A possible solution to the Milky Way's binary-deficient retrograde stellar population. Evidence that $\omega$ Centauri has formed in an extreme starburst

TL;DR

This paper investigates the low binary fraction among retrograde stars in the Milky Way, proposing that extreme star formation conditions in $ Centauri's progenitor explain this deficiency and predicting binary properties for future observations.

Contribution

It introduces a model linking $ Centauri's origin to extreme starburst conditions, explaining the retrograde binary deficiency and predicting binary distributions.

Findings

Retrograde binary fraction can be explained by high-density star formation conditions.

Extreme starburst scenarios with high star formation rates are consistent with observed binary deficiencies.

Predicted binary separation and mass ratio distributions will aid future observational tests.

Abstract

Context. The fraction of field binaries on retrograde orbits about the Milky Way is significantly lower compared to its prograde counterpart. Chemical and dynamical evidence suggests that the retrograde stellar population originates from $\omega$ Centauri, which is either the most massive globular cluster (GC) of the Milky Way or the putative core of a former dwarf galaxy. Aims. Star formation conditions required to produce the retrograde binary population are constrained assuming that the retrograde stellar population originates from $\omega$ Centauri's progenitor. Methods. We match the observed low binary fraction with dynamical population synthesis models, including a universal initial binary population and dynamical processing in star clusters, making use of the publicly available binary population synthesis tool BiPoS1. Results. It is found that either the GC progenitor of $\omega$ Centauri must have formed with a stellar density of $\approx 10^8 \; M_{sun} \; pc^{-3}$ or the $\omega$ Centauri dwarf galaxy's progenitor star cluster population must have formed in an extreme starburst with a star formation rate exceeding $1000 \; M_{sun} \; yr^{-1}$ and probably a top-heavy embedded-cluster mass function with suppressed low-mass cluster formation. The separation and mass ratio distribution for retrograde field binaries are predicted for comparison with future observations. Conclusions. A viable solution for the deficiency of binaries on retrograde orbits is presented, and star formation conditions for $\omega$ Centauri as well as orbital parameter distributions for the Milky Way's retrograde binary population are predicted. The dwarf galaxy origin for $\omega$ Centauri is tentatively preferred within the present context.