It's always the quiet ones: Single Degenerate Double Detonation Type Ia Supernova from Quiescent Helium Accretion

Abstract

We investigate a sub-Chandrasekhar mass double detonation pathway for Type Ia supernovae arising from single degenerate helium accreting carbon-oxygen white dwarfs. Building on our previous one dimensional study of recurrent helium novae (Hillman et al. 2025), we evolve a 0.7 solar mass white dwarf through steady accretion at 10^-8 Msun yr^-1 until it reaches 1.1 solar mass, yielding realistic, time evolved helium rich profiles. These profiles are mapped into FLASH simulations, incorporating nuclear burning for helium and carbon-oxygen detonation, in multi-dimensional hydrodynamic runs. A localized, modest temperature perturbation near the base of the helium shell robustly triggers an outward helium-shell detonation. The ensuing inward propagating shock converges in the carbon-oxygen core, igniting a secondary detonation that unbinds the star. We obtain a Ni56 yield of ~0.64 solar mass, an intermediate-mass element (Si-Ca) mass of ~0.41 solar mass, and maximum ejecta velocities approaching 22,000 km/s, values consistent with normal Type Ia supernovae. Our results demonstrate that recurrent helium accretors, typically quiescent over long timescales, can evolve under subtle, "quiet" conditions to trigger robust double detonations, supporting their role as viable progenitors of sub-Chandrasekhar mass Type Ia supernovae.

Figures (4)

• Figure 1: Long-term envelope and core evolution in quiescent accretion models. Helium mass (top) and bulk core carbon/oxygen masses (middle/bottom) versus time for several $(M_{\rm WD,i},\dot M)$ tracks in hillman2025. Our fiducial model (orange) starts at $M_{\rm WD,i}=0.7\,\mathrm{M_\odot}$ with $\dot M=10^{-8}\,\mathrm{M_\odot\,yr^{-1}}$ and grows to $1.1\,\mathrm{M_\odot}$. These models experienced uninterrupted prolonged helium accretion until indicating a SN ignition.
• Figure 2: Pre-ignition structure during uninterrupted helium accretion. Temperature (top) and density (bottom) as functions of radius at several epochs in our fiducial 1D Lagrangian model. The narrow temperature spike at $r\simeq2.5\times10^3$ km marks the naturally emerging base-of-shell peak reached a few timesteps before the 1D calculation becomes numerically unresolved ($T_{\max}\approx3\times10^8$ K). In the multi-D flash simulations we do not map this peak to an entire shell; instead, we represent it as a localized off-axis Gaussian hotspot to avoid artificially overheating a shell while preserving the physical trigger.
• Figure 3: Time sequence of the double detonation. Equatorial $(x\!-\!y)$ slices of temperature and density (top rows), followed by $z$–integrated species maps (He, C, O, Si, Fe, Ni), at $t=\{21.1,\,56.0,\,115.5,\,172.2,\,215.1,\,270.1,\,317.5,\,393.6\}$ ms. The helium–shell detonation wraps around the star while an inward shock compresses the CO core and triggers secondary ignition. He/C/O are depleted as Si/Fe/Ni are produced. By $\sim\!0.5$ s the detonation phase is complete and the ejecta accelerate outward.
• Figure 4: Abundance stratification in mass coordinates. Angle–averaged mass fractions binned by the tracers' initial enclosed–mass coordinate $M/M_\odot$, evaluated once the ejecta are homologous. The inner ejecta are dominated by nickel, followed by an IME mantle (Si, S, Ar, Ca), and a thin outer layer of shell ashes.