Mass Ratios in Wide Binary Stars

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Abstract
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) ∝ 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.

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