I study the progenitors and explosion mechanisms of supernovae through both observation and theory. My main tools are polarimetry of supernovae (spectropolarimetry and imaging polarimetry) and three-dimensional Monte Carlo radiative-transfer calculations.
Different explosion models for core-collapse supernovae predict different explosion geometries. Polarization is a way to study the shape of a spatially unresolved source, and I use it to place constraints on the explosion mechanism from the geometry of the explosion.
Polarization from aspherical supernova ejecta is nearly wavelength-independent, whereas polarization from light echoes by circumstellar dust depends on wavelength. I proposed a method to separate the two using this difference (Nagao et al. 2017b, 2018b) and have applied it to spectropolarimetry of Type II supernovae (2019, 2021, 2023b). These studies suggest that aspherical explosions are not unusual and that asphericity correlates with explosion energy (2024a, 2024b).
Polarimetry of the 03fg-like Type Ia supernovae 2021zny and 2022ilv showed aspherical explosions, which I discussed in the context of white-dwarf mergers (Nagao et al. 2024c).
Explosion energy of Type II supernovae versus the polarization timescale tpol (smaller values mean more aspherical explosions). From Nagao et al. 2024a.
Circumstellar material around a supernova was ejected by its progenitor shortly before the explosion, so its amount and distribution reflect the mass-loss history of the star in its final stage. I study this material through infrared echoes, scattered optical light echoes and emission from circumstellar interaction.
I have developed methods to estimate the amount and distribution of circumstellar material from infrared echoes, polarization and light curves (2017b, 2020, 2026a), and applied them to objects such as bright long-lived Type II supernovae (2025; Reynolds, Nagao et al. 2025a, b) and SN 2024qiw, whose progenitor underwent clumpy mass ejections 5–24 years before the explosion (2026b).
I analysed the near-infrared excess of the Type Ia supernova 2012dn as an infrared echo from circumstellar dust and estimated the mass-loss rate of the progenitor system (Nagao et al. 2017a, 2018a).
Schematic illustration of our interpretation of the observational properties of SN 2024qiw. (Nagao et al. 2026b)
The Rubin Observatory LSST will greatly increase the number of supernovae discovered. I plan to extend polarimetric observations to a wider range of objects and to continue analysis with radiative-transfer and radiation-hydrodynamics calculations, toward a better understanding of explosion mechanisms and the final evolution of progenitors.
NUTS Nordic-led supernova group using the NOT (2020–)
SOXS New transient programme at the NTT (2020–)
ePESSTO+ Supernova spectroscopic collaboration using the NTT (2019–)
ENGRAVE VLT follow-up of gravitational-wave counterparts (2019–)
CSC Finland–Japan Collaboration on radiation-hydrodynamics calculations of interacting supernovae (2023–)