Atomic Processes for Late-Stage Kilonovae

A kilonova is the phenomenon produced by the merging of two neutron stars. It became of great interest after the 2017 detection of gravitational waves produced by-such. Atomic processes govern many aspects of the light we observe from kilonovae and by which we test our models of them.

The heavy elements (e.g. lanthanides) produced by nuclear fusion in the early phase of a kilonova evolution, dominate. There is poor understanding of the atomic processes for these few-times ionized atoms, especially when investigating the later non-local thermodynamic equilibrium (LTE) phase of kilonovae.

A key quantity is the relative abundance of various ionization stages of the constituent elements. This results from competition between ionization and recombination. The process known as dielectronic recombination (DR) dominates here, as it does in virtually all non-LTE plasmas.

Currently, a constant recombination rate is used by kilonovae modellers. This is extremely crude compared to other areas. From solar and magnetic fusion modelling, it is clear that an accurate description of atomic processes is needed here to support the progression of the field.

However, there is a complexity surrounding these low-charged heavy elements and the atomic physics is both fascinating and challenging, which goes beyond data production.

The AUTOSTRUCTURE general atomic code used in the pioneering work on kilonovae by Kasen, Badnell and Barnes will be used to investigate the recombination and ionization of heavy elements key to kilonovae modelling and to assess the uncertainty in resultant DR rate coefficients. Our work will engage closely with our collaborative modelling groups.