Gaia DR3 Detectability of Unresolved Binary Systems

Gaia can not individually resolve very close binary systems, however, thecollected data can still be used to identify them. A powerful indicator ofstellar multiplicity is the sources reported Renormalized Unit Weight Error(ruwe), which effectively captures the astrometric deviations fromsingle-source solutions. We aim to characterise the imprints left on ruwecaused by binarity. By flagging potential binary systems based on ruwe, we aimto characterise which of their properties will contribute the most to theirdetectability. We develop a model to estimate ruwe values for observations ofGaia sources, based on the biases to the single-source astrometric trackarising from the presence of an unseen companion. Then, using the recipes fromprevious GaiaUnlimited selection functions, we estimate the selectionprobability of sources with high ruwe, and discuss what binary propertiescontribute to increasing the sources ruwe. We compute the maximum ruwe valuewhich is compatible with single-source solutions as a function of theirlocation on-sky. We see that binary systems selected as sources with a ruwehigher than this sky-varying threshold have a strong detectability window intheir orbital period distribution, which peaks at periods equal to the Gaiaobservation time baseline. We demonstrate how our sky-varying ruwe thresholdprovides a more complete sample of binary systems when compared to singlesky-averaged values by studying the unresolved binary population in the GaiaCatalogue of Nearby Stars. We provide the code and tools used in this study, aswell as the sky-varying ruwe threshold through the GaiaUnlimited Python package