Dynamics of wide binary stars: A case study for testing Newtonian dynamics in the low acceleration regime

Extremely wide binary stars represent ideal systems to probe Newtonian dynamics in the low acceleration regimes (< 10-10ms-2) typical of the external regions of galaxies. Here, we present a study of 60 alleged wide binary stars with projected separation ranging from 0.004pc to 1pc, probing gravitational accelerations well below the limit where dark matter or modified dynamics theories set in. Radial velocities with accuracy 100m/s were obtained for each star, in order to constrain their orbital velocity, that, together with proper motion data, can distinguish bound from unbound systems. It was found that about half of the observed pairs do have velocity in the expected range for bound systems, out of the largest separations probed here. In particular, we identified five pairs with projected separation > 0.15pc that are useful for the proposed test. While it would be premature to draw any conclusion about the validity of Newtonian dynamics at these low accelerations, our main result is that very wide binary stars seem to exist in the harsh environment of the solar neighborhood. This could provide a tool to test Newtonian dynamics versus modified dynamics theories in the low acceleration conditions typical of galaxies. In the near future the GAIA satellite will provide data to increase significantly the number of wide pairs that, with the appropriate follow-up spectroscopic observations, will allow the implementation of this experiment with unprecedented accuracy.