I investigate the binarity properties of field stars using more than 50 000 main-sequence stars with stellar mass from 0.4 to 0.85 M observed by LAMOST and Gaia in the solar neighbourhood. By adopting a power-law shape for the mass-ratio distribution with a power index of γ, I conduct a hierarchical Bayesian model to derive the binary fraction (fb) and γ for stellar populations with different metallicities and primary masses (m1). I find that fb is tightly anticorrelated with γ; that is, the populations with a smaller binary fraction contain more binaries with a larger mass-ratio, and vice versa. The high-γ populations, with γ > 1.2, have a lower stellar mass and higher metallicity, while the low-γ populations, with γ < 1.2, have a higher mass and lower metallicity. The fb of the high-γ group is anticorrelated with [Fe/H] but flat with m1. However, the fb of the low-γ group displays a clear correlation with m1 but is quite flat with [Fe/H]. The substantial differences are probably due to the dynamical processing that occurred when the binaries were in the embedded star clusters in their early lives. Dynamical processing tends to destroy binaries with a smaller primary mass, smaller mass-ratio, and wider separation. Consequently, the high-γ group containing stars with a smaller m1 is influenced more effectively and hence contains fewer binaries, many of which have a larger mass-ratio and shorter period. However, the low-γ group is less affected by the dynamical processing owing to the larger m1. These results suggest that dynamical processing works effectively and has significantly altered the present-day binary properties of field stars.

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