Line identifications for heavy elements of importance in kilonova modelling

Gravitational waves were first detected arising from a binary black hole merger, and subsequently also observed from a neutron star merger (NSM), GW170817. The electromagnetic counterpart to GW170817, resulting from the material ejected during the merger, was also identified and is called a kilonova. These are luminous at optical and infrared wavelengths, hence allowing the detailed study of NSMs. Of particular relevance to the present proposal is the role of NSMs in the production of heavy elements via rapid neutron capture (r-process). The r-process pathway is responsible for creating 50% of all the heavy elements (Z > 30), and is the only source of elements beyond bismuth at Z = 83. Several sites for the production of r-processed elements have been theorised, but NSMs are the only astrophysical sources where the production of r-process elements is confirmed, and also appear to be responsible for most of their creation. Observations of the expanding material from an NSM - i.e. kilonovae - hence provide the only possibility to detect and quantify the r-process in situ. This in turn will allow some of the biggest open questions in astrophysics to be addressed, namely how and where are the heavy elements from Fe to U made.

Unfortunately, the large outflow velocities of kilonovae ejecta (~0.1-0.3c) leads to spectral lines being both Doppler broadened and shifted from their rest wavelengths. Consequently, individual lines cannot be resolved and employed to determine element abundances, and researchers need to use radiative transfer codes to model the spectra, and assess the presence (or otherwise) of r-process elements. Particularly interesting elements to search for evidence of their presence are Pt, Au, Ce, La, Pb and Os, with all predicted to be important for modelling the spectra of kilonova. However, the atomic data required as input to kilonova models for these elements are very limited, including for line wavelengths. These are vital, as any absorption or emission due to such features may hence be missed by models if the lines are not included. One can use theoretical wavelengths, but these must be tested and calibrated via experimental measurements, to ensure that transitions are properly identified and that their (calculated) transition probabilities are corrected for the energy difference delta-E between theory and experiment (which scales as delta-E^3 for allowed lines).

The very limited line identification data available is illustrated by NIST Atomic Spectra Database results for Au, which lists only 2 lines of Au I-IV at wavelengths >800 nm, where theoretical work indicates one might expect to find strong, observable spectral features. Given this, we have initiated an ambitious programme to obtain accurate experimental line wavelengths for the I-IV ionisation stages of Pt, Au, Ce, La, Pb and Os. We will focus on the unexplored spectral region above 800 nm and in particular the important infrared range (1 - 5 microns), observed by VLT and JWST instruments, plus the upcoming ELT, and yet currently poorly modelled. For our programme we will use laser-generated plasmas of high purity samples in high vacuum, greatly reducing contamination and hence blending with other elements. We have already undertaken a proof-of-concept study on Au between 300 - 1000 nm, where we detected a total of 81 new Au I and Au II emission lines, including 7 in the 800 - 950 nm range. Our programme is focused on the identification of allowed lines, but we will also undertake some new experiments to try and identify non-allowed lines of species in the infrared region, which may appear during the later stages of kilonova when the plasma becomes optically thin. In addition we will seek input from the kilonova research community for line identification data for other elements, or wavelength regions, whose importance only arises during the project.