The dynamical properties of stellar systems in the Galactic disc

MNRAS, in press 1 Abstract. We postulate that stars in the Galactic field are born in aggregates of binary stars with half mass radii R0.5 and number of binaries Nbin which are dynamically equivalent to the dominant mode cluster (Nbin, R0.5) ≈ (200, 0.8 pc). Binary orbits are distributed according to an initial period distribution which is consistent with pre-main sequence data. Stellar masses are paired at random from the KTG(1.3) mass function. We develop a simple model for the redistribution of orbital angular momentum and energy in short-period proto-binary systems (pre-main sequence eigenevolution), which establishes the observed correlations between eccentricity, mass ratio and period. The evolution of orbital parameters owing to perturbations by neighbouring systems (stimulated evolution) within the dominant mode cluster places 1-2 per cent of all orbits into the eigenevolution region (P 100 days, e 0.1 approximately) of the eccentricity-period diagram. The number of such forbidden orbits at any time is a function of the stellar number density, the dynamical and the nuclear age of the cluster. Observations of binaries in clusters should reveal the odd binary with forbidden orbital parameters. Examples of such systems may be the pre-main sequence binaries P2486 and EK Cep and binaries in stellar clusters with eccentric orbits at periods smaller than the circularisation cutoff period. Eigenevolution is