The Merger Rate of Binary White Dwarfs in the Galactic Disk

We use multi-epoch spectroscopy of ∼4000 white dwarfs in the Sloan Digital Sky Survey to constrain the properties of the Galactic population of binary white dwarf systems and calculate their merger rate. With a Monte Carlo code, we model the distribution of ΔRV max , the maximum radial velocity shift between exposures of the same star, as a function of the binary fraction within 0.05 AU, f bin , and the power-law index in the separation distribution at the end of the common-envelope phase, α. Although there is some degeneracy between f bin and α, the 15 high-ΔRV max systems that we find constrain the combination of these parameters, which determines a white dwarf merger rate per unit stellar mass of 1.4 +3.4 −1.0 × 10 −13 yr −1 M −1 ☉ (1σ limits). This is remarkably similar to the measured rate of Type Ia supernovae (SNe Ia) per unit stellar mass in Milky-Way-like Sbc galaxies. The rate of super-Chandrasekhar mergers is only 1.0 +1.6 −0.6 × 10 −14 yr −1 M −1 ☉ . We conclude that there are not enough close binary white dwarf systems to reproduce the observed SN Ia rate in the "classic" double degenerate super-Chandrasekhar scenario. On the other hand, if sub-Chandrasekhar mergers can lead to SNe Ia, as has been recently suggested by some studies, they could make a major contribution to the overall SN Ia rate. Although unlikely, we cannot rule out contamination of our sample by M-dwarf binaries or non-Gaussian errors. These issues will be clarified in the near future by completing the follow-up of all 15 high-ΔRV max systems.