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

Yonadav Barry Ginat, Hagai B. Perets

Research output: Contribution to journalArticlepeer-review


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 active galactic nucleus discs, that are affected by the central massive black hole, or binary planetesimals in protoplanetary 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.

Original languageEnglish
Pages (from-to)190-194
Number of pages5
Issue number1
StatePublished - 1 Nov 2021


  • Binaries: close
  • Galaxy: centre
  • Galaxy: disc
  • Protoplanetary discs

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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