Speaker
Description
Observations of kilonovae, the electromagnetic counterparts to (binary) neutron star mergers, confirm their role as key sites for $r$-process nucleosynthesis, producing elements heavier than ion in the expanding ejecta following this event. The groundbreaking observation of the 2017’s kilonova (astronomical transient AT2017gfo) associated with the renowned gravitational wave event GW170817 provided robust evidence for the formation of lanthanides and, potentially, actinides. This conclusion was drawn from the temporal evolution of the kilonova’s luminosity curve across the ultraviolet, visible, and infrared regions of the electromagnetic spectrum. It marked the first observational confirmation that elements are there being synthesized through the rapid neutron-capture process ($r$-process), a mechanism proposed several decades ago. However, identifying specific features, which can be attributed to the absorption and emission lines of individual elements produced in the ejecta, in the luminosity curve remains a significant challenge, mainly due to the lack of atomic data for heavy atoms/ions.
The main difficulties in analyzing kilonova spectra are the severe lack of both atomic data for lanthanides and actinides, as well as collisional atomic data required to model the late stages ($>4$ days post-NSM) of the expanding ejecta. While it is reasonable to assume local thermodynamic equilibrium (LTE) and that atomic absorption processes dominate during the early epochs ($<1$ day after the merger), this assumption breaks down during the nebular phase (non-LTE). At these late stages, relevant physical processes include collisional processes such as electron-impact excitation and radiative recombination – all of which suffer from a scarcity of accurate atomic data.
To address this challenge, we have calculated key atomic parameters for lanthanides and actinides: for the former, atomic structure and recombination processes will be the main focus; for the latter, only atomic structure will be shown.