Asymmetrical tidal tails of open star clusters: stars crossing their cluster's prah challenge Newtonian gravitation

TL;DR

This study compares Newtonian and Milgromian dynamics in explaining asymmetrical tidal tails of open star clusters, finding Milgromian physics better matches observed asymmetries and predicts rapid cluster demise due to increasing orbital eccentricity.

Contribution

It demonstrates that Milgromian dynamics can explain observed tidal tail asymmetries in open clusters, unlike Newtonian gravity, and introduces a method to estimate cluster orbital eccentricity.

Findings

Milgromian dynamics reproduces observed tail asymmetries.

Newtonian gravity predicts near-symmetrical tails.

Clusters on circular orbits develop eccentricity and spin-up opposite to orbital angular momentum.

Abstract

After their birth a significant fraction of all stars pass through the tidal threshold (prah) of their cluster of origin into the classical tidal tails. The asymmetry between the number of stars in the leading and trailing tails tests gravitational theory. All five open clusters with tail data (Hyades, Praesepe, Coma Berenices, COIN-Gaia 13, NGC 752) have visibly more stars within dcl = 50 pc of their centre in their leading than their trailing tail. Using the Jerabkova-compact-convergent-point (CCP) method, the extended tails have been mapped out for four nearby 600-2000 Myr old open clusters to dcl>50 pc. These are on near-circular Galactocentric orbits, a formula for estimating the orbital eccentricity of an open cluster being derived. Applying the Phantom of Ramses code to this problem, in Newtonian gravitation the tails are near-symmetrical. In Milgromian dynamics (MOND) the asymmetry reaches the observed values for 50 < dcl/pc < 200, being maximal near peri-galacticon, and can slightly invert near apo-galacticon, and the K\"upper epicyclic overdensities are asymmetrically spaced. Clusters on circular orbits develop orbital eccentricity due to the asymmetrical spill-out, therewith spinning up opposite to their orbital angular momentum. This positive dynamical feedback suggests Milgromian open clusters to demise rapidly as their orbital eccentricity keeps increasing. Future work is necessary to better delineate the tidal tails around open clusters of different ages and to develop a Milgromian direct n-body code.