The dynamical properties of stellar systems in the Galactic disc

Abstract

We postulate that most stars are born in aggregates of binary systems which are dynamically equivalent to the `dominant mode cluster'. The initial binary orbits are consitent with pre-main sequence data. Stellar masses are paired at random from the KTG(1.3) mass function. We develop a simple model which establishes the observed correlations between eccentricity, mass ratio and period. Stimulated evolution of orbital parameters within the cluster places a few per cent of all orbits into the eigenevolution region of the eccentricity- period diagram. Observations should reveal the odd binary with eccentric orbits at periods smaller than the circularisation cutoff period. We show that the birth eccentricity distribution must be approximately dynamically relaxed because stimulated evolution in the dominant mode cluster cannot sufficiently thermalise a significantly different distribution. After disintegration of the dominant mode cluster we have a proportion of binaries which compares favourably with the observed proportion. The shape of the period distribution agrees well with the observed distribution. The mass-ratio distribution of G dwarf binaries is depleted at small mass ratios and has the shape of the main sequence distribution. We predict and tabulate the mass ratio distribution for main sequence binaries with a primary star less massive than $1.1\,M_\odot$. Our model Galactic field stellar population has a binary proportion among G-, K- and M-dwarfs in good agreement with the observational data. Too few triple and quadruple systems form by capture to account for the number of observed systems. We compare the specific angular momentum distribution of our initial binary star population with the observed distribution of specific angular momenta of molecular cloud cores.