Born to be wide: the distribution of wide binaries in the field and soft binaries in clusters

Abstract: Most stars, binaries, and higher multiplicity systems are thought to form in stellar clusters and associations, which later dissociate. Very wide binaries can be easily disrupted in clusters due to dynamical evaporation (soft binaries) and/or due to tidal disruption by the gravitational potential of the cluster. Nevertheless, wide binaries are quite frequent in the field, where they can sometimes play a key role in the formation of compact binaries, and serve as tools to study key physical processes. Here we use analytic tools to study the dynamical formation of soft binaries in clusters, and their survival as field binaries following cluster dispersion. We derive the expected properties of very wide binaries both in clusters and in the field. We analytically derive their detailed distributions, including wide-binary fraction as a function of mass in different cluster environments, binaries mass functions and mass ratios, and the distribution of their orbital properties. We show that our calculations agree well on most aspects with the results of N-body simulations, but show some different binary-fraction dependence on the cluster mass. We find that the overall fraction of wide binaries scales as $\propto N_\star^{-1}$ where $N_\star$ is the size of the cluster, even for non-equal mass stars. More massive stars are more likely to capture wide companions, with most stars above five solar mass likely to capture at least one stellar companion, and triples formation is found to be frequent.