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

Abstract

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∼ 18.5 and covers a circular ∼11 deg2 area. The accuracy of CtlgM proper motions, at about 0.3-0.5masyr-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 ∼165 cluster members among the stars in our sample, while 314 stars with σμ < 2.5masyr-1 have membership probabilities Pμ≥ 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 11598 objects down to V∼ 21. A total of 4273 objects are common between the two catalogues. The accuracy of proper motions in CtlgD ranges from 1.0 to 6masyr-1. A combination of both proper-motion catalogues was instrumental in confirming that Blanco 1 contains a large population of M dwarfs (∼150 down to M5V - 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. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.