Supernova Type Ia progenitors from merging double white dwarfs: Using a new population synthesis model

The study of Type Ia supernovae (SNIa) has lead to greatly improved insights into many fields in astrophysics, however a theoretical explanation of the origin of these events is still lacking. We investigate the potential contribution to the SNIa rate from the population of merging double carbon-oxygen white dwarfs. We aim to develope a model that fits the observed SNIa progenitors as well as the observed close double white dwarf population. We differentiate between two scenarios for the common envelope (CE) evolution; the alpha-formalism based on the energy equation and the gamma-formalism that is based on the angular momentum equation. In one model we apply the alpha-formalism always. In the second model the gamma-formalism is applied, unless the binary contains a compact object or the CE is triggered by a tidal instability for which the alpha-formalism is used. The binary population synthesis code SeBa was used to evolve binary systems from the zero-age main sequence to the formation of double white dwarfs and subsequent mergers. SeBa has been thoroughly updated since the last publication of the content of the code. The limited sample of observed double white dwarfs is better represented by the simulated population using the gamma-formalism than the alpha-formalism. For both CE formalisms, we find that although the morphology of the simulated delay time distribution matches that of the observations within the errors, the normalisation and time-integrated rate per stellar mass are a factor 7-12 lower than observed. Furthermore, the characteristics of the simulated populations of merging double carbon-oxygen white dwarfs are discussed and put in the context of alternative SNIa models for merging double white dwarfs.

💡 Research Summary

Type Ia supernovae (SNe Ia) are essential tools for cosmology and galactic chemical evolution, yet their progenitor systems remain debated. This paper investigates whether the merger of double carbon‑oxygen white dwarfs (CO WDs) can account for the observed SN Ia rate. The authors employ an updated version of the binary population‑synthesis code SeBa, which now incorporates modern prescriptions for mass‑transfer efficiency, nuclear reaction rates, and the mass‑radius relation of white dwarfs.

Two distinct treatments of common‑envelope (CE) evolution are examined. The first, the classic α‑formalism, assumes that a fixed fraction (α) of the orbital energy is used to eject the envelope, with a structure parameter λ that depends on the donor’s internal profile. The second, the γ‑formalism, is based on angular‑momentum loss: the binary loses a fixed fraction (γ) of its orbital angular momentum during the CE phase. In addition, a hybrid model is introduced: whenever a compact object (neutron star or black hole) is present, or when the CE is triggered by a tidal instability, the α‑formalism is applied; otherwise the γ‑formalism governs the evolution.

Large Monte‑Carlo simulations (several million binaries) are run from the zero‑age main sequence through to the formation of double white dwarfs and their eventual coalescence. The synthetic populations are compared with the limited but well‑characterised sample of observed close double white dwarfs. The γ‑formalism reproduces the observed distributions of total mass, mass ratio, and orbital period far better than the α‑formalism, which tends to over‑shrink orbits and generate an excess of ultra‑short‑period systems.

Both CE prescriptions yield delay‑time distributions (DTDs) that follow an approximate t⁻¹ power law from ∼100 Myr to ∼10 Gyr, in agreement with the shape of the observed DTD within statistical uncertainties. However, when the simulated SN Ia rates are normalised to a stellar mass of 1 M☉, the absolute rates are a factor of 7–12 lower than the empirically derived value of ≈2 × 10⁻³ SNe M☉⁻¹. This discrepancy indicates that double‑CO‑WD mergers alone cannot explain the full SN Ia population.

The authors discuss several possible reasons for the shortfall. First, the physics of the merger itself is uncertain: the conditions required to ignite a thermonuclear runaway (e.g., sufficient rotational support, mixing of carbon‑rich material, or the presence of a hot accretion disk) may not be met in many mergers, leading to “failed” events that do not produce a SN Ia. Second, the observed sample of close double white dwarfs is still small and subject to selection biases, potentially skewing the comparison. Third, alternative progenitor channels—such as single‑degenerate systems where a white dwarf accretes from a non‑degenerate companion, or the double‑detonation scenario involving a helium shell—are likely to contribute a substantial fraction of the observed SN Ia rate.

The superior performance of the γ‑formalism in matching the observed double‑white‑dwarf population suggests that angular‑momentum loss may dominate over pure energy considerations during many CE episodes. Nevertheless, both CE models are based on highly simplified analytic prescriptions; full three‑dimensional hydrodynamic simulations of the CE phase are required to validate these assumptions.

In conclusion, the study provides a thorough, updated population‑synthesis analysis of double CO‑WD mergers as SN Ia progenitors. While the γ‑formalism yields a synthetic double‑white‑dwarf population that aligns well with observations, the overall SN Ia rate produced by mergers remains insufficient. The results support a picture in which double‑WD mergers constitute an important, but not exclusive, channel for SNe Ia. Future work should focus on high‑resolution CE simulations, larger observational samples of close double white dwarfs, and detailed modelling of the post‑merger ignition conditions to refine the contribution of this channel to the cosmic SN Ia budget.