The separation distribution and merger rate of double white dwarfs: improved constraints

We obtain newand precise information on the double white dwarf (DWD) population and on its gravitational-wave-driven merger rate by combining the constraints on the DWD population from two previous studies on radial velocity variation. One of the studies is based on a sample of white dwarfs (WDs) from the Sloan Digital Sky Survey (SDSS, which with its low spectral resolution probes systems at separations a < 0.05 au) and the other is based on the ESO-VLT Supernova-Ia Progenitor surveY (SPY, which with its high spectral resolution is sensitive to a < 4 au). From a joint likelihood analysis, the DWD fraction among WDs is f(bin) = 0.095 +/- 0.020 (1 sigma, random) + 0.010 (systematic) in the separation range less than or similar to 4 au. The index of a power-law distribution of initialWD separations (at the start of solely gravitationalwave-driven binary evolution), N(a)da proportional to a(alpha)da, is alpha = -1.30 +/- 0.15 (1 sigma) + 0.05 (systematic). The Galactic WD merger rate per WD is R-merge = (9.7 +/- 1.1) x 10(-12) yr(-1). Integrated over the Galaxy lifetime, this implies that 8.5-11 per cent of all WDs ever formed have merged with another WD. If most DWD mergers end as more-massive WDs, then some 10 per cent of WDs are DWD-merger products, consistent with the observed fraction of WDs in a 'high-mass bump' in the WD mass function. The DWD merger rate is 4.5-7 times the Milky Way's specific Type Ia supernova (SN Ia) rate. If most SN Ia explosions stem from the mergers of some DWDs (say, those with massive-enough binary components) then similar to 15 per cent of all WD mergers must lead to a SN Ia.