Nebular spectra of kilonovae with detailed recombination rates. I. light r -process composition

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Banerjee, Smaranika and Jerkstrand, Anders and Badnell, Nigel and Pognan, Quentin and Ferguson, Niamh and Grumer, Jon (2025) Nebular spectra of kilonovae with detailed recombination rates. I. light r -process composition. The Astrophysical Journal, 992 (1). 19. ISSN 1538-4357 (https://doi.org/10.3847/1538-4357/adf6ba)

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Abstract

To investigate spectra of kilonovae in the nonlocal thermal equilibrium phase (t ≳ 1 week), we perform atomic calculations for dielectronic recombination (DR) rates for the light r-process elements Se (Z = 34), Rb (Z = 37), Sr (Z = 38), Y (Z = 39), and Zr (Z = 40) using the HULLAC code. For the different elements, our results for the DR rate coefficients for recombining from the ionization states of II to I, III to II, and IV to III vary between 2 × 10−12–5 × 10−11 cm3 s−1, 10−13–5 × 10−11 cm3 s−1, and 2 × 10−15–10−11 cm3 s−1, respectively, at a temperature of T = 10,000 K. Using this new atomic data (DR), we study the impact on kilonova model spectra at phases of t = 10 days and t = 25 days after the merger using the spectral synthesis code SUMO. Compared to models using the previous treatment of recombination as a constant rate, the new models show significant changes in ionization and temperature, and, correspondingly, in emergent spectra. With the new rates, we find that Zr (Z = 40) plays a yet more dominant role in kilonova spectra for light r-process compositions. Furthermore, we show that previously predicted mid-infrared (e.g., [Se III] 4.55 μm) and optical (e.g., Rb I 7802, 7949 Å) lines weaken in the new model. Instead, [Se I] 5.03 μm emerges as a signature. These results demonstrate the importance of considering the detailed microphysics for modelling and interpreting the late-time kilonova spectra.

ORCID iDs

Banerjee, Smaranika, Jerkstrand, Anders, Badnell, Nigel ORCID logoORCID: https://orcid.org/0000-0001-7418-7996, Pognan, Quentin, Ferguson, Niamh and Grumer, Jon;
  • Item type:Article
    ID code:94352
    Dates:
    Date
    Event
    10 October 2025
    Published
    1 October 2025
    Published Online
    30 July 2025
    Accepted
    30 January 2025
    Submitted
    Subjects:Science > Astronomy
    Department:Faculty of Science > Physics
    Depositing user: Pure Administrator
    Date deposited:06 Oct 2025 10:48
    Last modified:02 Feb 2026 17:11
    URI:https://strathprints.strath.ac.uk/id/eprint/94352
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