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oai:figshare.com:article/32633622 2026-10-01T17:01:38Z portal_1172 item_type_8 month_year_10_2026

Spectroscopic modelling of astrophysical mergers and kilonovae James Gillanders (24165789) PUREID: 382767622 atomic data line identification radiative transfer neutron stars AT2017gfo AT2018kzr neutron star mergers r-process rapid neutron capture process In this thesis, we modelled the spectra of the two fastest-declining electromagnetic transients observed to date, AT2017gfo and AT2018kzr. We performed radiative transfer spectral modelling to deduce ejecta properties of these transients. Our focus was on interpreting kilonovae, the electromagnetic counterpart associated with the decay of rapid neutron capture (<i>r</i>-process) elements. Only a single kilonova has been spectroscopically observed to date, AT2017gfo, and so we modelled its spectra, in order to fully interpret its ejecta properties. First, we undertook a detailed study exploring the effects platinum and gold (both third <i>r</i>-process peak elements) would have on typical kilonova spectra, and whether any of these features appeared in the spectra of AT2017gfo. We found no evidence for either of these species (we placed upper limits on their masses, <i>M</i>_Pt ≲ a few 10⁻³ M_⊙, and <i>M</i>_Au ≲ 10⁻² M_⊙), and so we concluded that the heaviest <i>r</i>-process species were not synthesised in any detectable quantity by AT2017gfo. Second, we comprehensively modelled the entire spectral sequence, using realistic composition profiles from theoretical simulations, to deduce the properties of the ejecta across its entire lifetime. We found that the early spectra (≲ 7 days) can be well-reproduced with a combination of first <i>r</i>-process peak elements, and lanthanides. We found we must invoke a disjoint in composition to obtain agreement with the early spectra, indicative of a ‘two-component’ model for AT2017gfo. We also modelled the late spectra, and identified a number of prominent emission features. We were able to reproduce the feature at ∼ 1𝜇m across all phases with the strong Sr II NIR triplet, although we require clumped material at late times. We also modelled the two NIR emission features at ∼ 1.58 and ∼ 2.07𝜇m, both as P-Cygni and pure emission features, but concluded that without complete atomic data that contains NIR line information for the heavy elements, we cannot uniquely identify their source. Finally, we modelled the fast-evolving transient, AT2018kzr. Its lightcurve declined comparably rapidly to AT2017gfo, indicating a compact merger scenario. Our analysis determined that it did not synthesise any <i>r</i>-process material, and that it was the result of the merger of an oxygen-neon white dwarf with a compact binary – either a neutron star or a stellar mass black hole. Although this transient is unrelated to <i>r</i>-process nucleosynthesis, understanding these types of merger systems is important for developing our understanding of fast-evolving transients that occupy the same parameter space as kilonovae. 2026-10-01T17:01:38Z Text Thesis 10.17034/32633622.v1 https://figshare.com/articles/thesis/Spectroscopic_modelling_of_astrophysical_mergers_and_kilonovae/32633622 All Rights Reserved