Direct Numerical Simulations of Oxygen-Flame-Driven Deflagration-to-Detonation Transition in Type Ia Supernovae

We present direct numerical simulations demonstrating deflagration-to-detonation transition (DDT) driven by oxygen flames in Type Ia supernova progenitors. Using the Castro hydrodynamics code coupled with the ``aprox13’’ 13-isotope nuclear network, we simulate combustion in isolated fuel regions where oxygen flames trail carbon flames. In a fiducial one-dimensional run at ${\rho }_0=3.5\times {10}^7\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$ we observe spontaneous DDT of the oxygen flame via the Zel’dovich gradient mechanism when the carbon-oxygen separation reaches $\sim 10\mathrm{k}\mathrm{m}$. The oxygen detonation then captures the carbon flame and triggers a stable carbon detonation. Systematic one-dimensional parameter scans show that successful carbon DDT requires upstream densities in the range $(3.1$–$3.6)\times {10}^7\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$ and a minimum carbon-flame thickness of $\gtrsim 20\mathrm{m}$. Two-dimensional simulations confirm DDT and demonstrate that the multidimensional cellular structure of the oxygen detonation can promote carbon detonation at somewhat lower densities than in one dimension. These results provide direct numerical evidence that oxygen-flame-driven DDT is physically plausible in turbulent white-dwarf environments and underscore the importance of multidimensional effects for Type Ia supernova explosion modeling.