Mass ratios in wide binary stars

The mass ratio distribution N(q) in wide binary stars has been found by Abt to depend on the spectral type of the primary. The stars with an O-type primary are rather equal in mass in comparison with binaries that have solar-type primaries. We investigate whether this results from the theory of stellar dynamics if the binary stars are remnants of three-body encounters. In case of solar-type primaries, the theoretical distribution is close to N(q) proportional to q(-1), as is found also in a careful analysis of observations by Trimble. However, the distribution for the O-type binaries has no simple explanation in the three-body theory, and it is likely that the observed distribution is closely linked to the process of formation of binaries. We also explain the binary period distribution as a result of stellar dynamical processes in star cluster environments. The agreement with the theory enforces the view that low-mass wide binaries may not be primordial but result from random stellar encounters.