Unresolved stellar companions with Gaia DR2 astrometry

Understand

For stars with unresolved companions, motions of the centre of light and that of mass decouple, causing a single-source astrometric model to perform poorly.

• We show that such stars can be easily detected with the reduced chi2 statistic, or RUWE, provided as part of Gaia DR2.
• We convert RUWE into the amplitude of the image centroid wobble, which, if scaled by the source distance, is proportional to the physical separation between companions (for periods up to several years).
• We test this idea on a sample of known spectroscopic binaries and demonstrate that the amplitude of the centroid perturbation scales with the binary period and the mass ratio as expected.