Wide binaries without viable bound Newtonian orbits

Context. Wide binaries offer a unique opportunity to test gravity in the low acceleration regime, where deviations from Newtonian dynamics may appear. Aims. We use high-resolution VLT-ESPRESSO archival spectra to study 26 wide binaries with projected separations larger than 13,000 AU. By combining precise radial velocities with Gaia proper motions and parallaxes, we aim to test whether these systems are consistent with Newtonian gravity in the low acceleration regime. Methods. We use multiple radial velocity measurements and stellar parameters to remove systems affected by unresolved triples, chance alignments, or young systems. For the remaining binaries, we combine radial velocities (corrected for convective shift and gravitational redshift) with Gaia proper motions, parallaxes, and positions to attempt bound Newtonian orbital solutions Results. Fourteen of the 26 initial systems were discarded: 12 due to radial velocity variability indicating unresolved close binaries, one hosting a faint Gaia companion, and one too young. Of the remaining 12, nine can be fitted with bound orbital solution, while three show velocity differences too large to be reconciled with any bound Newtonian orbit. Conclusions. For the three systems that cannot be fitted with a bound orbit, repeated radial velocity observations allow us to confidently exclude, with one possible exception, unresolved triple stellar companions or massive close-in planets as causes. Given their likely large 3-dimensional separations, these binaries may have been dynamically perturbed or disrupted by stellar encounters or Galactic tides, and may no longer be gravitationally bound. This highlights how utmost caution must be applied when studying wide binaries as isolated systems.

💡 Research Summary

This paper investigates whether very wide binary stars (projected separations > 13 kAU) can be described by bound Newtonian (Keplerian) orbits, using high‑resolution VLT‑ESPRESSO spectra combined with Gaia DR3 astrometry. The authors observed 26 binary systems between December 2020 and March 2022, obtaining 3–10 radial‑velocity (RV) measurements per star with typical single‑measurement uncertainties of 3–9 m s⁻¹. After correcting the RVs for convective blueshift and gravitational redshift, they derived precise three‑dimensional velocities for each component.

A first screening removed 12 systems that showed significant RV variability (indicative of unresolved close companions), one system with a faint Gaia companion, and one very young pair (< 1 Gyr). The remaining 12 binaries constitute the final sample (46 % of the original). For each of these, the authors used the masses, parallaxes, proper motions, and RVs (including their uncertainties) to search for bound Keplerian solutions, following the methodology of Saglia et al. (2025).

Nine of the twelve binaries admit viable bound solutions. Some of these require very high eccentricities (e > 0.9) and a range of inclination angles, with a median three‑dimensional separation to projected separation ratio of 1.7 (the edge‑on pair reaches ≈ 5.4). The other three systems (pairs 7, 11, 12) have total velocity differences that exceed the theoretical maximum for a bound Newtonian orbit, ν = √(2GM/r), by a large margin; the uncertainties on the velocity differences are < 10 %, ruling out measurement error.

The authors explore alternative explanations. Unseen stellar companions are unlikely because the RVs are stable and the Gaia renormalised unit weight error (RUWE) values are low, except for one star (TYC 5346‑457‑1) with a modestly elevated RUWE, which could hint at a face‑on tertiary that does not affect RVs. Massive close‑in planets are also improbable: no known planet hosts match the sample, and the RV precision would have detected any Jupiter‑mass companion on short periods. Cross‑matching with known open clusters and moving groups finds no association, eliminating the possibility that the pairs are merely co‑moving members of a dissolving association.

Consequently, the most plausible cause for the non‑bound systems is dynamical perturbation over gigayear timescales. The paper cites theoretical work (Hamilton & Modak 2024; Jiang & Tremaine 2010) showing that binaries with semi‑major axes ≳ 10⁴ AU experience disruption timescales of a few gigayears due to stellar encounters and Galactic tidal forces, while those with a > 10⁵ AU are disrupted rapidly (< 1 Gyr). Simulations also indicate that disrupted binaries can linger within the Jacobi radius (≈ 170 kAU for a 2 M⊙ system) in quasi‑periodic relative motion for many gigayears, consistent with the separations of the observed non‑bound pairs.

The study concludes that a non‑negligible fraction of very wide binaries are not gravitationally bound, even though they appear as common‑proper‑motion pairs. This cautions against using such systems as isolated testbeds for modified gravity theories without first confirming their dynamical binding. Future work should combine longer‑baseline RV monitoring with detailed dynamical modeling to better discriminate bound from unbound wide pairs.