Astrometric Identification of Nearby Binary Stars I: Predicted\n Astrometric Signals

Abstract

We examine the capacity to identify binary systems from astrometric errors\nand deviations alone. Until the release of the fourth Gaia data release we lack\nthe full astrometric time series that the satellite records, but as we show can\nstill infer the presence of binaries from the best fit models, and their error,\nalready available. We generate a broad catalog of simulated binary systems\nwithin 100 pc, and examine synthetic observations matching the Gaia survey's\nscanning law and astrometric data processing routine. We show how the Unit\nWeight Error (UWE) and Proper Motion Anomaly (PMA) vary as a function of\nperiod, and the properties of the binary. Both UWE and PMA peak for systems\nwith a binary period close to the time baseline of the survey. Thus UWE can be\nexpected to increase or remain roughly constant as we observe the same system\nover a longer baseline, and we suggest $UWE_{eDR3}>1.25$ and $\\Delta\nUWE/UWE_{eDR3}>-0.25$ as criteria to select astrometric binaries. For stellar\nbinaries we find detectable significant astrometric deviations for 80-90\\% of\nour simulated systems in a period range from months to decades. We confirm that\nfor systems with periods less than the survey's baseline the observed $UWE$\nscales $\\propto \\ \\varpi$ (parallax), $a$ (semi-major axis) and $\\Delta\n=\\frac{|q-l|}{(1+q)(1+l)}$ where $q$ and $l$ are the mass and light ratio\nrespectively, with a modest dependence on viewing angle. For longer periods the\nsignal is suppressed by a factor of roughly $\\propto P^{-2}$ (period). PMA is\nlargest in orbits with slightly longer periods but obeys the same approximate\nscaling relationships.\n