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I. Se I–X atomic calculation and expansion opacity for early stage kilonova spectral analysis

¹ Department of Physics, Sapienza University of Rome, P.le A. Moro 5, 00185 Rome, Italy
² National Institute for Astrophysics – Astronomical Observatory of Abruzzo, Via Maggini snc, 64100 Teramo, Italy
³ National Institute for Nuclear Physics – Section of Perugia, Via A. Pascoli, Perugia, Italy
⁴ Department of Physics and Earth Science, University of Ferrara, Via Saragat 1, I-44122 Ferrara, Italy
⁵ National Institute for Nuclear Physics – Section of Ferrara, Via Saragat 1, I-44122 Ferrara, Italy
⁶ Institute of Theoretical Physics and Astronomy, Vilnius University, Saulėtekio Ave. 3 Vilnius, Lithuania

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 26 July 2025
Accepted: 22 February 2026

Abstract

Context. In the multi-messenger era, kilonovae represent key sites of r-process nucleosynthesis, making opacity estimation and spectral analysis crucial for constraining their composition.

Aims. Since light r-process elements shape the early (∼0.5 − 1.5 d) ejecta opacity, we present a detailed study of selenium with a focus on atomic data calculation, expansion opacity estimation, and spectral analysis.

Methods. We calculated the selenium atomic data from Se I to Se X using the GRASP2018 code. We performed a systematic analysis and evaluation of their precision through a detailed comparison with the NIST ASD, and other works available in the literature. We then used these atomic data to estimate expansion opacity at different temperatures (e.g. T = 5000 K,  10 000 K,  20 000 K,  100 000 K) and densities (e.g. ρ = 10⁻¹³ g cm⁻³,  3 × 10⁻¹² g cm⁻³). We performed spectral analysis with the Monte Carlo radiative transfer code POSSIS, with a pre-computed opacity grid calculated with new densities and temperatures, ranging from −19.5 to −4.5 g cm⁻³ in log-scale and from 1000 to 51 000 K, respectively. In the analysis, we considered two scenarios: one in which the opacity contribution comes from 100% selenium ejecta, and another in which selenium only partially contributes to the total opacity (∼10% of the total mass).

Results. The selenium atomic calculations show a good agreement with NIST ASD, with accurate energy levels and transitions determined alongside atomic data for higher ionisation stages not fully covered by NIST ASD. The expansion opacities calculated with these new selenium data exhibit differences in comparison to existing works in the literature. Selenium spectral features can only be observed in the kilonova scenario consisting of 100% selenium. When selenium accounts for about 10% of the total kilonova mass, these features become undetectable. Finally, all selenium results are now available in the new open-source MARTINI platform dedicated to element nucleosynthesis.