Wide Binaries as a Modified Gravity test: prospects for detecting triple-system contamination

Abstract

Recent studies have shown that velocity differences of very wide binary stars, measured to high precision with GAIA, can provide an interesting test for modified-gravity theories which emulate dark matter; in essence, MOND-like theories (with external field effect included) predict that wide binaries (wider than ~ 7 kAU) should orbit ~ 15% faster than Newtonian for similar orbits; such a shift is readily detectable in principle in the sample of 9,000 candidate systems selected from GAIA EDR3 by Pittordis and Sutherland (2022; PS22). However, the main obstacle at present is the observed "fat tail" of candidate wide-binary systems with velocity differences at ~1.5 - 6x circular velocity; this tail cannot be bound pure binary systems, but a possible explanation of the tail is triple or quadruple systems with unresolved or undetected additional star(s). While this tail can be modelled and statistically subtracted, obtaining an accurate model for the triple population is crucial for a robust test for modified gravity. In this paper we explore prospects for observationally constraining the triple population: we simulate a population of hierarchical triple systems "observed" as in PS22 at random epochs and viewing angles; for each simulated system we evaluate various possible methods for detecting the third star, including GAIA astrometry, radial velocity drift, and several imaging methods from direct Rubin images, speckle imaging and coronagraphic imaging. Results are generally encouraging, in that typically 90% of the triple systems in the key regions of parameter space are detectable; there is a moderate "dead zone" of cool brown-dwarf companions at ~ 25-100 AU separation which are not detectable with any of our baseline methods. A large but feasible observing campaign can greatly clarify the effect of the triple/quadruple population and make the gravity test decisive.