2025MNRAS.538.1663G

Kilonovae are key to advancing our understanding of r-process nucleosynthesis. To date, only two kilonovae have been spectroscopically observed, AT 2017gfo and AT 2023vfi. Here, we present an analysis of the JWST spectra obtained +29 and +61 d post-merger for AT 2023vfi (the kilonova associated with GRB 230307A). After re-reducing and photometrically flux-calibrating the data, we empirically model the observed X-ray to mid-infrared continua with a power law and a blackbody, to replicate the non-thermal afterglow and apparent thermal continuum $\gtrsim 2$ $\mu$m. We fit Gaussians to the apparent emission features, obtaining line centroids of $20218_{-38}^{+37}$, $21874 \pm 89$, and $44168_{-152}^{+153}$ Å, and velocity widths spanning $0.057 - 0.110$ c. These line centroid constraints facilitated a detailed forbidden line identification search, from which we shortlist a number of r-process species spanning all three r-process peaks. We rule out Ba II and Ra II as candidates and propose Te I-III, Er I-III, and W III as the most promising ions for further investigation, as they plausibly produce multiple emission features from one (W III) or multiple (Te I-III, Er I-III) ion stages. We compare to the spectra of AT 2017gfo, which also exhibit prominent emission at $\sim 2.1$ $\mu$m, and conclude that [Te III] $\lambda 21050$ remains the most plausible cause of the observed $\sim 2.1$ $\mu$m emission in both kilonovae. However, the observed line centroids are not consistent between both objects, and they are significantly offset from [Te III] $\lambda 21050$. The next strongest [Te III] transition at 29 290 Å is not observed, and we quantify its detectability. Further study is required, with particular emphasis on expanding the available atomic data to enable quantitative non-LTE spectral modelling.