A possible solution to the Milky Way’s binary-deficient retrograde stellar population

Evidence that ω Centauri has formed in an extreme starburst

¹ Erzbischöfliches Clara-Fey-Gymnasium Bonn-Bad Godesberg, Rheinallee 5, 53175 Bonn, Germany
e-mail: astro.michi@yahoo.com
² Helmholtz-Institut für Strahlen- und Kernphysik, University of Bonn, Nussallee 14-16, 53115 Bonn, Germany
e-mail: pavel@astro.uni-bonn.de
³ Charles University in Prague, Faculty of Mathematics and Physics, Astronomical Institute, V Holešovičkách 2, 18000 Praha, Czech Republic
e-mail: joerg@sirrah.troja.mff.cuni.cz

Received: 22 July 2021
Accepted: 3 November 2021

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 ω 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 ω 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 ω Cen must have formed with a stellar density of ≈10⁸ M_⊙ pc⁻³ or the ω Centauri dwarf galaxy’s progenitor star cluster population must have formed in an extreme starburst with a star formation rate exceeding 1000 M_⊙ 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 ω Centauri as well as orbital parameter distributions for the Milky Way’s retrograde binary population are predicted. The dwarf galaxy origin for ω Centauri is tentatively preferred within the present context.