Pre-SN O–C Shell Merger Dynamics

• Pre-supernova O–C shell merger is a convective-reactive event where oxygen and carbon burning shells merge, radically altering the star’s nucleosynthetic pathways.
• 3D hydrodynamic simulations reveal enhanced convective velocities, entrainment rates, and mixing asymmetries that drive the production of odd-Z elements and radioactive isotopes.
• The merger significantly impacts core structure and explosion dynamics by reducing compactness, seeding instabilities, and leaving distinct chemical fingerprints in supernova remnants.

A pre-supernova O–C shell merger refers to the convective-reactive event occurring in the final hours to days in the evolution of a massive star, where the convective oxygen-burning shell dynamically interacts with—and ultimately coalesces into—a neighboring carbon-burning shell. This violent episode destratifies the compositional onion-shell structure, radically altering the core's thermal and nucleosynthetic state immediately prior to gravitational collapse and supernova explosion. O–C shell mergers are now established as critical sites for the production of odd-Z elements (e.g., K, Sc), p-process isotopes, and certain radioactive species, with major implications for progenitor structure, hydrodynamic instabilities, and the observable chemical and kinematic structure of supernova remnants.

1. Physical Mechanisms and Dynamical Criteria

In massive stars of initial mass $M_{\rm ini} \sim 8$–$25\,M_\odot$, the terminal nuclear burning stages produce concentric convective shells: typically O-burning at $T\sim(1.5$–$2.8)\,$GK adjacent to C-burning at lower temperature and lower mean molecular weight. Under classical quasi-static evolution, these shells are separated by radiative, entropy-increasing buffer zones that are (in 1D) marginally stable according to the Schwarzschild or Ledoux criteria.

Destratification—and full shell merger—occurs when one or more of the following are satisfied:

• Convective boundary erosion: Turbulent eddies entrain stable fluid, thinning the boundary layer. The entrainment velocity $u_e$ scales as $u_e \sim A v_c \mathrm{Ri}_B^{-n}$, where $\mathrm{Ri}_B$ is the local bulk Richardson number (buoyancy jump). When $\mathrm{Ri}_B\lesssim10^{2-3}$, boundaries become susceptible to merger.
• Feedback from nuclear burning: Ingestion of fresh C/Ne material into the hotter O-shell triggers explosive burning, releasing energy and light particles (protons, $\alpha$'s) that steepen local gradients and drive further turbulence. The process becomes convective-reactive, characterized by a Damköhler number $\mathrm{Da}\sim1$ ($\tau_{\mathrm{mix}} \sim \tau_{\mathrm{burn}}$).
• Convective turnover times: 3D simulations yield $v_{\rm conv}\sim(1$–$5)\times10^7\,{\rm cm\,s}^{-1}$, with turnover times $\tau_{\rm conv} \sim 10$–$100$ s, allowing the mixing-burn cycle to run on pre-SN timescales~[$\lesssim10^4$ s before collapse; cf. (Sato et al., 10 Jul 2025, Rizzuti et al., 2024)].

The critical O–C shell merger event then forms a single, macroscopic convective zone encompassing the prior C, Ne, and O shells, with highly enhanced mixing and energy production.

2. 3D Hydrodynamics, Mixing, and Instabilities

Detailed 3D hydrodynamic simulations (e.g., PPMstar, MESA-3D) of pre-SN O–C mergers reveal dynamics fundamentally distinct from 1D mixing-length-theory (MLT) predictions:

• Convective velocities: 3D models consistently yield $v_{\rm 3D}/v_{\rm MLT}\sim5$–10, reflecting far more vigorous interior mixing than previously assumed [$2407.15544$].
• Entrainment scaling: The mass entrainment rate at the convective boundary is proportional to the local convective luminosity, $\dot{M}_{\rm entrain} \propto v_{\rm conv}$, achieving values $\gtrsim 10^{-4}-10^{-3}M_\odot\,\mathrm{s}^{-1}$ at peak luminosities [$1808.04014$, $1704.05985$].
• Clumpiness correction: 1D models lacking subgrid clumpiness underestimate the nuclear burning by factors up to 2, as the local enhancement in fuel abundance increases reaction rates nonlinearly.
• Instability regime: If the feedback from nuclear burning is strong enough, global non-radial oscillations (Mach numbers $\gtrsim 0.2$–0.3) and dipolar ($\ell=1$) asymmetries arise, as seen in both simulation and inferred from the structure of, e.g., Cassiopeia A's remnant [$2507.07563$].

