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Wide binaries without viable bound Newtonian orbits

L. Pasquini, R. Saglia, F. Patat, L. Berni, D. Bossini, L. Magrini, H. Ludwig, M. T. Murphy, J. R. de Medeiros, J. Chanamé

TL;DR

This work tests Newtonian gravity in the low-acceleration regime using 26 wide binaries with projected separations exceeding 13 kAU, combining high-precision VLT-ESPRESSO radial velocities with Gaia proper motions and parallaxes. After removing unresolved multiples and non-members, 12 binaries remain, among which 9 yield bound Keplerian orbit solutions while 3 display total velocity differences incompatible with bound Newtonian orbits. The non-bound cases are argued to arise from dynamical processes such as stellar encounters and Galactic tides that disrupt wide binaries, rather than a failure of Newtonian gravity. The results urge caution when interpreting wide binaries as isolated gravity probes and point to future data from Gaia DR4 and ongoing ESPRESSO campaigns to further clarify the dynamics of these systems.

Abstract

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.

Wide binaries without viable bound Newtonian orbits

TL;DR

This work tests Newtonian gravity in the low-acceleration regime using 26 wide binaries with projected separations exceeding 13 kAU, combining high-precision VLT-ESPRESSO radial velocities with Gaia proper motions and parallaxes. After removing unresolved multiples and non-members, 12 binaries remain, among which 9 yield bound Keplerian orbit solutions while 3 display total velocity differences incompatible with bound Newtonian orbits. The non-bound cases are argued to arise from dynamical processes such as stellar encounters and Galactic tides that disrupt wide binaries, rather than a failure of Newtonian gravity. The results urge caution when interpreting wide binaries as isolated gravity probes and point to future data from Gaia DR4 and ongoing ESPRESSO campaigns to further clarify the dynamics of these systems.

Abstract

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.
Paper Structure (11 sections, 1 figure, 3 tables)

This paper contains 11 sections, 1 figure, 3 tables.

Table of Contents

  1. Introduction
  2. Sample and observations
  3. Methods
  4. Bound Newtonian solutions
  5. Discussion
  6. Contamination by unseen companions or giant planets:
  7. Common proper motion pairs from clusters and associations:
  8. Perturbed, unbound orbits:
  9. Conclusions
  10. Observed systems not suitable for our test
  11. Tables

Figures (1)

  • Figure 1: r/s (3D/projected separation) ratio vs distance $\chi ^2$ distribution for the four binaries with no Keplerian solutions: three from this study and one from the HARPS sample (WB24, shown in the top-left panel; Saglia+2025). For all stellar pairs, the most probable r/s are large, from 5 to 50. When multiplying these ratios by the projected separations ($s$), the 3D minimum separation of the three pairs analysed in this paper is $\sim$ 100 KAU.