Dynamical population synthesis: constructing the stellar single and binary contents of galactic field populations

The galactic field's late-type stellar single and binary populations are calculated on the observationally well-constrained supposition that all stars form as binaries with invariant properties in discrete star formation events. A recently developed tool (Marks, Kroupa & Oh) is used to evolve the binary star distributions in star clusters for a few million years until an equilibrium situation is achieved which has a particular mixture of single and binary stars. On cluster dissolution the population enters the galactic field with these characteristics. The different contributions of single stars and binaries from individual star clusters, which are selected from a power-law-embedded star cluster mass function, are then added up. This gives rise to integrated galactic field binary distribution functions (IGBDFs), resembling a galactic field's stellar content (dynamical population synthesis). It is found that the binary proportion in the galactic field of a galaxy is larger the lower the minimum cluster mass, M_{ecl, min}, the lower the star formation rate, SFR, the steeper the embedded star cluster mass function (described by index β) and the larger the typical size of forming star clusters in the considered galaxy. In particular, period, mass ratio and eccentricity IGBDFs for the Milky Way (MW) are modelled using M_{ecl, min}= 5 M_⊙, SFR = 3 M_⊙ yr^-1 and β= 2 which are justified by observations. For r_h≈ 0.1-0.3 pc, the half-mass radius of an embedded cluster, the aforementioned theoretical IGBDFs agree with independently observed distributions, suggesting that the individual discrete star formation events in the MW generally formed compact star clusters. Of all late-type binaries, 50 per cent stem from M_ecl≲ 300 M_⊙ clusters, while 50 per cent of all single stars were born in M_ecl≳ 10⁴ M_⊙ clusters. Comparison of the G-dwarf and M-dwarf binary populations indicates that the stars are formed in mass-segregated clusters. In particular, it is pointed out that although in the present model all M-dwarfs are born in binary systems, in the MW's Galactic field the majority ends up being single stars. This work predicts that today's binary frequency in elliptical galaxies is lower than that in spiral and dwarf galaxies. The period and mass-ratio distributions in these galaxies are explicitly predicted.