Binaries are softer than they seem: Effects of an external potential on the scattering dynamics of binaries

TL;DR

This paper revises Heggie's law by incorporating external potentials, revealing that in certain environments, binary systems can soften instead of harden due to finite Hill radii affecting scattering outcomes.

Contribution

It introduces a modified understanding of binary evolution in external potential environments, supported by analytical solutions and N-body simulation comparisons.

Findings

Binary hardening is slowed or reversed in environments with finite Hill radii.

The mean hardening rate depends non-trivially on the environment's Hill radius.

Analytical models accurately reproduce N-body simulation results.

Abstract

Binary evolution is influenced by dynamical scattering with other stars in dense environments. Heggie's law states that, due to their environments, hard binaries (whose orbital energy surpasses the typical energy of single stars) tend to harden (increase their orbital energy), while soft binaries typically soften. Here, we show that Heggie's law sometimes needs to be revised, by accounting for an external potential, for example, for binaries in nuclear stellar discs or AGN discs, that are affected by the central massive black hole, or binary planetesimals in proto-planetary discs, affected by the host star. We find that in such environments, where the Hill radius is finite, binary-single scattering can have different outcomes. In particular, a three-body encounter could be cut short due to stars being ejected beyond the Hill radius, thereby ceasing to participate in further close interactions. This leads to a systematic difference in the energy changes brought about by the encounter, and in particular slows binary hardening, and even causes some hard binaries to soften, on average, rather than harden. We use our previously derived analytical, statistical solution to the bound chaotic three-body problem to quantitatively characterise the revision of the hardening-softening phase transition and evolution of binaries. We also provide an analytical calculation of the mean hardening rate of binaries in any environment (also reproducing the results of detailed N-body simulations). We show that the latter exhibits a non-trivial dependence on the Hill radius induced by the environment.