Towards the field binary population: Influence of orbital decay on close binaries

Abstract

Surveys of the binary populations in the solar neighbourhood have shown that the periods of G- and M-type stars are log-normally distributed. However, observations of young binary populations suggest a log-uniform distribution. Clearly some process(es) change the period distribution over time. Most stars form in star clusters, in which two important dynamical processes occur: i) gas-induced orbital decay of embedded binary systems and ii) destruction of soft binaries in three-body interactions. The emphasis here is on orbital decay which has been largely neglected so far. Using a combination of Monte-Carlo and dynamical nbody modelling it is demonstrated here that the cluster dynamics destroys the number of wide binaries, but leaves short-period binaries basically undisturbed even for a initially log-uniform distribution. By contrast orbital decay significantly reduces the number and changes the properties of short-period binaries, but leaves wide binaries largely uneffected. Until now it was unclear whether the short period distribution of the field is unaltered since its formation. It is shown here, that orbital decay is a prime candidate for such a task. In combination the dynamics of these two processes, convert an initial log-uniform distribution to a log-normal period distribution. The probability is 94% that the evolved and observed period distribution were sampled from the same parent distribution. This means binaries can be formed with periods that are sampled from the log-uniform distribution. As the cluster evolves, short-period binaries are merged to single stars by the gas-induced orbital decay while the dynamical evolution in the cluster destroys wide binaries. The combination of these two equally important processes reshapes a initial log-uniform period distribution to the log-normal period distribution, that is observed in the field (abridged).

9 pages, 9 figures

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