Unresolved Binaries and Multiples in the Intermediate Mass Range in Open Clusters: Pleiades, Alpha Per, Praesepe, and NGC 1039

In this study, we continue our project to search for unresolved binary and multiple systems in open clusters exploiting the photometric diagram (H-W2)-W1 versus W2-(BP-K) first introduced in Malofeeva et al. In particular, here we estimate the binary and multiple star ratios and the distribution of the component mass ratio q in the Galactic clusters Alpha Persei, Praesepe, and NGC 1039. We have modified the procedure outlined in our first study making star counts automatic and by introducing bootstrapping for error estimation. Basing on this, we reinvestigated the Pleiades star cluster in the same mass range as in our previous work and corrected an inaccuracy in the mass ratio q distribution. The binary and multiple star ratio in the four clusters is then found to lie between 0.45 ± 0.03 and 0.73 ± 0.03. On the other hand, the ratio of systems with multiplicity more than 2 is between 0.06 ± 0.01 and 0.09 ± 0.02. The distribution of the component mass ratio q is well fitted with a Gaussian having the mode between 0.22 ± 0.04 and 0.52 ± 0.01 and the dispersion between 0.10 ± 0.02 and 0.35 ± 0.07. All clusters show a large number of the very low-mass secondary components in the binary systems with primary components below 0.5 M ⊙.