A gap in the double white dwarf separation distribution caused by the common-envelope evolution astrometric evidence from Gaia

The trajectory of the center of light of an unresolved binary is different from that of its center of mass. Binary-induced stellar centroid wobbling can therefore be detected as an excess in the goodness-of-fit of the single-star astrometric model. We use reduced χ² of the astrometric fit in the Gaia Early Data Release 3 to detect the likely unresolved double white dwarfs (DWDs). Using parallax-based distances we convert the excess of reduced χ² into the amplitude of the centroid wobble δa, which is proportional to the binary separation a. The measured δa distribution drops towards larger wobble amplitudes and shows a break around log₁₀δa ≍ -0.7 where it steepens. The integral of the distribution yields DWD fraction of 6.5 ± 3.7 per cent in the range 0.01<a(au)<2 . Using synthetic models of the Galactic DWDs we demonstrate that the break in the δa distribution corresponds to one side of a deep gap in the DWD separation distribution at around a ≍ 1 au. Model DWDs with separations less than several au shrink dramatically due to (at least one) common envelope phase, reshaping the original separation distribution, clearing a gap and creating a pile-up of systems with a ≍ 0.01 au and log₁₀δa < -2. Our models reproduce the overall shape of the observed δa distribution and its normalization, however the predicted drop in the numbers of DWDs beyond the break is steeper than in the data.