Titre de la dissertation: « Multi-platform computations of radiative properties and opacities in moderately-charged lanthanides in the context of early-phase kilonovae following neutron star mergers ».

Promoteurs de thèse: Monsieur Pascal Quinet et Monsieur Patrick Palmeri

 Résumé de la dissertation: The first detection of the gravitational waves, produced by the coalescence of two neutron stars, was observed on August 17, 2017, by the interferometers LIGO (USA) and VIRGO (Italy). This event, named GW170817, was followed by a short gamma-ray burst detected 1.7 seconds later by the FERMI and INTEGRAL space telescopes. This collision also produced an electromagnetic signal powered by the ejection of hot and radioactive matter, known as a kilonova. The spectral analysis of the latter recorded by several tens of telescopes, operating in the infrared, the visible, the ultraviolet and the X-ray wavelength ranges, revealed the presence of heavy elements. In this context, the lanthanides (Z = 57 – 71) play an important role because, given their rich spectra, they significantly contribute to the kilonova opacity. In order to interpret and model the spectrum of a kilonova, it is therefore crucial to precisely know the radiative parameters characterising these elements. While the determination of such parameters has already been the subject of various studies over the recent years, the latter only concern the first ionisation degrees (up to 3+) and are therefore limited to the analysis of kilonovae in a temperature range below 20000 K. To extend the modelling of this type of celestial object to higher temperatures, corresponding to the early phase of kilonovae (less than a day post-merger), it is essential to know the radiative parameters of lanthanide ions in higher charge stages for which few studies have been conducted so far. Our PhD thesis aims to make a significant contribution in this field as it consists of a detailed study of the radiative processes characterising moderately-charged lanthanide ions (from 4+ to 9+) and to deduce the corresponding astrophysical opacities for typical early-phase kilonova ejecta conditions 0.1 day after the merger. As there is almost no experimental data available for these ions, our calculations are based on a multi-platform approach involving different independent theoretical methods, namely the pseudo-relativistic Hartree-Fock (HFR), the fully relativistic Multiconfiguration Dirac-Hartree-Fock (MCDHF) and the Configuration Interaction Many-Body Perturbation Theory (CI+MBPT) methods. Due to the absence of sufficient experimental data, this approach is the only way to estimate the accuracy of the results obtained through systematic comparisons between distinct computational procedures. Atomic data were determined for moderately-charged (V—X) ions from Z = 57 to 62 and (V—VII) ions from Z = 63 to 71 using the HFR method. For some specific ions, radiative parameters were also determined by MCDHF and CI+MBPT methods in order to benchmark our HFR results. After a comparison between our theoretical transition probabilities and oscillator strengths with the available experimental data, our HFR results were used to compute astrophysical opacities (expansion and Planck mean opacities) for these elements. This allowed us to determine which elements contribute the most to the kilonova opacity. For many lanthanide ions, our study allowed also to provide the ground configuration levels which were unclearly determined up to now. Finally, it was found that, for some lanthanide ions, namely those with Z = 63 – 71 and ionisation stages VIII – X, the configurations were so complex (with unfilled 4f and 5p subshells) that it was extremely complicated, or even impossible, to make the theoretical calculations, the huge size of the energy matrices making the diagonalization of the Hamiltonian extremely challenging. In these cases, in order to obtain the spectroscopic parameters required to compute opacities, we investigated another approach, the so-called Resolved Transition Arrays (RTA) method, based on statistical simulations rather than using full computational methods. Such method was tested for Sm VIII and Eu VI, two ions whose atomic data were already calculated by computational methods, in order to validate the obtained opacities by comparing them with the results deduced from full atomic calculations. Using compact formulae, we were therefore able to simulate atomic data for Dy VIII (Z = 66) in order to estimate the corresponding expansion opacity. The numerous new results obtained in our work represent a significant contribution to the study of atomic parameters and the determination of opacities affecting kilonovae spectra for moderately ionized lanthanide atoms. To do this, the combination of different theoretical methods based on pseudo- and fully-relativistic atomic structure computational approaches, on the one hand, and a statistical approach, on the other hand, was implemented for the first time, allowing to deduce radiative data for billions of spectral lines in lanthanide ions from 4+ to 9+ and to estimate the corresponding expansion and Planck mean opacities in early phase kilonovae following neutron star mergers.