INVERSE DYNAMICAL POPULATION SYNTHESIS AND STAR-FORMATION

Recent observations of pre-main-sequence stars suggest that all stars may form in multiple systems. However, in the Galactic field only abou t 50 per cent of all systems are binary stars. We investigate the hypo thesis that stars form in aggregates of binary systems and that the dy namical evolution of these aggregates leads to the observed properties of binary stars in the Galactic field. A thorough analysis of star co unt data implies that the initial stellar mass function rises monotoni cally with decreasing mass and that it can be approximated by three po wer-law segments. Together with our assumption that the birth mass-rat io 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 inver se dynamical population synthesis we assume that the initial distribut ion of periods is flat in log(10)P, where P is the orbital period in d ays, and 3 less than or equal to log(10)P less than or equal to 17.5. This is consistent with pre-main-sequence data. We distribute N-bin = 200 binaries in aggregates with half-mass radii 0.077 less than or equ al to R(0.5)less than or equal to 2.53 pc, corresponding to the range from tightly clustered to isolated star formation, and follow the subs equent evolution of the stellar systems by direct N-body integration. We find that hardening and softening of binary systems do not signific antly increase the numbers of orbits with log(10)P<3 and log(10)P>7.5, respectively. After the cluster with R(0.5) approximate to 0.8 pc dis integrates we obtain a population which consists of about 60 per cent binary systems with a period distribution for log(10)P>4, as is observ ed, and in which the G dwarf binaries have a mass-ratio distribution w hich agrees with the observed distribution. This result indicates that the majority of Galactic field stars may originate from a clustered s tar-formation mode, characterized by the dominant-mode cluster which h as initially (N-bin, R(0.5)) approximate to (200, 0.8 pc). We invert t he orbit depletion function and obtain an approximation to the initial binary star period distribution for star formation in the dominant-mo de cluster. Comparison with the measured distribution of orbits for pr e-main-sequence stars formed in the distributed mode of star formation suggests that the initial distribution of binary star orbits may not depend on the star-formation environment. If a different stellar mass function to the one we adopted is assumed then inverse dynamical popul ation synthesis cannot solve for an aggregate in which the initial bin ary star population evolves to the observed population in the Galactic field. This implies that the Galactic field stellar mass function may be related to the stellar density at birth in the most common, or dom inant, mode of star formation.