The fate of wide binaries in the solar neighborhood: Encounters with stars and molecular clouds

Abstract

The time evolution of wide binaries by stochastic encounters with field stars and molecular clouds in the solar neigborhood is computed with a hybrid Monte Carlo scheme. Specifically, a Fokker-Planck analysis is used for distant encounters and the effects of close encounters are treated encounter by encounter. This approach has a considerable advantage in speed over direct integration methods, enabling the evolution to be computed for a wide range of initial semimajor axes, a 0, and environmental parameters. We present the probability that a binary has achieved a new semimajor axis, P [a; a 0, τ], for several values of a 0 and for several characteristic times such as τ = 2.5 × 10 8, 3.0 × 10 9, and 4.5 × 10 9 years. We also present the probability of survival as a function of time for a 0 between 0.01 and 1 pc. The former indicates how an initial distribution of binaries may be expected to evolve and the latter provides a semimajor axis cutoff which may be compared with observations. In order to use the observations together with the Monte Carlo results as a diagnostic probe, we investigate the sensitivity of these simulations to assumptions about the solar neighborhood. As an example, we consider the lifetime and stability of the proposed solar companion. Approximate analytic scaling of all key quantities with assumed environmental parameters is derived and compared with the simulations. The hybrid simulations reveal two physical effects not taken into account by the scaling relations.