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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| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
IOP Publishing
2024-01-01
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| Series: | The Astrophysical Journal |
| Subjects: | |
| Online Access: | https://doi.org/10.3847/1538-4357/ad96b3 |
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| Summary: | 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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| ISSN: | 1538-4357 |