Binaries in the field: fossils of the star formation process?
  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$_\odot$.
