An Upper Limit to the Space Density of Short-period Noninteracting Binary White Dwarfs

We have searched for short-period, detached binary stars in which both stars are white dwarfs by looking for radial velocity variations in spectroscopically identified DA and DB white dwarfs. The search was sensitive to binaries with orbital periods between 30 s and 3 hr and, within that range, we would have detected roughly 90% of all binaries, depending on the distribution of their orbital periods, mass ratios, and spectral types. We observed 44 stars without finding any binaries. The fraction of white dwarfs that are binaries is less than 1/20 with a 90% probability and less than 1/37 with a 70% probability. Other surveys sensitive to longer periods have also failed to find a large population of binary white dwarfs. The space density of binary white dwarfs is too low to account for the rate of Type I supernovae in the Galaxy and too low to agree comfortably with recent estimates of their numbers from theoretical calculations of the evolution of close binaries. Binary white dwarfs are the dominant source of the background gravitational waves at periods between about 30 s and 1 hr. The background is likely to be much lower than previously thought.