THE MERGER RATE OF BINARY WHITE DWARFS IN THE GALACTIC DISK

We use multi-epoch spectroscopy of similar to 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 Delta RVmax, 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, alpha. Although there is some degeneracy between f(bin) and alpha, the 15 high-Delta RVmax systems that we find constrain the combination of these parameters, which determines a white dwarf merger rate per unit stellar mass of 1.4(-1.0)(+3.4) x 10(-13) yr(-1)M(circle dot)(-1) (1 sigma limits). This is remarkably similar to the measured rate of Type Ia supernovae (SNe Ia) per unit stellar mass inMilky-Way-like Sbc galaxies. The rate of super-Chandrasekhar mergers is only 1.0(-0.6)(+1.6) x 10(-14) yr(-1)M(circle dot)(-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-Delta RVmax systems.