Nucleosynthesis in Two-Dimensional Delayed Detonation Models of Type Ia Supernova Explosions

The nucleosynthetic characteristics of various explosion mechanisms of Type Ia supernovae (SNe Ia) is explored based on three two-dimensional explosion simulations representing extreme cases: a pure turbulent deflagration, a delayed detonation following an approximately spherical ignition of the initial deflagration, and a delayed detonation arising from a highly asymmetric deflagration ignition. Apart from this initial condition, the deflagration stage is treated in a parameter-free approach. The detonation is initiated when the turbulent burning enters the distributed burning regime. This occurs at densities around $10^{7}$ g cm$^{-3}$ – relatively low as compared to existing nucleosynthesis studies for one-dimensional spherically symmetric models. The burning in these multidimensional models is different from that in one-dimensional simulations as the detonation wave propagates both into unburned material in the high density region near the center of a white dwarf and into the low density region near the surface. Thus, the resulting yield is a mixture of different explosive burning products, from carbon-burning products at low densities to complete silicon-burning products at the highest densities, as well as electron-capture products synthesized at the deflagration stage. In contrast to the deflagration model, the delayed detonations produce a characteristic layered structure and the yields largely satisfy constraints from Galactic chemical evolution. In the asymmetric delayed detonation model, the region filled with electron capture species (e.g., $^{58}$Ni, $^{54}$Fe) is within a shell, showing a large off-set, above the bulk of $^{56}$Ni distribution, while species produced by the detonation are distributed more spherically (abridged).

💡 Research Summary

This paper investigates the nucleosynthetic output of Type Ia supernovae using three two‑dimensional hydrodynamic explosion models that span the extremes of possible ignition conditions. The first model is a pure turbulent deflagration (DO) in which the flame propagates solely by subsonic turbulent burning. The second and third models are delayed‑detonation scenarios: one with an approximately spherical ignition of the deflagration (DDS) and another with a highly asymmetric ignition (DDA). Apart from the initial flame geometry, the deflagration phase is treated without adjustable parameters; the turbulent flame speed is computed directly from a sub‑grid turbulence model. Detonation is triggered automatically when the turbulent flame enters the distributed burning regime, which in these simulations occurs at a relatively low density of ≈10⁷ g cm⁻³. This density is an order of magnitude lower than the values traditionally assumed in one‑dimensional, spherically symmetric studies, and it fundamentally changes the way the detonation wave propagates. In the two‑dimensional runs the detonation front advances both inward into the high‑density core and outward into the low‑density outer layers, producing a mixture of burning products ranging from carbon‑burning ashes at low density to complete silicon‑burning yields (⁵⁶Ni, ⁵⁶Fe) at the highest densities. Electron‑capture isotopes such as ⁵⁸Ni and ⁵⁴Fe are synthesized primarily during the deflagration stage and remain embedded in the ejecta.

The pure deflagration model yields a relatively homogeneous composition dominated by intermediate‑mass elements and a modest amount of ⁵⁶Ni; its electron‑capture products are centrally concentrated, but the overall Fe‑peak yield is insufficient to match Galactic chemical evolution (GCE) constraints. By contrast, both delayed‑detonation models develop a pronounced layered structure: an outer shell of carbon‑burning products, a middle zone rich in Si, S, Ca, and an inner core dominated by iron‑peak nuclei. The DDS model, with its near‑spherical ignition, produces a fairly symmetric distribution of ⁵⁶Ni and electron‑capture species, and its integrated yields of Fe‑peak and α‑elements satisfy the abundance ratios observed in the Milky Way.

The asymmetric delayed‑detonation (DDA) model reveals a striking geometric offset: the region enriched in electron‑capture isotopes forms a shell that is displaced upward relative to the bulk of the ⁵⁶Ni distribution. Meanwhile, the detonation‑produced material retains a more spherical geometry. This offset reproduces the off‑center Ni/Fe enhancements inferred from nebular‑phase spectra of some SNe Ia and offers a natural explanation for observed polarization and line‑profile asymmetries.

Overall, the study demonstrates that initiating detonation at low density in multidimensional simulations yields a richer and more realistic nucleosynthetic outcome than traditional 1‑D models. The layered ejecta structure and the compatibility of the yields with GCE constraints support delayed detonation as a viable explosion mechanism for normal SNe Ia. Moreover, the sensitivity of the electron‑capture isotope distribution to the initial flame geometry suggests that observed asymmetries in supernova remnants could be used to diagnose the ignition configuration of the progenitor white dwarf. Future work extending these calculations to full three‑dimensional geometry and coupling them with radiative‑transfer models will be essential for connecting the predicted nucleosynthetic signatures to observable light curves and spectra.