Yet modern transient surveys have begun to uncover a handful of nearby PISN candidates, and SN 2018ibb stands out as one of the best identified to date. This Type Ic supernova exhibited a rise time exceeding 100 days and a peak luminosity around 10^44 erg s^-1, both consistent with theoretical PISN light-curve predictions. Its spectroscopic properties also matched expectations, including the strong [O III] emission characteristic of pair-instability explosions. However, late-phase spectra revealed an unexpected blue flux excess below approximately 5000 Å, a feature not predicted in standard PISN models. This excess was suggested to originate from the interaction between the supernova ejecta and dense circumstellar matter (CSM) surrounding the progenitor, implying an additional luminosity source beyond the radioactive decay of nickel-56.

To investigate this puzzle, a team led by Takashi J. Moriya of the National Astronomical Observatory of Japan developed synthetic light-curve models of PISNe interacting with dense CSM. The researchers adopted a PISN progenitor model computed with the KEPLER hydrodynamical stellar evolution code, using a bare helium star whose explosion energy and nickel-56 mass were determined self-consistently rather than treated as free parameters. The subsequent light curves were computed with the STELLA multi-frequency radiation hydrodynamics code, which solves hydrodynamic equations coupled with radiative transfer using the variable Eddington factor method across 100 wavelength bins. Based on the estimated metallicity of the host galaxy, they identified a model whose bolometric light curve matched the overall behavior of SN 2018ibb around peak luminosity, with properties consistent with previous independent estimates.

The team attached a wind-like CSM density structure on top of the ejecta, characterized by a mass-loss rate and CSM velocity. They assumed a CSM velocity of 1,000 km s^-1, corresponding to typical terminal wind speeds of Wolf-Rayet stars, and varied the mass-loss rate to adjust the CSM density. Within the investigated range, the CSM interaction had little effect on the light curve near peak. However, after approximately 150 days since explosion, the synthetic light curves with CSM began to deviate from those without, with increasing CSM density producing higher luminosity at later times.

The critical breakthrough came from matching the observed luminosity evolution of SN 2018ibb from 150 days after peak. The researchers found that a mass-loss rate of 0.01 (v_CSM/1,000 km s^-1) M_sun yr^-1 reproduced the bolometric luminosity well in the 300-day window following that epoch. More strikingly, SN 2018ibb showed a distinct luminosity break at around 300 days from peak. By varying the CSM radius while keeping the mass-loss rate fixed, the team demonstrated that this break is determined by the CSM radius, and the observed break could be reproduced by setting the dense CSM radius at 8.5×10^16 cm.

This radius carries profound implications for the progenitor’s final moments. Assuming a wind velocity of 1,000 km s^-1, the mass-loss rate must have increased to the inferred value for a period of 28 (v_CSM/1,000 km s^-1)^-1 years before explosion to place the CSM at the required distance. After the luminosity break, the synthetic light curves returned to the CSM-free prediction, indicating that the ejecta had swept past the dense material. However, the observed bolometric light curve of SN 2018ibb showed a flattening after the break, suggesting the supernova began interacting with less dense CSM beyond that boundary.

The team validated their numerical results using an analytic model of interaction-powered supernova luminosity, approximating the ejecta density structure with two power-law components. They found that the combined luminosity of the CSM-free PISN and the analytic interaction contribution reproduced the overall evolution until the break, with a kinetic-to-radiative conversion efficiency that matched the numerical model well. Notably, the CSM interaction luminosity contributed negligibly around peak, meaning PISN properties and CSM properties can be independently constrained from different phases of the light curve. The photospheric velocity evolution was also largely unaffected by CSM interaction, with the model’s g-band velocities roughly consistent with Fe II absorption-line measurements from SN 2018ibb.

The inferred mass-loss rate is far higher than expected from hot stellar winds of hydrogen-free stars, and if sustained over long timescales, the progenitor would have been stripped below the mass threshold required for a PISN explosion. The enhancement must therefore have occurred only in the final short period before detonation, consistent with the light-curve-derived timescale. Intriguingly, the progenitor model was found to trigger carbon shell burning after core contraction approximately 30 years before explosion, matching the estimated enhancement timescale. The core contraction with shell burning initiates progenitor expansion, which may drive the mass-loss enhancement. Alternatively, as the star approaches the dynamically unstable configuration triggered by pair creation, it may also enter a state conducive to enhanced mass loss.

Other mechanisms remain possible. The CSM could have formed through eruptive mass loss or binary mass transfer rather than a steady wind. Another intriguing possibility is that the mass-loss rate was not actually enhanced, but material lost from the progenitor accumulated near the stellar surface due to strong magnetic fields confining the stellar wind. Such magnetically confined material could remain close to the star and affect the PISN light curve in the manner observed. Further investigation is needed to determine whether magnetically confined CSM can extend to the inferred radius.

