Electron-impact excitation collision strengths and radiative parameters in heavy elements for nebular-phase kilonova modeling

Gravitational waves emitted following a neutron star merger (NSM) were observed for the first time in 2017 (GW170817). An electromagnetic signal known as a kilonova (KN) emitted by newly synthesized r-process elements during such an event was also detected (AT2017gfo). More recently, the spectrum of a new KN was observed by the JWST at late times.

NSMs are candidate sites of heavy element production, a process that remains incompletely understood so far. KN modeling has thus become a topic of significant scientific interest. During the first few days after the merger, the KN ejecta is in its photospheric phase, and the LTE assumption is valid, simplifying the calculation of atomic energy level populations through Boltzmann and Saha equations. About a week post-merger, the KN ejecta begins its nebular phase, where non-LTE effects become important, making the determination of energy level populations extremely complex.

To model nebular-phase KN spectra, it is thus necessary to account for all radiative and collisional processes that can (de)populate the various energy levels of the ejecta ions, such as radiative transitions, electron impact collisions, dielectronic recombinations, photoionization and radiative recombinations. In this work, we model the electron-impact excitation process in heavy ions (potentially) observed in KN spectra, such as Sr II and Te I-III, by means of two different approaches: the Plane Wave Born approximation as implemented in the pseudo-relativistic Hartree-Fock (HFR) method and a Distorted Waves approach. The resulting collision strengths are compared with values available in the literature that were calculated using the more complex R-matrix method to assess the validity of our approximations in the purpose of large-scale calculations in all heavy elements (in particular, lanthanides and actinides) suspected to be present in the KN ejecta. In addition, radiative parameters for allowed and forbidden transitions are also computed using HFR.