Supernova Ejecta Reveal Stellar Mass and Nucleosynthesis via Infrared Lines.

Radiative transfer calculations of core-collapse supernovae reveal distinct infrared emission evolution between Type II and Ibc events. Ibc supernovae exhibit increasing infrared luminosity, dominated by lines like [NeII]12.81µm, correlating with progenitor mass and ejecta ionization. Nickel lines trace explosive nucleosynthesis, though iron and cobalt lines show diverse behaviour.

The aftermath of a massive star’s death, a core-collapse supernova, continues to radiate detectable energy long after the initial explosion. Understanding the spectral signatures of this nebular phase – weeks to months post-explosion – provides crucial insights into the physics of these events, including the progenitor star’s mass, the extent of explosive nucleosynthesis, and the distribution of heavy elements. Recent research, detailed in ‘Infrared diagnostics of late-time core-collapse supernova spectra’ by Luc Dessart from the Institut d’Astrophysique de Paris, CNRS-Sorbonne Université, and colleagues, presents detailed radiative transfer calculations modelling infrared emission from supernova ejecta. The study focuses on the evolution of spectral lines from atoms and ions, excluding the contribution of molecules and dust, to better interpret observations and constrain models of stellar death.

Infrared Emission Reveals Supernova Ejecta Composition and Dynamics

Radiative transfer calculations demonstrate how infrared emission evolves in the ejecta of both Type II and Type Ibc supernovae between 200 and 500 days post-explosion, providing crucial insights into the aftermath of massive star deaths. These models, which exclude the effects of molecules and dust, analyse the light emitted by atoms and ions within the expanding remnants of massive stars, revealing fundamental differences in their composition and dynamics. The research focuses on progenitors with initial masses ranging from 10 to 40 solar masses, providing insights into the diverse outcomes of core-collapse supernovae and expanding our understanding of stellar evolution.

Calculations reveal distinct evolutionary pathways between Type II and Type Ibc supernovae, highlighting the importance of explosion mechanism and progenitor properties in shaping the observed emission. Type II models maintain optical luminosity throughout the observed period, indicating continued energy release from radioactive decay and ongoing thermal emission. Conversely, Type Ibc models exhibit increasing infrared brightness, ultimately concentrating 80% of their luminosity in this wavelength range by 500 days. This difference arises from the higher kinetic energy-to-mass ratio in Type Ibc ejecta, facilitating gamma-ray escape and resulting in lower density and increased ionisation, fundamentally altering the emission processes within the remnant. These findings underscore the importance of considering both thermal and non-thermal emission mechanisms when modelling supernova remnants. Non-thermal emission arises from energetic particles, while thermal emission is produced by the heated material itself.

The strength of the [NeII] 12.81µm emission line correlates with progenitor mass, providing a potential diagnostic tool for estimating the initial mass of the exploded star. This line radiates up to 20% of the total luminosity in Type Ibc models after 300 days, demonstrating its prominence in the infrared spectrum of these events and making it a valuable probe of the ejecta’s physical conditions. Detailed modelling of this line’s profile can provide information about the velocity structure and density distribution of the ejecta.

Infrared lines originating from nickel are useful tracers of explosive nucleosynthesis, highlighting the importance of understanding the nuclear processes that occur during supernovae. These events are responsible for the synthesis of many of the elements heavier than iron, which are essential for the formation of planets and life. By studying the abundance and distribution of these elements in supernova remnants, researchers can gain insights into the conditions under which these elements are formed and the mechanisms that drive the explosive nucleosynthesis.

The diversity observed in the infrared iron and cobalt lines underscores the complexity of supernova remnants and the need for sophisticated modelling techniques to accurately interpret the observed spectra. Researchers must consider the effects of radiative transfer – the process by which energy is transported through the ejecta – non-equilibrium ionization, and line blending when analysing these lines. Furthermore, they must account for the effects of dust absorption and emission, which can significantly alter the observed spectra. Advanced modelling techniques, such as Monte Carlo simulations and time-dependent radiative transfer calculations, are essential for accurately interpreting the observed spectra and extracting meaningful information about the physical conditions within the remnant.

Future observations with upcoming observatories, such as the James Webb Space Telescope and the Extremely Large Telescope, will provide unprecedented opportunities to study supernova remnants in the infrared and optical wavelengths. These observations will allow researchers to map the velocity structure, density distribution, and chemical composition of supernova remnants with unprecedented detail, providing crucial insights into the processes that govern massive star explosions and the origin of elements. These observations will also allow researchers to study the interaction of supernova remnants with the surrounding interstellar medium, providing insights into the feedback processes that regulate star formation and the evolution of galaxies.

The distinct spectral evolution observed in Type II and Type Ibc supernovae underscores the importance of considering the progenitor’s initial mass and explosion mechanism when modelling supernova remnants. Type II supernovae, originating from massive stars that retain their hydrogen envelope, exhibit different emission characteristics compared to Type Ibc supernovae, which result from the collapse of stars that have lost their outer layers. This difference in progenitor properties leads to variations in the ejecta’s composition, density, and velocity structure, ultimately affecting the observed emission. Detailed modelling of these differences can provide valuable insights into the processes that govern stellar evolution and the formation of different types of supernovae.

The correlation between the [NeII] 12.81µm emission line strength and progenitor mass provides a potential method for estimating the initial mass of exploded stars, offering a valuable tool for studying the stellar populations in different galaxies. By calibrating this relationship with theoretical models and observations of nearby supernovae, researchers can use this line to estimate the masses of supernovae at larger distances, providing insights into the star formation history and chemical evolution of these galaxies. This approach complements other methods for estimating supernova masses, such as light curve modelling and spectroscopic analysis.