As PISN candidates accumulate, signatures of unexpected CSM interaction are emerging repeatedly. SN 2023vbw, another recent PISN candidate, also shows CSM interaction features. Because PISN progenitors require low overall mass loss to sustain their enormous masses, they were not expected to be surrounded by dense CSM. However, a significant fraction of supernovae from massive stars are now recognized to experience mass-loss enhancement within decades before explosion. PISN progenitors may undergo similar short-term enhanced mass loss, affecting their observational properties. Most predicted PISN characteristics to date ignore dense CSM, but models likely require updating. The presence of dense CSM is expected to make PISNe bluer than previously predicted, which could influence expected discovery rates at high redshifts and reshape our understanding of how the most massive stars die across cosmic time.

Subject of Research: Properties of the circumstellar matter around the pair-instability supernova candidate SN 2018ibb revealed by its light curve

Article Title: Properties of the circumstellar matter around the pair-instability supernova candidate SN 2018ibb revealed by its light curve

Article References: Moriya, T. J., Schulze, S., Heger, A., Blinnikov, S. I., & Potashov, M. S. (2026). Properties of the circumstellar matter around the pair-instability supernova candidate SN 2018ibb revealed by its light curve. Frontiers in Astronomy and Space Sciences, 13, Article 1944397. https://doi.org/10.3389/fspas.2026.1944397

Image Credits: AI Generated

DOI: 10.3389/fspas.2026.1944397

Keywords: pair-instability supernovae, SN 2018ibb, circumstellar matter, mass loss, massive stars, light curves, supernova ejecta interaction, stellar evolution, Properties, circumstellar, matter, around

The research, led by Amandine Passin and corresponding author Fabienne Lagarde of the Institut des Molécules et des Matériaux du Mans at Le Mans Université, together with colleagues from the French industrial plastics research centre CT-IPC, set out to answer a deceptively simple question: does the initial formulation of a plastic determine its environmental fate? Although additives such as stabilisers, antioxidants and processing aids are incorporated into polymers specifically to improve durability, their influence on the generation of microplastics and nanoplastics, and on the leaching of chemical compounds into water, has remained surprisingly poorly characterised. Understanding this link is critical because microplastics and their dissolved or gaseous degradation products follow very different pathways through ecosystems and pose distinct challenges for environmental monitoring.

To probe this question, the team compared two polypropylene formulations under controlled laboratory conditions. The first was a reference polypropylene containing no added additives, representing the bare polymer matrix. The second, designated PP + 6, contained six industrially representative additives chosen to reflect the kinds of compounds commonly used in real-world plastic products. Plastic pellets from both formulations were subjected to accelerated ultraviolet weathering designed to mimic prolonged sunlight exposure, and the weathered pellets were subsequently placed in water and agitated mechanically to reproduce the physical wear that plastics experience in rivers, oceans and soils. This combination of photochemical ageing followed by mechanical stress allowed the researchers to simulate, in compressed laboratory timescales, the sequential insults that plastic debris suffers outdoors.

The analytical toolkit behind the study was deliberately multi-technique. Gravimetric measurements tracked how much mass each formulation lost over time, while morphometric analysis and scanning electron microscopy revealed how particle sizes and surface textures evolved. Total organic carbon analysis quantified the soluble degradation products dissolved into the water phase, allowing the team to distinguish between material that broke off as intact microplastic and nanoplastic particles and material that left the polymer as dissolved organic species. By following these endpoints in parallel, the researchers could build a mass-balance picture of degradation rather than focusing narrowly on particle counts alone.

The results revealed a dramatic and unexpected divergence between the two formulations. The additive-free reference polypropylene degraded through a clearly defined three-phase process, ultimately losing a cumulative 82 plus-or-minus 8 percent of its mass after 50 days of ultraviolet exposure. In the first phase, degradation was dominated by the release of volatile compounds and soluble species, as ultraviolet photons cleaved polymer chains and oxidation reactions produced small molecules that either evaporated or dissolved. The second phase was marked by surface ablation, in which thin layers of the embrittled polymer flaked away as microplastic particles. The third and most dramatic phase saw a sharp acceleration in the release of microplastics, nanoplastics and soluble products as entire granules fragmented, a process that alone accounted for 62 plus-or-minus 7 percent of the total mass loss measured at the 50-day mark.

The additive-containing formulation told a strikingly different story. PP + 6 degraded far more slowly, reaching only a cumulative mass loss of 24 plus-or-minus 3 percent after the same 50 days of ultraviolet weathering, and its degradation followed two distinct phases rather than three. The six additives, by scavenging radicals and shielding the polymer from photo-oxidation, effectively bought the material time, delaying chain scission and postponing the catastrophic fragmentation seen in the additive-free sample. From a durability standpoint, the additives did precisely what they were designed to do. But the study makes clear that slowing degradation is not the same as eliminating its environmental consequences.

Indeed, even as PP + 6 resisted fragmentation, the researchers observed a continuous increase in two other forms of pollution. Emissions of volatile compounds rose steadily throughout the weathering period, and the additives themselves leached progressively into the aqueous phase. This means that a plastic product which appears, to the naked eye, to be weathering gracefully may nonetheless be quietly releasing its chemical constituents into surrounding water over months and years. Because many plastic additives are known or suspected to be ecotoxic, endocrine-active or persistent, this slow-release pathway represents an environmental exposure route that particle-focused monitoring programmes can easily miss entirely.

