A new line on the wide binary test of gravity

Abstract

The relative velocity distribution of wide binary (WB) stars is sensitive to the law of gravity at the low accelerations typical of galactic outskirts. I consider the feasibility of this wide binary test using the 'line velocity' method. This involves considering only the velocity components along the direction within the sky plane orthogonal to the systemic proper motion of each WB.I apply this technique to the WB sample of Hernandez et. al. (2019), carefully accounting for large-angle effects at one order beyond leading. Based on Monte Carlo trials, the uncertainty in the one-dimensional velocity dispersion is ≈ 100 m/s when using sky-projected relative velocities. Using line velocities reduces this to ≈ 30 m/s because these are much less affected by distance uncertainties.My analysis does not support the Hernandez et. al. (2019) claim of a clear departure from Newtonian dynamics beyond a radius of ≈ 10 kAU, partly because I use 2σ outlier rejection to clean their sample first. Nonetheless, the uncertainties are small enough that existing WB data are nearly sufficient to distinguish Newtonian dynamics from Modified Newtonian Dynamics. I estimate that ≈ 1000 WB systems will be required for this purpose if using only line velocities.In addition to a larger sample, it will also be important to control for systematics like undetected companions and moving groups. This could be done statistically. The contamination can be minimized by considering a narrow theoretically motivated range of parameters and focusing on how different theories predict different proportions of WBs in this region.

Probing the radial acceleration relation and the strong equivalence principle with the Coma cluster ultra-diffuse galaxies

“…Its influence can be important for the stability and secular evolution of galaxies even when it is weak (Banik et al 2020), and it can create interesting features such as asymmetric tidal tails of globular clusters (Thomas et al 2018). The EFE is an observational necessity to allow, e.g., the dynamics of wide binary stars to remain consistent with MOND (Pittordis & Sutherland 2019, Banik 2019, although see also Hernandez et al 2021). Because of this EFE, a rotationallysupported (pressure-supported) system in isolation is expected to have a higher rotational velocity (velocity dispersion) than the same system around a massive host (e.g.…”

Detailed numerical implementation of the wide binary test

“…Indeed, RVs are often not available at all, corresponding to an uncertainty equal to the velocity dispersion in the Solar neighbourhood (Appendix E). Where known at the few km/s level or better, they can be used to correct for perspective effects, thereby improving the accuracy of the v sky determination (Banik 2019). In what follows, we will not consider the 3D v, so any instances of v are intended as a shorthand for v sky .…”

Modified Newtonian Gravity, Wide Binaries and the Tully-Fisher Relation

“…For this reason, Banik and Zhao [80] recommended to only use data out to a separation of 20 kAU-0.1 pc-and, certainly, data for separations larger than 1 pc would be contaminated by many false binaries. A more careful analysis including outlier rejection has been carried out by Banik [83]. In this study, the author concludes that the widest sky-projected separation bins should not be used.…”