Supernova SN 2024afyu: A Key to Pair-Instability Explosions ✛
Context
Pair-instability supernovae (PISNe) are a long-predicted but never unambiguously confirmed death mechanism for very massive stars. Alongside this work, the archive hosts studies of supernovae with unusual light curves, helium shells, and neutrino signals, but most are explained by alternative energy sources: interaction with circumstellar material, magnetars, or accretion onto a black hole. SN 2024afyu stands out because its late-time spectra contain lines that are difficult to reproduce without an anomalously large amount of sulfur and silicon, directly pointing to a thermonuclear explosion of a massive core.
Methods
The authors assembled an unprecedented dataset: 48 spectroscopic epochs from 48 days before explosion to 554 days after, obtained with eight telescopes including Keck, LBT, and the Nordic Optical Telescope. Photometry covers the range from ultraviolet to near-infrared. A key technique was identifying the [S II] emission lines at 4070 Å and 1.03 μm, which arise from the same upper level, allowing the identification to be checked via the luminosity ratio. Then, using LTE formulas, they estimated the mass and temperature of the sulfur, comparing with PISN and ordinary core-collapse supernova models.
Results
The sulfur mass in the ejecta is about 3 solar masses, two to three orders of magnitude larger than in typical core-collapse supernovae. The temperature of the sulfur zone is estimated at 3670–3750 K, consistent with PISN models but not with ordinary supernovae. The nickel-56 mass, estimated from the late-time luminosity, lies in the range 0.4–1.0 solar masses. The light curve of SN 2024afyu is broader and slower than that of ordinary supernovae, and its peak absolute magnitude, Mr = −18.9 mag, makes it the faintest among PISN candidates. The spectra show a unique combination of strong [Ca II] lines without accompanying oxygen lines, which has never been observed before.
Implications
This work follows the general search for PISNe but differs from its neighbors by relying not on luminosity or light-curve shape but on detailed spectral analysis of elemental abundances. It is the first supernova whose sulfur and silicon lines are so strong that they require masses achievable only in a pair-instability explosion. This approach reduces dependence on model assumptions about the energy injection mechanism and offers a more direct test of nucleosynthesis.
Future development
The topic may evolve toward more detailed radiative transfer modeling that includes element mixing and varying metallicities. New PISN calculations for progenitor masses in the range 70–130 solar masses will help clarify which parameter combinations reproduce the observed spectra of SN 2024afyu. Mid-infrared observations will also be needed to test the hypothesis of dust formation.
Impact
The results will affect the interpretation of other long-lived supernovae and the selection criteria for PISN candidates in future sky surveys such as LSST.
Next steps
The next step will be to obtain later spectra of SN 2024afyu to track the evolution of ionization and the possible emergence of new lines, as well as to search archival data for pre-explosion activity.
Key open problems
This work is directly connected to the unresolved problem of identifying pair-instability supernovae and to the question of the upper mass limit for stars that can be completely disrupted without forming a black hole.
The knowledge machine's view
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The authors collected 48 spectroscopic epochs of SN 2024afyu, spanning from 48 to 554 days after explosion. They identified the [S II] emission lines at 4070 Å and 1.03 μm, and from their ratio estimated a sulfur mass of about 3 solar masses at a temperature of 3670–3750 K. This required a revision of standard core-collapse scenarios and pointed to a pair-instability explosion.
Where its strength is
The strength of the work lies in the direct spectroscopic determination of an anomalously large sulfur mass, which provides a more reliable test of nucleosynthesis than indirect indicators from the light curve.
Where it could go next
- It might be interesting to compare the conclusions with the work on the neutrino signal from a hidden compact object in SN 2021foa: if PISNe leave no remnant, the absence of a neutrino beacon in SN 2024afyu could serve as additional evidence.based on: Neutrino Lighthouse: The Hidden Heart of Supernova SN 2021foa arXiv:2606.06409v1
- Closely related is the study of helium envelopes in thermonuclear explosions; one could check how the presence of helium in the progenitor would affect the synthesis of sulfur and silicon in PISNe.based on: Helium in the Thermonuclear Fire of a Supernova arXiv:2605.07827
- In the archive there is an analysis of a double-peaked supernova explained by interaction with the circumstellar medium; it is worth exploring whether part of the sulfur emission in SN 2024afyu could arise from shock excitation of the shell, rather than only from the ejecta.based on: The Star That Exploded Twice arXiv:2605.16487
Worth trying
- Check whether the late-time spectra of SN 2024afyu show neutral oxygen lines, which could indicate incomplete burning and constrain the progenitor mass.
- Compute synthetic spectra for PISN models with different helium abundances to understand how a helium envelope would affect the observed sulfur lines.
- Search archival data for signs of pre-explosion variability or mass loss that could explain part of the emission through interaction with the circumstellar medium.
Why this may matter
If SN 2024afyu is indeed a pair-instability supernova, it would be the first direct observational confirmation of such a mechanism, and would allow calibration of models of nucleosynthesis and chemical evolution of galaxies.
Nearby works in our archive
- A Star's Death in a Layered Cocoonsimilarity 0.55
- Neutrino Lighthouse: The Hidden Heart of Supernova SN 2021foasimilarity 0.54
- The Star That Exploded Twicesimilarity 0.54
- The Hidden Glow Before the Blastsimilarity 0.54
- A Neutron Star's Pulse in X-Ray Burstssimilarity 0.54
- How One Supernova United Different Types of Explosionssimilarity 0.52
- Helium in the Thermonuclear Fire of a Supernovasimilarity 0.52
The archive is sparse around this work: the closest review is only 0.55, against a usual 0.64. It looks like few have drilled nearby.
The [S II] lines at 4070 Å and 1.03 μm arise from the same energy level, so their luminosity ratio must be strictly fixed. The observed ratio of 1.41–1.44 almost exactly matched the theoretical value of 1.55, providing a strong argument for the identification.
Key numbers
- sulfur mass: ~3 solar masses
- nickel-56 mass: 0.4–1.0 solar masses
- rise time: 85 days
- peak absolute magnitude: Mr = −18.9 mag
- sulfur zone temperature: 3670–3750 K