A gap in the double white dwarf separation distribution caused by the common-envelope evolution: astrometric evidence from <i>Gaia</i>

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