Dynamical friction on binary stars in dark matter dominated environments

TL;DR

This paper investigates how dynamical friction affects binary stars in dark matter-rich environments, deriving formulas for orbital evolution and suggesting binaries could help probe dark matter in faint galaxies.

Contribution

It provides new formulas and timescales for binary orbital evolution under dynamical friction in dark matter environments, considering the center-of-mass motion.

Findings

Significant eccentricity oscillations in some binaries.

Gradual decrease of semi-major axes due to dynamical friction.

Potential bias in velocity dispersion measurements from unresolved binaries.

Abstract

We study binary stars moving through a uniform dark matter background and experiencing dynamical friction. The centre-of-mass motion of the pairs is taken into account. We derive formulas and timescales for the secular evolution of the orbital parameters for both wide and close binaries. We apply these results to environments typical of dark matter dominated ultra-faint dwarf galaxies and show that some binaries undergo significant eccentricity oscillations, while their semi-major axes decrease more gradually. We consider a simple binary star population and find that dynamical friction, notably, can enhance the bias from unresolved binaries in velocity dispersion measurements. With future, more detailed theoretical studies and improving observational capabilities, binary stars may serve as a tool to probe the dark matter content of some of the faintest galaxies.