Binaries in the field: fossils of the star formation process?

Abstract

Recent observations of binary stars in the Galactic field show that the binary fraction and the mean orbital separation both decrease as a function of decreasing primary mass. We present $N$-body simulations of the effects of dynamical evolution in star-forming regions on primordial binary stars to determine whether these observed trends can be explained by the dynamical processing of a common binary population. We find that dynamical processing of a binary population with an initial binary fraction of unity and an initial excess of intermediate/wide separation (100 - 10${}^4$ au) binaries does not reproduce the observed properties in the field, even in initially dense ($\sim 10^3$M${}_{\odot }$ pc${}^{-3}$) star-forming regions. If instead we adopt a field-like population as the initial conditions, most brown dwarf and M-dwarf binaries are dynamically hard and their overall fractions and separation distributions are unaffected by dynamical evolution. G-dwarf and A-star binaries in the field are dynamically intermediate in our simulated dense regions and dynamical processing does destroy some systems with separations $>$100 au. However, the formation of wide binaries through the dissolution of supervirial regions is a strong function of primary mass, and the wide G-dwarf and A-star binaries that are destroyed by dynamical evolution in subvirial regions are replenished by the formation of binaries in supervirial regions. We therefore suggest that the binary population in the field is $indicative$ of the primordial binary population in star-forming regions, at least for systems with primary masses in the range 0.02 - 3.0M${}_{\odot }$.

17 pages, 11 figures, accepted for publication in MNRAS

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