2017MNRAS.464.1607Y

Using population synthesis, we study a double-degenerate (DD) scenario for Type Ia supernovae (SNe Ia), aiming to estimate the maximum possible contribution to the rate of SNe from this scenario and the dependence of the delay-time distribution (DTD) on it. We make an extreme assumption that all mergers of super-Chandrasekhar pairs of CO white dwarfs (WDs) and mergers of CO WDs more massive than 0.47 M_☉ with hybrid or helium WDs more massive than 0.37 M_☉ produce SNe Ia. The models are parametrized by the product of the common envelope efficiency and the parameter of binding energy of stellar envelopes, α_ce λ, which we vary between 0.25 and 2. The best agreement with observations is obtained for α_ce λ = 2. A substantial contribution to the rate of SNe Ia is provided by the pairs with a hybrid WD. The estimated Galactic rate of SNe Ia is 6.5 x 10^–3 yr^–1 (for the mass of the bulge and thin disc equal to 7.2 x 10¹⁰ M_☉), which is comparable to the observational estimate (5.4 ± 0.12) x 10^–3 yr^–1. The model DTD for 1 <= t <= 8 Gyr satisfactorily fits the DTD for SNe Ia in the field galaxies (Maoz, Mannucci & Brandt). For this epoch, the model DTD is ∝t^–1.64. At earlier and later epochs, our DTD has a deficit of events, as in other studies. Marginal agreement with the observational DTD is achieved even if only CO+CO WDs with M₁ >= 0.8 M_☉ and M₂ >= 0.6 M_☉ produce SNe Ia. A better agreement of observed and modelled DTD may be obtained if tidal effects are weaker than assumed and/or the metallicity of the population is much lower than solar.