Proper motions from Schmidt plates – II. The Hyades

Abstract

We have used COSMOS scans of photographic plates taken by the Palomar Oschin Schmidt telescope (as part of the original Palomar Sky Survey) and by the UK Schmidt telescope to obtain proper motions for ∼ 450 000 stars within a 112-degree2 region covering part of the Hyades cluster. With epoch differences of 33 to 37 yr, we have obtained proper motions accurate to 6–12 milliarcsec yr−1 and have identified 393 candidate Hyads to a limiting magnitude of |$M_v \sim + 15.5$|⁠, including at least two new white dwarf candidates. The main-sequence luminosity function determined from our sample is similar to that defined by local field stars, with a broad maximum at |$M_v \sim + 12$|⁠. Deriving distances from µu, the proper motion towards the convergent point, and the space velocity determined by Gunn et al., we find stars with motions consistent with membership at distances of more than 15 pc from the cluster centre. Moreover, both the line-of-sight and surface density distributions show evidence for significant mass segregation – indeed, the overall proper motion distribution suggests a tight core centred within a much broader distribution. We derive a total mass of 410–480 M⊙ and a gravitational binding radius of ∼ 10.5 pc.