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The dynamical properties of stellar systems in the Galactic disc

1995; Oxford University Press; Volume: 277; Issue: 4 Linguagem: Inglês

10.1093/mnras/277.4.1507

1365-2966

Pavel Kroupa,

Astro and Planetary Science

We postulate that stars in the Galactic field are born in aggregates of binary stars with half mass radii R 0.5 and number of binaries N bin which are dynamically equivalent to the dominant mode cluster (N bin , R 0.5 ) ≈ (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 expected to depopulate the eigenevolution region within 10 5 yrs for pre-main sequence binaries, but main sequence binaries with forbidden orbits should remain in the eigenevolution region for times of order 10 9 yrs.We show that the binary star population must have a birth eccentricity distribution which is 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 population of Galactic field systems with orbital parameters as observed, with a surviving proportion of binaries of f tot = 0.48 ± 0.03 which compares favourably with the observed proportion f obs tot = 0.47 ± 0.05.The rise of the period distribution to a maximum at log 10 P ≈ 4.8 reflects approximately the initial distribution, whereas the decay for log 10 P > 4.8 results from stimulated evolution.The mass-ratio distribution of G dwarf binaries is depleted at small mass ratios and has the shape of the main sequence distribution despite initially being the KTG(1.3)mass function.We predict and tabulate the mass ratio distribution for main sequence binaries with a primary star less massive than 1.1 M ⊙ .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.An example of a triple system which may have formed by capture in the birth aggregate may be Proxima Cen-α Cen A/B.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.According to our model about 40 per cent of all late-type stars are single after cluster dissolution but had companions with log 10 P ≥ 6 at birth.These stars are expected to have circumstellar disks (and possible planetary systems) extending to at least about 40 AU.