Three-dimensional spectral modelling of astrophysical transients : unravelling the nucleosynthetic content of supernovae and kilonovae

Project Information

SUPERSPEC

Grant agreement ID: 803189

DOI

10.3030/803189

Project closed

EC signature date 18 December 2018

Start date 1 October 2019

End date 30 September 2025

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 500 000,00

EU contribution

€ 1 500 000,00

Coordinated by

STOCKHOLMS UNIVERSITET
Sweden

Periodic Reporting for period 4 - SUPERSPEC (Three-dimensional spectral modelling of astrophysical transients : unravelling the nucleosynthetic content of supernovae and kilonovae)

Reporting period: 2024-04-01 to 2025-09-30

Summary of the context and overall objectives of the project

The SUPERSPEC project has developed and applied a new generation of models for spectral formation in supernovae and kilonovae - the element factories of the Universe. Arising when massive stars collapse, and when neutron stars merge, respectively, these transients enrich cosmos with elements throughout the periodic table. The diagnosis of this production occurs through advanced spectral analysis with Non-Local-Thermodynamic-Equilibrium (NLTE) models. With high-accuracy models developed over the project, applied to the analysis of new high-quality observations, we have taken a leap forward in building up a systematic understanding for the origin of the elements and the relation between different progenitors and explosive transient types. In addition has three-dimensional structures been directly inferred through the links between morphology and line profiles.

SUPERSPEC has seen the construction of codes capabable to model kilonovae in 1D, and supernovae in 3D, both in full collisional-radiative NLTE. This has defined a significant advancement in the modelling capabilities of the field. The 3D code breaks new ground by using an innovative domain decomposition technique, which allows the splitting of the star over several nodes on the compute cluster. This allows more memory allocation per region in the domain, in turn allowing for more accurate microphysics.

With these new codes we have made the first studies of spectral formation in kilonovae with consideration of NLTE, with in general big differences to LTE modelling seen. We have been able to identify which processes are important, and which elements produce which kind of signatures. This in turn has allowed a better interpretation of observed data, as well as taking a leading role in designing new observation plans. The models have allowed clarification of what can be learned from which lines, at which epochs, in kilonovae. The work has included development of large amounts of ab-initio atomic data, both for levels and transition rates, but also new recombination rates, more than doubling the available amount for trans-iron elements. Several new collision strengths have also been calculated.

The 3D code has been used to study spectral formation using state-of-the-art hydrodynamic simulations (through collaborators in Garching) - the first time this has been done for core-collapse supernovae in 3D. Explosions of both Type I (H-poor) and Type II (H-rich) supernovae have been investigated, and compared to observations. This work has delivered new constraints on allowed combinations of ejecta mass and explosion energy for Type I supernovae. It has demonstrated that current neutrino-driven explosion simulations deliver promising agreement regarding line profiles when compared to observed ones. It has also firmly anchored the association between low-mass H-rich progenitors and so called subluminous Type II supernovae.

The SUPERSPEC project has made an important contribution to better understand the origin of the elements in the Universe - a pillar of modern astrophysics and for our sense of our origins in cosmos. It has played an important role in connecting areas in the emerging field of multi-messenger astronomy, and delivered new technologies with cross-disciplinary use.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The work performed in SUPERSPEC has resulted in:

* Development of time-dependent physics, r-process atomic data, and thermalization physics of kilonovae. Papers: Pognan et al. 2022a, 2022b.

* Dielectronic recombinatiom rates for trans-iron elements. Paper: Banerjee et al. 2025, 2026 in prep, Furgoson et al. 2026, submitted.

* Application of the new kilonova code to compute the first suite of NLTE kilonova models. These models have allowed us to better understand
the evolution of physical conditions (temperature, ionization) and spectral formation in kilonovae. They have also allowed to determine which elements generate
which spectral signatures, and how these can be used to determine abundances and/or emission volumes.
Papers: Pognan et al. 2024, 2025, Jerkstrand et al. 2026 (under review), Pognan et al. 2026 (under review).

* Development of a new 3D spectral synthesis code for supernovae. Papers: van Baal et al. 2023, van Baal & Jerkstrand, submitted.

* Application of the 3D code to modelling hydrogen-stripped (Type I) supernovae. Papers: van Baal et al. 2023, 2024.

* Application of the 3D code to model low-mass Type II supernovae. Paper: van Baal 2026, submitted.

* Abundance analysis: Paper: Temim et al. 2024, Schweyer et al. 2025, Aamer 2025.

* The modelling of superluminous supernovae (SLSNe), in particular for the case of a magnetar central engine source. Paper: Omand & Jerkstrand 2023.

* Build-up of nebular data samples, and analysis of these. Paper: Prentice et al. 2022, Boström, Jerkstrand et al, in prep.

* A scattering-by-scattering line transfer code has been developed to study the Sobolev approximation and its degree of validity. Paper: Jerkstrand, in prep.

* An in-depth code comparison to test and validate codes and methods. Paper: Blondin et al. 2022.

Apart from the publications listed above, dissemination has also occurred at a large number of international conferences and workshops.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Our new modelling tools define a new state-of-the-art, and enables very accurate models to be developed both in 1D and 3D.