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.

Automated summary

The identification of binary stars in resolved stellar populations is challenging, but it can be estimated effectively using the photometric diagram constructed with pseudocolors. In the Pleiades star cluster, the binary star ratio is between 0.54 and 0.70, while the ratio of systems with a multiplicity of more than two is between 0.10 and 0.14. The distribution of the component mass ratio follows a power law, and there are expected to be a large number of brown dwarfs among secondary components with mass below 0.5 times solar mass.