Unresolved Binaries in the Intermediate Mass Range in the Pleiades Star Cluster*

The identification of binary stars of different mass ratios in resolved stellar populations is a challenging task. We show how the photometric diagram constructed with the pseudocolors (H-W2)-W1 versus W2-(BP-K) can be employed to estimate the binary and multiple star ratios and the distribution of their component mass ratio q effectively. As an application, we investigate the Pleiades star cluster in the range of primary component mass between 0.5 and 1.8 M (circle dot). The binary star ratio is found to be between 0.54 +/- 0.11 and 0.70 +/- 0.14. On the other hand, the ratio of systems with a multiplicity of more than two is between 0.10 +/- 0.00 and 0.14 +/- 0.01. The distribution of the component mass ratio q has been approximated by a power law with the exponent between -0.53 +/- 0.10 and -0.63 +/- 0.22. Below 0.5 M (circle dot), we expect a large number of brown dwarfs among secondary components.