Merging white dwarfs and SN Ia
  Using population synthesis, we study a double-degenerate (DD) scenario for 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 $M_\odot$ with hybrid or helium WDs more massive than 0.37$M_\odot$ produce SNe Ia. The models are parametrized by the product of the common envelope efficiency and the parameter of binding energy of stellar envelopes $\alpha_{ce}\lambda$, which we vary between 0.25 and 2. The best agreement with observations is obtained for $\alpha_{ce}\lambda$=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 $6.5 10^{-3}$ per yr (for the mass of the bulge and thin disk equal to $7.2 10^{10} M_\odot$), which is comparable to the observational estimate $(5.4\pm0.12) 10^{-3}$ per yr. The model DTD for 1 to 8 Gyr range satisfactorily fits DTD for SNe Ia in the field galaxies (Maoz et al. 2012). For this epoch, model DTD is a power low with an index -1.64. At earlier and later epochs our DTD has a deficit of events, as in other studies. Marginal agreement with observational DTD is achieved even if only CO+CO WD with $M_1\geq0.8\,M_\odot$ and $M_2\geq0.6\,M_\odot$ produce SNe Ia. A better agreement of observed and model DTD may be obtained if tidal effects are weaker than assumed and/or metallicity of population is much lower than solar. 
L.R. Yungelson and A.G. Kuranov
astro-ph.SR