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 chi(2) 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 chi(2) into the amplitude of the centroid wobble delta a, which is proportional to the binary separation a. The measured delta a distribution drops towards larger wobble amplitudes and shows a break around log(10)delta a approximate to -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 delta a distribution corresponds to one side of a deep gap in the DWD separation distribution at around a approximate to 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 approximate to 0.01 au and log(10)delta a < -2. Our models reproduce the overall shape of the observed delta a distribution and its normalization, however the predicted drop in the numbers of DWDs beyond the break is steeper than in the data.