Eccentricity evolution in equal-mass binaries for power-law dynamical friction models

Abstract

The eccentricity of a binary is driven by radiative mechanisms such as gravitational wave emission and environmental effects like mass accretion and dynamical friction. In this work, we consider a subsonic dynamical friction model and generalize this to accommodate models with power-law dependence on the velocity. For equal-mass binaries, we find that dynamical friction in the subsonic case does not increase the eccentricity, which would have been necessary to compete against the eccentricity-decreasing effects of gravitational wave emission and produce an eccentricity peak. We also consider a ~v^−n model for dynamical friction which generalizes the subsonic case, and find that this does not monotonically evolve with decreasing semi-major axis, and includes eccentricity-increasing and eccentricity-decreasing regimes, for n > 0.