Entrainment, multi-phase burning, and asymmetric (even hemispheric) compositional patterns are robust features of the merged shell, with significant consequences for both structure and subsequent explosion.

3. Nucleosynthesis, SPAr Reaction Flows, and Yield Implications

The O–C shell merger creates an environment in which a distinctive suite of nucleosynthetic processes operate:

• Proton-capture-driven energy (SPAr process): Proton captures on $^{32,34}$S, $^{31}$P, and $^{38}$Ar dominate the local nuclear energy output at the base of the merged shell, exceeding the classical C/O fusion energy contribution by factors $\sim400$ ($\epsilon_{\rm SPAr}/\epsilon_{\rm C+O}\approx400$) at $T\sim2.8$ GK, $\rho\sim2.5\times10^6\,\mathrm{g\,cm}^{-3}$ [$2509.13749$]. This drives further mixing and energy feedback in a runaway fashion.
• Odd-Z element and p-process isotope production: The ingestion and burning of C-rich material, coupled with intense capture flows, produce enhanced yields of K, Sc, Cl, and P, often by overproduction factors $OP_{\rm m}\sim10$–$30$ (up to $>1$ dex) compared to non-merger models [$1704.05985$, $2504.18867$].
• Radioactive isotopes: 3D macro-physical mixing enables pre-explosive synthesis of $^{44}$Ti and other radioactive species ($^{22}$Na, $^{51}$Cr, $^{60}$Fe) at levels that can rival or exceed their explosive yields; for $^{44}$Ti, the predicted production spans almost 5 dex as entrainment and mixing physics are varied [$2512.17705$].

These nucleosynthetic signals directly address the persistent underproduction of odd-Z elements in galactic chemical evolution models, and provide a unique fingerprint of the pre-SN merger in the chemical structure of both SN remnants and presolar grains.

4. Observational Diagnostics and Remnant Signatures

Multiple lines of observational evidence now support the occurrence of pre-SN O–C shell mergers:

• X-ray spectroscopy of young SNRs: Abnormally high Mg/Ne mass ratios ($\gtrsim1$ versus the canonical $\sim0.2-0.3$) in O-rich ejecta—as inferred in the SNR N49B—are direct fingerprints of shell merger events that burned Ne to Mg prior to collapse [$2403.04156$].
• Composition inhomogeneities in ejecta: Cassiopeia A exhibits O-rich knots with wide-ranging Ne/Mg and Si/Mg ratios, spatially correlated patterns of Si-rich and Ne-rich ejecta, and large-scale asymmetry—predicted by 3D merger models and interpreted as fossil evidence of a merger occurring $\lesssim10^4$ s pre-collapse [$2507.07563$].
• Asymmetric distributions of radioactive isotopes: The non-homogeneous (clumpy, asymmetric) spatial distribution of $^{44}$Ti in Cassiopeia A, mapped with NuSTAR, matches predictions from asymmetric pre-SN merger mixing [$2512.17705$].
• Yield trends in metal-poor stars: Chemical abundances of K and Sc in the Galactic disk and metal-poor stars are reproduced quantitatively only in models including the contribution of O–C shell mergers [$2504.18867$, $1704.05985$].

These signatures allow X-ray, gamma-ray, and isotopic data from SNRs and ancient stars to probe the internal burning and mixing dynamics of pre-SN progenitors.

