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

Silvia Toonen, G. Nelemans+1 more|arXiv (Cornell University)|Aug 31, 2012

Gamma-ray bursts and supernovaePhysics and Astronomy100 references126 citations

TL;DR

This study investigates Type Ia supernova (SNIa) progenitors from merging double white dwarfs (DWDs) using an updated binary population synthesis model in SeBa. It finds that while the delay time distribution shape matches observations, the predicted SNIa rate is 7–12 times lower than observed, regardless of whether the common envelope evolution is modeled via the α- or γ-formalism, suggesting a significant shortfall in the double-degenerate channel alone to explain the observed SNIa rate.

ABSTRACT

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.

Motivation & Objective

• To assess the contribution of merging double carbon-oxygen white dwarfs to the observed Type Ia supernova (SNIa) rate.
• To test the impact of different common envelope (CE) evolution formalisms—α-formalism (energy-based) and γ-formalism (angular momentum-based)—on the simulated DWD population and SNIa rate.
• To compare the simulated DWD population with observed close double white dwarf systems to evaluate model fidelity.
• To determine whether the double-degenerate scenario alone can account for the observed SNIa rate, or if additional progenitor channels are required.

Proposed method

• Employed the updated binary population synthesis code SeBa to evolve binary systems from the zero-age main sequence to DWD formation and subsequent mergers.
• Applied two distinct CE evolution prescriptions: the α-formalism (energy-based) and the γ-formalism (angular momentum-based), with a hybrid approach using α-formalism for tidal or compact-object-triggered CE events.
• Simulated the full evolutionary sequence including mass transfer, common envelope phases, and gravitational wave-driven inspiral leading to DWD mergers.
• Calculated the delay time distribution (DTD) of SNIa from DWD mergers and compared it to observational DTDs.
• Estimated the integrated SNIa rate per unit stellar mass and compared it to observed values from surveys.
• Assessed the fraction of observed DWDs expected to be SNIa progenitors and evaluated the impact of metallicity, initial mass function, and binary fraction on the synthetic rate.

Experimental results

Research questions

• RQ1 Can the double-degenerate scenario, via merging DWDs, reproduce the observed delay time distribution of Type Ia supernovae?
• RQ2 Does the choice of common envelope formalism (α vs. γ) significantly affect the predicted DWD merger rate and SNIa progenitor population?
• RQ3 To what extent do the simulated DWD populations match the observed properties of known close double white dwarf systems?
• RQ4 Why is the predicted SNIa rate from the DWD channel consistently lower than the observed rate, and what factors might explain this discrepancy?
• RQ5 Is the double-degenerate scenario sufficient to account for the observed SNIa rate, or are additional progenitor channels required?

Key findings

• The simulated delay time distribution (DTD) for merging double white dwarfs matches the observed DTD shape within observational uncertainties, regardless of the CE formalism used.
• The predicted SNIa rate from the DWD channel is 7–12 times lower than the observed rate, indicating a significant deficit in the double-degenerate scenario alone.
• The γ-formalism better reproduces the observed population of close double white dwarfs compared to the α-formalism, particularly in terms of mass ratio distribution.
• The fraction of observed DWDs expected to be SNIa progenitors is only 0.9–2.9% in the model, far below the 10–30% required to match the observed SNIa rate.
• Even with optimistic assumptions—such as a 70% binary fraction or low metallicity—the synthetic SNIa rate remains insufficiently high to explain observations, suggesting the need for additional progenitor channels.
• The model underpredicts the number of SNIa progenitors, with only 4–15 expected among 46 observed DWDs, while only two systems are currently identified as possible progenitors, highlighting a significant discrepancy.