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 dis- tribution. 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 num- ber 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 rel- ative 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.