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Recent theoretical and observational studies both argue that the merging ofdouble carbon-oxygen white dwarfs (WDs) is responsible for at least some TypeIa supernovae (SNe Ia). Previous (standard) studies of the anticipated SNbirthrate from this channel have assumed that the merger process isconservative and that the primary criterion for explosion is that the mergedmass exceeds the Chandrasekhar mass. Han & Webbink (1999) demonstrated thatmass transfer and merger in close double WDs will in many cases benon-conservative. Pakmor et al. (2011) further suggested that the mergerprocess should be violent in order to initiate an explosion. We have thereforeinvestigated how the SN Ia birthrate from the double-degenerate (DD) channel isaffected by these constraints. Using the binary-star population-synthesismethod, we have calculated the DD SN Ia birthrate under conservative andnon-conservative approximations, and including lower mass and mass-ratio limitsindicated by recent smoothed-particle-hydrodynamic calculations. The predictedDD SN Ia rate is significantly reduced by all of these constraints. Withdynamical mass loss alone (violent merger) the birthrate is reduced to 56% ofthe conservative rate. Requiring that the mass ratio $q>2/3$ further reducesthe birthrate to 18% that of the standard assumption. An upper limit of 0.0061SNuM, or a Galactic rate of $4.6 \times 10^{-4}{\rm yr}^{-1}$, might berealistic.

Some constraints on the lower mass limit for double-degenerate progenitors of Type Ia supernovae