Deep inside a massive star, before it explodes, layers sit on top of each other like the layers of an onion. Sometimes two adjacent layers, the carbon and oxygen layers, mix together. It's like how layers of broth and vegetables in a pot of soup blend when you stir them with a spoon. This mixing changes which chemical elements the star will eject into space when it explodes.
Scientists compared eight sets of explosion models with observations of the supernova remnant Cassiopeia A. This remnant is like a crime scene photograph, allowing us to reconstruct what happened inside the star. Models with layer merging yield a high argon-to-neon ratio (greater than 0.1) because neon is destroyed while argon is produced. Other models give a lower ratio. Observations in X-ray and infrared wavelengths match the merging models best.
The same approach was applied to supernova 1987A. There, based on the shift in titanium-44 lines, layer merging was likely not the main source of this isotope. This means different supernovae can have different histories: in some, layers merged; in others, they didn't. This helps astronomers read the chemical history of the universe more accurately.
as written by the authors arXiv:2603.24758
The merger between the carbon (C) and oxygen (O) shells hours to days before the collapse of a massive star significantly changes its nucleosynthesis, which is reflected in the elemental ratios observed in supernova remnants (SNRs). We present a nucleosynthesis study of $$^{44}$$Ti production in core-collapse supernovae (CCSNe), highlighting large silicon (Si), sulfur (S), calcium (Ca), and, most importantly, argon (Ar) to neon (Ne) ratios as diagnostics for carbon-oxygen (C--O) shell mergers. We compare yields from eight different sets of CCSNe models to observations of Cassiopeia A (Cas A), and show that C--O shell mergers are consistently the models that best match X-ray and infrared observations. These models produce high Ar/Ne ratios ($$\gtrsim 0.1$$), due to $$^{20}$$Ne depletion and production of $$^{36}$$Ar and $$^{38}$$Ar, while lower ratios are obtained from non-merger cases. Based on the Ar/Ne diagnostic, we compare the range of expected $$^{44}$$Ti produced by C--O shell mergers, which is up to $$\sim 20 - 30 \%$$ of the overall $$^{44}$$Ti, but expected to be located outside the reverse shock. Based on the sets of models considered, the photon flux expected from the $$^{44}$$Ti synthesized in the C--O shell merger in Cas A is below the $$NuSTAR$$ and $$COSI$$ detection limits, compatible with current limits locating most of the $$^{44}$$Ti interior to the reverse shock, but might be detectable from proposed missions like $$ASCENT$$. Finally, for the SNR of 1987A, a dominant C--O merger origin of the observed $$^{44}$$Ti is unlikely based on the observed redshift in its $$^{44}$$Ti line.
Titanium-44, the isotope in question, is radioactive and decays with a half-life of about 60 years. Astronomers use its glow in supernova remnants as a clock to determine the age of the explosion.