Journal article

A deep proper-motion survey of the nearby open cluster Blanco 1

We provide two comprehensive catalogues of positions and proper motions in the area of open cluster Blanco 1. The main catalogue, CtlgM, contains 6271 objects down to V similar to 18.5 and covers a circular similar to 11 deg2 area. The accuracy of CtlgM proper motions, at about 0.3-0.5 mas yr-1 for well-measured stars, permits an excellent segregation between the cluster and field stars. The vector-point diagram of proper motions indicates an estimated total of similar to 165 cluster members among the stars in our sample, while 314 stars with Sigma(mu) < 2.5 mas yr-1 have membership probabilities P-mu >= 1 per cent. We also explored the astrometric potential of the Catalogue of Objects and Measured Parameters from All Sky Surveys (COMPASS) data base in order to obtain additional proper motions for fainter stars in the area of Blanco 1. This effort produced the second catalogue of proper motions, CtlgD, containing 11 598 objects down to V similar to 21. A total of 4273 objects are common between the two catalogues. The accuracy of proper motions in CtlgD ranges from 1.0 to 6 mas yr-1. A combination of both proper-motion catalogues was instrumental in confirming that Blanco 1 contains a large population of M dwarfs (similar to 150 down to M5 V - the limit of our survey). In many respects, Blanco 1 is a scaled down 'twin' of the Pleiades. The noted discrepancy between the distance from a new Hipparcos parallax of Blanco 1 and the cluster's photometric distance, at least partially, might be due to the apparent correlation between parallax and proper motion in right ascension for the ensemble of cluster members.


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