Internal kinematics of GAIA eDR3 wide binaries

Abstract: Using the recent GAIA eDR3 catalogue we construct a sample of solar neighbourhood isolated wide binaries satisfying a series of strict signal-to-noise data cuts, exclusion of random association criteria and detailed colour-magnitude diagram selections, to minimise the presence of any kinematic contaminating effects having been discussed in the literature to date. Our final high-purity sample consists of 423 binary pairs within 130 pc of the sun and in all cases high-quality GAIA single-stellar fits for both components of each binary (final average RUWE values of 0.99), both also restricted to the cleanest region of the main sequence. We find kinematics fully consistent with Newtonian expectations for separations, $s$, below 0.009 pc, with relative velocities scaling with $\Delta V \propto s^{-1/2}$ and a total binary mass, $M_{b}$, velocity scaling consistent with $\Delta V \propto M_{b}^{1/2}$. For the separation region of $s> 0.009$ pc we obtain significantly different results, with a separation independent $\Delta V \approx 0.5$ km/s and a $\Delta V \propto M_{b}^{0.24 \pm 0.21}$. This situation is reminiscent of the low acceleration galactic baryonic Tully-Fisher phenomenology, and indeed, the change from the two regimes we find closely corresponds to the $a \lesssim a_{0}$ transition. These results are at odds not only with Newtonian expectations, but also with MOND predictions, where the presence of an external field effect implies only small deviations from Newtonian dynamics are expected for Solar Neighbourhood wide binaries.

• The paper demonstrates that wide binaries follow Newtonian dynamics below 0.009 pc but show unexpected velocity behavior beyond this limit.
• Methodology included rigorous selection from the GAIA eDR3 catalog using strict parallax, RUWE, and velocity criteria to ensure a pure sample of 423 binary pairs.
• Results suggest deviations from standard predictions at low accelerations, indicating possible modified gravitational dynamics that merit further investigation.

Overview of the Paper "Internal Kinematics of GAIA eDR3 Wide Binaries"

The paper discusses the internal kinematics of wide binary stars in the solar neighborhood using the GAIA eDR3 catalog. The authors, Hernandez, Cookson, and Cortes, construct a carefully curated sample of 423 wide binary pairs within 130 parsecs (pc) of the Sun. These pairs were selected based on strict criteria to reduce kinematic contamination from unresolved tertiary components, random associations, and to ensure high-quality parallax and proper motion data.

Methodology and Sample Selection

The authors employed a series of rigorous selection criteria to isolate a high-purity sample of wide binaries:

1. Initial Sample Construction: The GAIA eDR3 catalog is utilized to identify wide binary candidates within 200 pc, focusing on systems with high-quality parallax measurements and signal-to-noise ratios.
2. Exclusion of Random Associations: A critical step involved removing binaries where either star appeared in multiple pairs, ensuring an isolation sphere of at least 0.5 pc around the binaries.
3. Kinship-Based Pruning: Relative radial velocity cuts (< 4 km/s) were employed to exclude unbound, transient stellar pairs likely drawn from the local field population.
4. Further Data Refinement: Strict RUWE (< 1.2) and color-magnitude diagram cuts were implemented to minimize contamination from unresolved stellar companions, leading to an exceptionally clean sample span including only the most robust data points.

Key Findings

The analysis yielded kinematic results consistent with Newtonian dynamics for binary separations below 0.009 pc, where relative velocities scaled with separation as expected from Newtonian physics ($\Delta V \propto s^{-1/2}$) and total binary mass. However, a significant deviation from this trend was observed at larger separations (s > 0.009 pc), where the velocities appear independent of separation ($\Delta V \approx 0.5$ km/s) and scale with mass as $\Delta V \propto M_b^{0.24 \pm 0.21}$. This transition closely aligns with the acceleration threshold $a \lesssim a_0$, reminiscent of the low acceleration MOND regime and the galactic baryonic Tully-Fisher relation, yet the data do not fully conform to MOND predictions due to external field effects.

Implications and Future Directions

The paper's findings challenge traditional Newtonian expectations and pose difficulties for standard MOND interpretations due to the external field effect, suggesting a potentially novel dynamical behavior at stellar mass scales. The observed mass-velocity scaling in the low acceleration regime could imply modifications in gravity that differ fundamentally from current theories, demanding further observational and theoretical studies.

Future investigations leveraging enhanced datasets from forthcoming GAIA releases, with increased astrometric precision and extended baseline, are essential. Such datasets could confirm the current results, refine the underlying phenomena, and potentially distinguish between competing hypotheses regarding the kinematics of wide binaries in low acceleration environments.

This study highlights the utility of wide binaries as a natural laboratory for testing gravitational models at small-scale systems, extending our understanding of dynamics beyond the traditional realms explored at galactic scales.