The Single-Degenerate Channel Leads to Type Iax and Not Type Ia Supernovae due to Premature Ignition

Abstract

Type Ia supernovae (SNe Ia) are a critical tool for cosmology and galactic enrichment, yet the progenitor systems of normal SNe Ia remain a central puzzle. The long-debated single-degenerate (SD) channel, where a white dwarf (WD) accretes mass from a companion, faces major observational conflicts. Here, we present 3D hydrodynamic simulations that resolve these tensions by showing a fundamental dichotomy: accreting WDs predominantly ignite prematurely at sub-Chandrasekhar masses, producing low-energy, incomplete explosions consistent with Type Iax supernovae. Only WDs reaching a narrow mass threshold of 1.37 solar mass undergo complete destruction, characteristic of normal SNe Ia. This "safety valve" mechanism effectively recasts the SD channel as the main pathway to SNe Iax, not normal SNe Ia, providing a unified explanation for the observed scarcity of progenitor signatures in the latter and suggesting alternative channels dominate normal SNe Ia production.

Figures (8)

• Figure 1: Time evolution of the deflagration front and density structure for representative models.Top row:$1.330~M_{\odot}$ WD (partial deflagration). Snapshots are shown at $2.5$ and $4.9$ seconds. Bottom row:$1.374~M_{\odot}$ WD (complete disruption). Snapshots are shown at $8.4$ and $12.3$ seconds. The color map changes from red ($0.1$ g cm$^{-3}$) up to yellow ($10^5$ g cm$^{-3}$). The contours represent the temperature at $5$ different values. The asymmetric burning in the partial deflagration case contrasts with the nearly symmetric energy release in the complete disruption case.
• Figure 2: Relationship between white dwarf mass and ejected mass. The blue line represents models with an ignition offset of $88$ km, while the orange line represents models with an ignition offset of $168$ km. Note the sharp transition in ejected material at approximately $1.370~M_{\odot}$, corresponding to the boundary between partial deflagrations and complete disruptions. There is a strong dependency of the ejected material on the ignition offset; for a given offset, we see a much more subtle change in the ejected mass.
• Figure 3: Relationship between white dwarf mass and $^{56}\text{Ni}$ yield. The blue line represents models with an ignition offset of $88$ km, while the orange line represents models with an ignition offset of $168$ km. Note the sharp transition in nickel production at approximately $1.370~M_{\odot}$, corresponding to the boundary between partial deflagrations and complete disruptions. A clear dependency between the nickel yield from the explosion and the initial WD mass is evident.
• Figure 4: Relationship between WD mass and mean ejecta velocity. The blue line represents ignition points centered at $z=88$ km, and the yellow line represents the center at $z=168$ km. The destroyed WDs (models w1374-z88 and w1370-z168) show velocities typical of Type Ia supernovae ($\sim 10,000$ km s$^{-1}$). The partially exploded WDs have typical velocities of Type Iax supernovae, slowly ranging between $4000-5000$ km s$^{-1}$.
• Figure 5: Relationship between the WD mass and the kick velocity of the bounded material. The blue line represents ignition points centered at $z=88$ km and the yellow line represents the center at $z=168$ km. The destroyed WDs (models w1374-z88 and w1370-z168) are omitted from this plot as there is no kick velocity in these cases.