstrate that the Th feature may be detectable if the bulk of the ejecta in the line-forming region is dominated by relatively light r -process elements with the mixture of a small fraction of very neutron-rich material. Such conditions may be realized in the mergers of unequal-mass NSs or black hole–NS binaries. To detect the Th absorption features, observations from space (such as with the JWST) or high-altitude sites are important as the wavelength region of the Th features overlaps with that affected by strong telluric absorption. |
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| institution | Kabale University |
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| language | English |
| publishDate | 2024-01-01 |
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| spelling | doaj-art-8f13038f23a94f3c8736c99289d971092024-12-27T14:49:16ZengIOP PublishingThe Astrophysical Journal1538-43572024-01-0197819910.3847/1538-4357/ad96b3Thorium in Kilonova Spectra: Exploring the Heaviest Detectable ElementNanae Domoto0https://orcid.org/0000-0002-7415-7954Shinya Wanajo1https://orcid.org/0000-0002-4759-7794Masaomi Tanaka2https://orcid.org/0000-0001-8253-6850Daiji Kato3https://orcid.org/0000-0002-5302-073XKenta Hotokezaka4https://orcid.org/0000-0002-2502-3730Astronomical Institute, Tohoku University , Aoba, Sendai 980-8578, JapanAstronomical Institute, Tohoku University , Aoba, Sendai 980-8578, Japan; Max-Planck-Institut für Gravitationsphysik (Albert-Einstein-Institut) , Am Mühlenberg 1, D-14476 Potsdam-Golm, Germany; Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS), RIKEN , Wako, Saitama 351-0198, JapanAstronomical Institute, Tohoku University , Aoba, Sendai 980-8578, Japan; Division for the Establishment of Frontier Sciences, Organization for Advanced Studies, Tohoku University , Sendai 980-8577, JapanNational Institute for Fusion Science , 322-6 Oroshi-cho, Toki 509-5292, Japan; Interdisciplinary Graduate School of Engineering Sciences, Kyushu University , Kasuga, Fukuoka 816-8580, JapanResearch Center for the Early Universe, Graduate School of Science, University of Tokyo , Bunkyo, Tokyo 113-0033, Japan; Kavli IPMU (WPI), UTIAS, The University of Tokyo , Kashiwa, Chiba 277-8583, JapanKilonova spectra provide us with direct information about rapid neutron-capture nucleosynthesis ( r -process) in neutron star (NS) mergers. In this paper, we study the signatures of elements beyond the third r -process peak expected to be produced in neutron-rich ejecta in the photospheric spectra of kilonova. We select Ra ii , Ac iii , and Th iii as our candidates because they have a small number of valence electrons and low-lying energy levels, which tend to result in strong absorption features. We systematically calculate the strength of bound–bound transitions of these candidates by constructing a line list based on the available atomic database. We find that Th iii is the most promising species, showing strong transitions at near-infrared wavelengths. By performing radiative transfer simulations, we find that Th iii produces broad absorption features at ∼18000 Å in the spectra when the mass ratio of actinides to lanthanides is larger than the solar r -process ratio and the mass fraction of lanthanides is ≲6 × 10 ^−4 . Our models demonstrate that the Th feature may be detectable if the bulk of the ejecta in the line-forming region is dominated by relatively light r -process elements with the mixture of a small fraction of very neutron-rich material. Such conditions may be realized in the mergers of unequal-mass NSs or black hole–NS binaries. To detect the Th absorption features, observations from space (such as with the JWST) or high-altitude sites are important as the wavelength region of the Th features overlaps with that affected by strong telluric absorption.https://doi.org/10.3847/1538-4357/ad96b3R-processRadiative transfer simulationsAtomic physicsTransient sources |
| spellingShingle | Nanae Domoto Shinya Wanajo Masaomi Tanaka Daiji Kato Kenta Hotokezaka Thorium in Kilonova Spectra: Exploring the Heaviest Detectable Element The Astrophysical Journal R-process Radiative transfer simulations Atomic physics Transient sources |
| title | Thorium in Kilonova Spectra: Exploring the Heaviest Detectable Element |
| title_full | Thorium in Kilonova Spectra: Exploring the Heaviest Detectable Element |
| title_fullStr | Thorium in Kilonova Spectra: Exploring the Heaviest Detectable Element |
| title_full_unstemmed | Thorium in Kilonova Spectra: Exploring the Heaviest Detectable Element |
| title_short | Thorium in Kilonova Spectra: Exploring the Heaviest Detectable Element |
| title_sort | thorium in kilonova spectra exploring the heaviest detectable element |
| topic | R-process Radiative transfer simulations Atomic physics Transient sources |
| url | https://doi.org/10.3847/1538-4357/ad96b3 |
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