Detonations in Sub-Chandrasekhar Mass C+O White Dwarfs

Explosions of sub-Chandrasekhar-mass white dwarfs are one alternative to the standard Chandrasekhar-mass model of Type Ia supernovae. They are interesting since binary systems with sub-Chandrasekhar-mass primary white dwarfs should be common and this scenario would suggest a simple physical parameter which determines the explosion brightness, namely the mass of the exploding white dwarf. Here we perform one-dimensional hydrodynamical simulations, associated post-processing nucleosynthesis and multi-wavelength radiation transport calculations for pure detonations of carbon-oxygen white dwarfs. The light curves and spectra we obtain from these simulations are in good agreement with observed properties of Type Ia supernovae. In particular, for white dwarf masses from 0.97 - 1.15 Msun we obtain 56Ni masses between 0.3 and 0.8 Msun, sufficient to capture almost the complete range of Type Ia supernova brightnesses. Our optical light curve rise times, peak colours and decline timescales display trends which are generally consistent with observed characteristics although the range of B-band decline timescales displayed by our current set of models is somewhat too narrow. In agreement with observations, the maximum light spectra of the models show clear features associated with intermediate mass elements and reproduce the sense of the observed correlation between explosion luminosity and the ratio of the Si II lines at 6355 and 5972 Angstroms. We therefore suggest that sub-Chandrasekhar mass explosions are a viable model for Type Ia supernovae for any binary evolution scenario leading to explosions in which the optical display is dominated by the material produced in a detonation of the primary white dwarf.

💡 Research Summary

This paper investigates whether detonations of sub‑Chandrasekhar‑mass carbon‑oxygen white dwarfs (WDs) can reproduce the observed properties of Type Ia supernovae (SNe Ia). The authors construct a set of one‑dimensional hydrodynamic models with WD masses ranging from 0.81 M⊙ to 1.15 M⊙, all ignited centrally. The detonation front is treated with a level‑set method, using realistic high‑density detonation speeds and Chapman‑Jouguet speeds at low densities. Post‑processing tracer‑particle nucleosynthesis is performed for a baseline 50/50 C/O composition and, for the 1.06 M⊙ model, an additional case with 7.5 % ²²Ne to explore metallicity effects. The resulting ⁵⁶Ni yields span 0.01–0.81 M⊙, covering the range required for normal to bright SNe Ia, while intermediate‑mass elements (IMEs) and oxygen increase with WD mass. Inclusion of ²²Ne boosts stable iron‑group element (IGE) production at the expense of ⁵⁶Ni and IMEs, illustrating the sensitivity to progenitor metallicity.

Radiative transfer calculations are carried out with the Monte‑Carlo code ARTIS, employing a large atomic line list (~8.2 × 10⁶ lines) and NLTE ionization treatment. For the four models that produce >0.05 M⊙ of ⁵⁶Ni (0.88–1.15 M⊙), synthetic light curves in U, B, V, R, I, J, H, and K bands are generated, together with maximum‑light spectra. The models yield B‑band rise times of 18–20 days, peak absolute magnitudes from –16.6 mag (0.88 M⊙) to –19.9 mag (1.15 M⊙), and B‑V colours at maximum that are slightly redder than those of Chandrasekhar‑mass models but consistent with observations. The decline‑rate parameter Δm₁₅(B) varies between 1.34 and 1.77 mag, reproducing the observed width‑luminosity relation qualitatively, though the range is narrower than seen in real SNe Ia.

Spectroscopically, the models display the characteristic Si II λ6355 absorption, with velocities from ~6,000 km s⁻¹ (low‑mass model) to ~12,500 km s⁻¹ (high‑mass model), matching observed values. The ratio of the weaker Si II λ5972 to λ6355 line decreases with increasing luminosity, mirroring the empirical trend. Other IME features (Ca II, S II) are present, and O I λ7773 appears prominently in the faintest models, as observed in some low‑luminosity SNe Ia. The synthetic spectra compare well with data from SN 2004eo and SN 2005cf.

The authors conclude that pure detonations of sub‑Chandrasekhar C+O WDs, without a massive nickel‑rich helium shell, can naturally produce the required mixture of IGEs and IMEs and match the bulk photometric and spectroscopic properties of normal SNe Ia. Limitations include the narrow Δm₁₅ range and slightly redder colours, likely reflecting the idealized one‑dimensional setup and the neglect of any outer shell material. Future work should explore multi‑dimensional effects, realistic triggering mechanisms, and modest helium or unburned material in the outer ejecta to broaden the parameter space and improve agreement with the full diversity of observed SNe Ia.