Nominal thresholds for good astrometric fits, and prospects for binary detectability, for the full extended Gaia mission

Abstract The full extended Gaia mission spans slightly over 10 years, whilst the current data releases represent only a fraction of that timescale, 34 months in Gaia’s third data release (DR3). A longer time baseline improves astrometric fits by lowering the noise and making consistently bad fits (for example, due to binarity) more apparent. In this paper, we use simulated binaries from the Gaia Universe Model to examine the long-term astrometric behaviour of single stars and stellar binaries. We calculate nominal upper limits on the spread of goodness of astrometric fits for well-behaved single stars. Specifically, for the RUWE parameter, we predict thresholds of DR4 (RUWE₋₈₌=1. 15) and DR5 (RUWE₋₈₌=1. 11), based on the full mission nominal scanning law. These limits help identify poor astrometric fits and flag potential binary systems. We find that the number of detectable short-period binaries increases by 5-10% per new data release, suggesting detections may be possible for orbital periods down to days. The number of detectable long-period systems increases by 10-20%, with periods up to 100 years causing significant deviations in low and moderate-eccentricity binaries. Very eccentric systems with much longer periods (thousands of years) can still be detected if they pass through periapse during the observing window. The detectability of most systems is unaffected by the light ratio, although it is reduced for twin binaries. This is because astrometric motion is primarily a dynamical effect and depends mainly on the mass ratio, causing twin binaries to remain largely undetected. By contrast, the extended time baseline significantly enhances the detected binary population across the main-sequence and among young white dwarves. Finally, we compare our results to the analytic estimate for the spread in UWE, derived from a χ-distribution moderated by the number of observations. These agree with our inferred population limits, though they suggest that we may be biased by a small number of poorly sampled systems. In regions of the sky that are more frequently observed, lower limits could be employed, potentially bringing even more binaries above the threshold for detectability.