The broader significance of the work lies in its demonstration that plastic is not a monolithic pollutant. Two samples of the same base polymer, differing only in their additive packages, can follow fundamentally different degradation trajectories, generate different proportions of microplastics, nanoplastics, dissolved organics and volatile emissions, and therefore pose different environmental risks. Most laboratory studies of plastic weathering to date have used simplified, additive-free or minimally formulated materials for the sake of experimental control, yet the study shows that such model systems can dramatically overstate or mischaracterise the behaviour of the formulated plastics that actually populate the environment. Environmental fate models, exposure assessments and risk frameworks that ignore formulation chemistry may consequently be built on shaky ground.

The findings also complicate the emerging policy conversation around plastics. As negotiators and regulators consider rules governing plastic composition, additives and recyclability, this research suggests that decisions about what goes into a plastic product have consequences far beyond the use phase of the item. An additive package that extends service life may simultaneously delay microplastic generation while extending the period over which chemicals leach out, shifting the timing and nature of the environmental burden rather than simply reducing it. Conversely, formulations that fragment quickly may flood ecosystems with particles but exhaust their leachable additives sooner. Neither pathway is inherently benign, and the authors argue that both microplastic and nanoplastic generation and soluble and volatile emissions must be considered together when assessing the true environmental impact of plastic materials.

For the researchers, the next frontier is extending this mass-balance approach to other polymer types, other additive combinations and real environmental matrices, where sunlight, temperature, salinity, microbes and mechanical forces interact in far more complex ways than any accelerated weathering chamber can reproduce. But the central message of the study already stands: when it comes to how plastic ages in the environment, formulation is destiny. The invisible ingredients blended into a plastic pellet at the factory gate shape not only how long the product lasts, but what it becomes, particle by particle and molecule by molecule, as the sun slowly takes it apart.

Polypropylene is particularly vulnerable to photo-oxidation because its backbone contains tertiary carbon atoms, where hydrogen abstraction by UV-generated radicals initiates a self-propagating chain reaction. Once oxygen is incorporated, the polymer forms carbonyl groups that absorb light and accelerate further degradation, a process known as auto-accelerating photo-oxidation. This inherent chemical susceptibility explains why polypropylene items left outdoors become brittle, chalky and prone to crumbling within months, and why stabiliser packages are considered indispensable in nearly every commercial application of the resin.

The distinction between microplastics and nanoplastics matters scientifically because the two fractions behave differently once released. Particles in the micrometre range tend to settle, aggregate with natural organic matter and be ingested by filter feeders, whereas nanoplastic particles have far higher surface-area-to-mass ratios, can cross biological barriers more readily and are notoriously difficult to detect with conventional sampling methods. By tracking fragmentation phases gravimetrically rather than relying solely on particle counting, the study sidesteps some of the analytical blind spots that have hampered earlier weathering experiments, in which a large share of degrading material simply vanished from the measured mass budget.

Total organic carbon measurements offer a complementary advantage: they capture the dissolved fraction of degradation, including short-chain oxidation products and leached additives that carry no particle signature at all. Environmental monitoring programmes, which overwhelmingly target intact particles collected by nets and filters, are structurally blind to this soluble pool. The finding that volatile emissions also rise continuously during weathering adds a third, even less visible compartment, since evaporated fragments enter the atmosphere and may undergo further photochemical transformation far from the site of release.

The accelerated weathering approach used in the study compresses years of outdoor exposure into weeks, a standard technique in polymer science, though translating laboratory doses into real environmental lifetimes remains an acknowledged challenge. Even so, the relative comparison between the two formulations is robust, because both materials experienced identical conditions. The threefold difference in cumulative mass loss between the additive-free and additive-containing pellets therefore reflects genuine formulation effects rather than experimental artefact.

Because the work was published as an open-access article in Microplastics and Nanoplastics, with supplementary material available for readers seeking the full morphometric and carbon datasets, other laboratories can replicate the mass-balance framework directly. Extending it to polyethylene, polystyrene and formulated bioplastics, and to seawater matrices where salt and biofilms alter leaching behaviour, would allow the field to build the comparative database of formulation-specific degradation pathways that current risk assessments conspicuously lack.

Subject of Research: How plastic additives influence UV-induced degradation of polypropylene and the release of microplastics, nanoplastics and chemical compounds

Article Title: Influence of additives on UV-induced degradation of polypropylene: micro and nanoplastic formation and additives release

Article References: Passin, A., Glais, M., Arib, C., Montembault, V., Falher, T., & Lagarde, F. (2026). Influence of additives on UV-induced degradation of polypropylene: micro and nanoplastic formation and additives release. Microplastics and Nanoplastics. https://doi.org/10.1186/s43591-026-00227-z

Image Credits: AI Generated

DOI: 10.1186/s43591-026-00227-z

Keywords: polypropylene, plastic additives, UV degradation, microplastics, nanoplastics, additives leaching, volatile emissions, plastic weathering, polymer degradation, environmental pollution, plastic formulation, mass loss