Inverse dynamical population synthesis and star formation

MNRAS, in press 1 Abstract. Recent observations of pre-main sequence stars suggest that all stars may form in multiple systems. However, in the Galactic field only about 50 per cent of all systems are binary stars. We investigate the hypothesis that stars form in aggregates of binary systems and that the dynamical evolution of these aggregates leads to the observed properties of binary stars in the Galactic field. A thorough analysis of star count data implies that the initial stellar mass function rises monotonically with decreasing mass and that it can be approximated by three power law segments. Together with our assumption that the birth mass-ratio distribution is not correlated this leads to a contradiction with the distribution of secondary masses in Galactic field binaries with G-dwarf primaries which have too few low-mass companions. For the inverse dynamical population synthesis we assume that the initial distribution of periods is flat in log10P, where P is the orbital period in days, and 3 ≤log10P ≤ 7.5. This is consistent with pre-main sequence data. We distribute Nbin = 200 binaries in aggregates with half mass radii 0.077 ≤ R0.5 ≤ 2.53 pc, corresponding to the range from tightly clustered to isolated star formation, and follow the subsequent evolution of the stellar systems by direct N-body integration. We find that hardening and softening of binary systems do not significantly increase the