PI: Stuart Sim

Joshua Pollin, Stuart Sim, Luke Shingles, Fionntán Callan, Christine Collins, Connor Ballance, Fiona McNeill, Catherine Ramsbottom

Astrophysics Research Centre, Queen’s University Belfast, Northern Ireland

The James Webb Space Telescope (JWST) provides an exciting new opportunity to understand the physical mechanisms responsible for Type Ia supernovae. In particular, the high sensitivity and mid-infrared spectroscopic capabilities make it possible for JWST to measure emission lines produced from a range of elements and ions than cannot be studied with optical or near-infrared data alone. Particularly valuable are datasets gathered at a few hundred days after explosion during the so-called “nebular phase”. The low optical depths at these times enable us to see emission coming from the entire ejecta allowing us to study the full range of elements produced by explosive nucleosynthesis. Thus, the JWST spectra provide a new window to understand the structure and composition of the explosion ejecta, which is key to testing theoretical models for supernovae.

To interpret these new observations, we have used DiRAC to perform theoretical simulations of spectrum formation across the entire optical to infrared wavelength ranges using our radiative transfer code, ARTIS (Shingles et al. 2020, https://github.com/artis-mcrt/artis, https://zenodo.org/records/18670358). In particular, our first study (Pollin et al. 2025) has used modern state-of-the-art hydrodynamical simulations of white dwarf mergers (Pakmor et al. 2023) and, for the first time, considers the full three-dimensional structure of the ejecta predicted by such models at nebular phases. Figure 1 illustrates four example emission lines that are observable with JWST – in the figure, we compare observations of the supernova SN2021aefx (Kwok et al. 2023) to the spectra obtained from one of our explosion simulations for a range of observer orientations. The observations clearly show a difference in the morphology of the line profiles when comparing iron-peak elements (e.g. cobalt and nickel, which show clearly peaked rounded profiles – see left two panels in Figure 1) to lower mass species (e.g. argon or sulphur, which show flat-topped, broad profiles – see right panels in Figure 1). Our multi-dimensional simulations illustrate that the models can successfully reproduce these characteristics, lending credence to the structures predicted by theory (see also Blondin et al. 2023). Moreover, by carrying out full three-dimensional calculations, we also show that explosion models can predict significant variation depending on the observer’s line of sight. For example, this model shows clear shifts in the line centres for cobalt and nickel lines, depending on the observer angle, and a variety of sub-structure associated with the flat-topped profiles of intermediate mass elements. Such behaviour, which arises naturally from modern explosion models, places constraints on viable explosion scenarios and supports both the interpretation of current JWST observations and the planning of future JWST programmes. By identifying the spectral diagnostics and wavelength regions most sensitive to ejecta geometry and ionisation structure, these predictions help to discriminate between competing models and thereby advance our understanding of the explosion mechanism underlying Type Ia supernovae.

This work used the DiRAC Memory Intensive service (Cosma8) at Durham University, managed by the Institute for Computational Cosmology on behalf of the STFC DiRAC HPC Facility (www.dirac.ac.uk). The DiRAC service at Durham was funded by BEIS, UKRI and STFC capital funding, Durham University and STFC operations grants. DiRAC is part of the UKRI Digital Research Infrastructure.

References:

• Blondin et al., 2023, A&A, 678, 170
• Kwok et al., 2023, ApJ, 944, L3
• Pakmor et al., 2022, MNRAS, 517, 5260
• Pollin et al., 2025, arXiv:2507.05000
• Shingles et al., 2020, MNRAS, 492, 2029

Figure 1. Synthetic profiles (coloured lines) for emission profiles of selected features in JWST spectra from Pollin et al. (2025). The lines plotted all correspond to observer lines-of-sight in the equatorial plane of a white dwarf merger (identified by the azimuthal angle). Overplotted are profiles obtained from an artificial, spherically symmetric 1D version of the model (green dashed) and the measurements from JWST for SN2021aefx (Kwok et al. 2023).