5. Consequences for Core-Collapse Supernova Explosibility

Shell mergers exert a dominant influence on the final-state structure of the star at the point of collapse:

• Reduced core compactness: Merged-shell models exhibit lower pre-SN compactness parameters, $\xi_M$, and align with the "easy-to-explode" zone in Ertl's diagnostic mapping of SN engine success [$2403.04156$].
• Seed perturbations for explosion: Large-scale ($\ell=1$–3) velocity and compositional perturbations, reflective of high Mach numbers and aspheric flow, seed post-bounce hydrodynamic instabilities (e.g., neutrino-driven convection, SASI), thereby enhancing the likelihood and vigor of shock revival [$2507.07563$, $1808.04014$].
• Density and entropy profile smoothing: The flattening of the entropy jump and reduction of the density contrast at the C/O/Ne interfaces ensure that the collapse shock encounters smoother gradients, which can alter mass accretion rates, critical neutrino luminosity, and explosion energetics [$2407.15544$, $2504.18867$].
• Retention of merger products: The chemical structure established by the merger—especially odd-Z and radioactive species—persists through shock propagation due to the spatial extent and mixing of the merged shell.

A plausible implication is that O–C shell mergers not only set the chemical preconditions for the explosion but also act as a principal progenitor of supernova asymmetry, explosion geometry, neutron star natal kicks, and emergent nucleosynthetic signatures.

6. Frequency, Predictive Diagnostics, and Theoretical Uncertainties

The occurrence of pre-SN O–C shell mergers is model-dependent but has now been statistically characterized:

• Frequency: Full shell mergers are observed in $\sim$10–20% of published 1D evolutionary models spanning $M_{\rm ini}=9-25\,M_\odot$ and $Z=0$–$Z_\odot$; the condition is set primarily by the CO core mass and central C fraction after helium burning [$2504.18867$].
• Predictive criterion: $90\%$ of models with an O–C merger have $X_{C12}<0.277$ and $M_{CO}<4.90\,M_\odot$, with averages $M_{CO}=4.02\,M_\odot$, $X_{C12}=0.176$. This empirical rectangle is a necessary, but not sufficient, threshold for merger formation [$2504.18867$].
• Physical origin: Mergers are favored where the entropy and compositional barrier between O and C shells is small, i.e., at low $M_{CO}$ and $X_{C12}$, resulting in close mass spacing and low radiative barriers.
• 3D uncertainties: The ultimate yield and dynamical outcomes depend sensitively on the adopted macro-physical mixing (diffusion coefficients, boundary entrainment rates), with orders-of-magnitude spread in predicted $^{44}$Ti and odd-Z element production as these parameters are varied [$2512.17705$, $2407.15544$]. Full-fidelity requires further development of 3D convection-burn models.

Persistent uncertainties exist in the extrapolation from 1D to 3D treatments, the efficiency of entrainment, and the quantitative mapping from simulation to observed abundance patterns and explosion morphologies.

---

References:

• (Sato et al., 2024) Destratification in the Progenitor Interior of the Mg-rich Supernova Remnant N49B
• (Roberti et al., 17 Sep 2025) The SPAr process: proton captures powering carbon-oxygen shell mergers in massive stars
• (Ritter et al., 2017) Convective-reactive nucleosynthesis of K, Sc, Cl and p-process isotopes in O-C shell mergers
• (Sato et al., 10 Jul 2025) Inhomogeneous stellar mixing in the final hours before the Cassiopeia A supernova
• (Andrassy et al., 2018) 3D hydrodynamic simulations of C ingestion into a convective O shell
• (Roberti et al., 26 Apr 2025) The Occurrence and Impact of Carbon-Oxygen Shell Mergers in Massive Stars
• (Issa et al., 19 Dec 2025) Pre-supernova O-C shell mergers could produce more $^{44}\mathrm{Ti}$ than the explosion
• (Rizzuti et al., 2024) Shell mergers in the late stages of massive star evolution: new insight from 3D hydrodynamic simulations