Thesis title: Modelling of early-stage kilonova ejecta opacity in the multi-messenger era
In the multi-messenger era, the observation of an electromagnetic counterpart of the GW170817 event, known as kilonovae, provided evidence that the coalescence of binary neutron star systems represents a key site of r-process nucleosynthesis. Kilonovae are quasi-thermal electromagnetic transients powered by the radioactive decay of heavy neutron-rich isotopes synthesized in the aftermath of binary neutron star mergers. Due to the complexity of the phenomenon, essential information about the kilonova ejecta composition can be achieved thanks to models that incorporate radiative transfer simulations together with accurate numerical opacity estimations in view of spectral analysis characterisation.
Since light r-process elements are major contributors to the opacity of the early ($\sim\,0.5$-$1.5\,\mathrm{d}$) ejecta, a detailed study of the selenium element with a focus on atomic data calculation, expansion opacity estimation, and spectral analysis is presented.
In this thesis, the selenium (Se) atomic data are calculated from Se I to Se X using the \texttt{GRASP2018} code. A systematic analysis and evaluation of their precision is performed by a detailed comparison with the \texttt{NIST ASD}, and other works available in the literature. These atomic data are then used to estimate the expansion opacity at different temperatures (e.g., $T=5\,000\,\mathrm{K},\,10\,000\,\mathrm{K},\,20\,000\,\mathrm{K},\,100\,000\,\mathrm{K}$) and densities (e.g., $\rho = 10^{-13}\,\mathrm{g\,cm^{-3}},\,
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3\times10^{-12}\,\mathrm{g\,cm^{-3}}$). The selenium atomic calculations show good agreement with \texttt{NIST ASD}, with accurate energy levels and transitions determined alongside atomic data for higher ionization stages not fully covered by \texttt{NIST ASD}. The expansion opacities are computed with improved accuracy compared to those in the existing literature works.
Finally, spectral analysis has been performed with the Monte Carlo radiative transfer code \texttt{POSSIS} with a precomputed opacity grid calculated with new densities -19.5 $\le$ log $\rho$ $\le$ -4.5 $\mathrm{g\,cm^{-3}}$ on the logarithmic scale at steps of log $\rho$ = 0.5 and temperatures $1\,000$ $\le$ $T$ $\le$ $51\,000\,$ K with an increase of 500 K at each density iteration. In the analysis, two scenarios are considered: one in which the opacity contribution comes from 100\% selenium ejecta, and another in which selenium contributes only partially to the total opacity ($\sim$ 10\% of the total mass). Selenium spectral features can only be observed when KN is 100\% selenium, while pushing to 10\%, these features become undetectable.
All selenium results are now available on a new open-source \texttt{MARTINI} platform dedicated to element nucleosynthesis.