Merging white dwarfs and SN Ia

TL;DR

This study uses population synthesis to evaluate the double-degenerate scenario for Type Ia supernovae, estimating their contribution to the galactic rate and analyzing the delay-time distribution's dependence on model parameters.

Contribution

It provides the first detailed population synthesis analysis of the DD scenario, estimating the maximum contribution to SNe Ia rates and fitting the delay-time distribution to observations.

Findings

Estimated galactic SNe Ia rate of 6.5 x 10^-3 per year.

Model delay-time distribution fits observations for 1-8 Gyr with a power-law index of -1.64.

Hybrid white dwarf mergers significantly contribute to SNe Ia rates.

Abstract

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.