Computations of forbidden transition probabilities in lanthanide ions of interest for kilonova nebular phase analysis

In 2017, a gravitational waves signal (GW170819) associated with a neutron star merger was detected by the LIGO/Virgo collaboration. The coalescence of these two compact objects released a large quantity of matter in the space which was the site of nuclear reactions producing chemical elements heavier than iron such as lanthanides. This ejecta produced a transient electromagnetic phenomenon, most known as a kilonova. In the early phase, the spectrum of the kilonova is dominated by the millions of allowed transitions arising from these heavy elements. In the late times, when the kilonova is said to be in his nebular phase, the temperature and the density of the ejecta decrease so much that the ionization stage does not exceed the doubly charged species. Only low-lying levels, such as metastable levels, are populated giving rise to so-called emission forbidden lines, such as the magnetic dipole (M1) and electric quadrupole (E2) transitions. More recently, in March 2023, the James Webb Space Telescope reported spectroscopic observations of a transient afterglow similar to the AT2017gfo kilonova. Studies of the late recorded spectra showed some spectral features that could be explained by different candidate forbidden transitions like neodymium, erbium, or also other heavy elements. Element identification in nebular phase spectra is quite challenging since atomic data and especially forbidden line lists for heavy elements are scarce in the literature.

In order to extend the study of kilonovae in their nebular phase, we carried out new calculations of transition probabilities for M1 and E2 lines between low-lying levels in singly and doubly ionized lanthanide atoms. Given the lack of data in the literature, two theoretical methods were used in this work to model the atomic structure of these ions and to compute the radiative parameters. The first one is the fully-relativistic Multi-Configurational-Dirac-Hartree-Fock (MCDHF) method with the GRASP2018 code. The computed values were then compared to the results deduced from pseudo-relativistic Hartree-Fock (HFR) calculations in order to check their reliability. This allowed us to obtain a new consistent set of atomic data and to highlight the most intense lanthanides forbidden lines which are likely to be observed in the infrared spectrum of kilonovae in their nebular phase.