Internal kinematics of Gaia eDR3 wide binaries

ABSTRACT Using the recent Gaia eDR3 catalogue, we construct a sample of solar neighbourhood isolated wide binaries satisfying a series of strict signal-to-noise data cuts, exclusion of random association criteria, and detailed colour–magnitude diagram selections to minimize the presence of any kinematic contaminating effects having been discussed in the literature to date. Our final high-purity sample consists of 423 binary pairs within 130 pc of the sun and in all cases high-quality Gaia single-stellar fits for both components of each binary (final average RUWE values of 0.99), both also restricted to the cleanest region of the main sequence. We find kinematics fully consistent with Newtonian expectations for separations, s, below 0.009 pc, with relative velocities scaling with ΔV ∝ s−1/2 and a total binary mass, Mb, velocity scaling consistent with $\Delta V \propto M_{\mathrm{ b}}^{1/2}$. For the separation region of s > 0.009 pc, we obtain significantly different results, with a separation independent ΔV ≈ 0.5 km s−1 and a $\Delta V \propto M_{\mathrm{ b}}^{0.24 \pm 0.21}$. This situation is reminiscent of the low acceleration galactic baryonic Tully–Fisher phenomenology, and indeed, the change from the two regimes we find closely corresponds to the a ≲ a0 transition. These results are at odds not only with Newtonian expectations, but also with MOND predictions, where the presence of an external field effect implies only small deviations from Newtonian dynamics are expected for solar neighbourhood wide binaries.