Destruction of Wide Binaries by the Milky Way

Abstract

Wide binary stars (those with separations > 0.1 pc) have low binding energies and are therefore very sensitive to perturbations from encounters with stars, molecular gas, and even dark matter. Given their sensitivity, wide binaries are valuable tools to help constrain the distribution and nature of dark matter in our galaxy - especially in the galactic halo. We use the galactic dynamics software Gala to model a smooth Milky Way potential both with and without a galactic bar. We then use the tidal fields from Gala to model wide binary evolution with the N-body integrator, REBOUND. We were especially interested in establishing the impact of binary eccentricity (e), binary inclination (theta), binary initial separation (d_0), galactic radius (r), galactic orbital velocity (v_t and v_z), and disk crossing time (a.k.a. z oscillation time, T_z), on binary disruption. At d_{final} = 3 pc, we declare binaries gravitationally unbound, or "disrupted", as a result of perturbations. Of special interest was establishing the d_0 for which 50% of binaries disrupt (d_{LD50}). While our small sample size of 2,880 orbits (half with and half without the bar) covered enough initial conditions to give us an idea of important trends and correlations, it was not enough to develop a robust d_{LD50} relationship. We found, though, that a larger fraction, dn, of binaries were disrupted with increasing e and theta. Also, when the binary period equals T_z, the binary is in the last third to 1% of its life. We also saw that T_z is shorter for lower r and lower v_z, that short T_z is correlated with higher dn, and that there is a larger dn for galactic orbits with lower minimum r. Inclusion of the galactic bar also causes a larger dn.