Proper motion measurements as indicators of binarity in open clusters

Abstract

We analyze 9 open clusters with ages in the range 70 Myr to 3.2 Gyr using UCAC2 proper motion data and 2MASS photometry, which allows us to reach stellar masses down to ≈0.7 M⊙. We employ in this work an approach in which the background proper motion contribution is statistically subtracted in order to obtain the cluster's intrinsic proper motion distribution. For each cluster we consider the projected velocity distributions in the core and off-core regions separately. In the projected velocity distribution of all sample clusters we find a well-defined low-velocity peak, as well as an excess in the number of stars at larger velocities. The low-velocity peak is accounted for by the random motion of the single stars, while the high-velocity excess can be attributed to the large velocity changes produced by a significant fraction of unresolved binaries in a cluster. We derive kinematic parameters of the single-star distribution, in particular the projected velocity dispersion. The relatively large velocity dispersions derived in this work may reflect the non-virialized state of the clusters. Based on the relative number of high-velocity (binary) and single stars, we inferred for the sample clusters unresolved binary fractions in the range 15% ≤ fbin ≤ 54%, for both core and off-core regions. Stars with a projected velocity exceeding the maximum reached by the single-star distribution are identified in 2MASS J × (J - H) colour-magnitude diagrams. The asymmetry observed in the distribution of these stars around the main sequence is consistent with models of main-sequence widening resulting from unresolved binaries combined with 2MASS photometric uncertainties. The present results suggest that care must be taken when applying proper-motion filters to sort out members, especially binaries in a star cluster. This paper shows that proper motions turn out to be a useful tool for identifying high-velocity stars as unresolved binary cluster members, and as a consequence, map and quantify the binary component in colour-magnitude diagrams.