Fingerprints of the ejecta
The nebular phase of a supernova occurs at late times (more than ~100 days after peak brightness), when the ejected material has expanded and thinned enough that we can finally see into the supernova’s inner layers. Radioactive decay continues to deposit energy into this gas, causing it to glow. At this stage, we observe emission lines from individual elements, and the shapes of those lines reveal how the newly forged elements are distributed throughout the ejecta. Because different explosion models predict different spatial distributions, nebular spectroscopy serves as a powerful forensic tool for testing which models best match real supernovae. The mid-infrared (MIR) region is especially valuable for this work—it provides clean, isolated lines from each of the key element groups.
Nebular line profiles can be thought of as flattened (2D) CAT scans of the supernova ejecta. The extremely high expansion velocities (up to ~10,000 km/s) Doppler-broaden the emission lines, so the observed line profile effectively maps the cross-sectional brightness at each line-of-sight velocity slice. In the simplest case—a uniform, glowing sphere of gas (left video)—those cross-sections are circles (A = πr2), producing a smooth, parabolic line shape. If instead some material is missing from the center, forming a thick spherical shell (right video), the smaller inner parabola is subtracted from the larger one, flattening the top of the line profile. We indeed observe such flat-topped profiles in the intermediate-mass elements of normal Type Ia supernovae, indicating that these elements reside in the outer layers of the ejecta.
Layered ejecta in normal SN Ia
The above element distributions were derived from the nebular line profiles of the normal Type Ia supernova SN 2024gy. Although the geometry is more complex than a uniform sphere, the same physical principles apply. The ejecta show a clear layered structure: the stable iron-group elements (traced by 58Ni, produced at the highest densities), the radioactive iron-group elements (traced by 56Co, from slightly lower-density burning), and the intermediate-mass elements (traced by Ar and Ca, formed at still lower densities) each occupy distinct regions. This stratification indicates that the explosion involved a detonation—a fast, supersonic burning phase that prevents strong mixing between layers. Detailed analysis of novel features in the [Ni II] and [Ni III] lines (singly and doubly ionized nickel) further suggests that SN 2024gy resulted from a near-Chandrasekhar-mass white dwarf that exploded via a delayed detonation.
