Pair-instability supernova candidate SN 2018ibb reveals dense circumstellar matter through its light curve

Pair-instability supernovae (PISNe) are among the most violent thermonuclear explosions predicted in astrophysics, arising when the cores of very massive stars become so hot that gamma rays spontaneously convert into electron-positron pairs. This pair creation drains radiation pressure from the stellar interior, triggering a dynamical collapse that ignites runaway oxygen burning and tears the entire star apart. The core mass required for such an explosion is estimated to lie between roughly 100 and 140 solar masses, with the exact threshold depending sensitively on the carbon fraction in the core. The corresponding zero-age main-sequence masses range from about 80 to 250 solar masses when mass loss and rotation are negligible. Because strong stellar winds strip away mass as metallicity increases, PISNe are expected to occur almost exclusively in metal-poor environments, making them exceptionally rare in the local Universe.

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