Gaia Data Release 1: Open cluster astrometry: performance, limitations, and future prospects

Context. The first Gaia Data Release contains the Tycho-Gaia AstrometricSolution (TGAS). This is a subset of about 2 million stars for which,besides the position and photometry, the proper motion and parallax arecalculated using Hipparcos and Tycho-2 positions in 1991.25 as priorinformation.Aims. We investigate the scientific potential and limitations of theTGAS component by means of the astrometric data for open clusters.Methods. Mean cluster parallax and proper motion values are derivedtaking into account the error correlations within the astrometricsolutions for individual stars, an estimate of the internal velocitydispersion in the cluster, and, where relevant, the effects of the depthof the cluster along the line of sight. Internal consistency of the TGASdata is assessed.Results. Values given for standard uncertainties are still inaccurateand may lead to unrealistic unit-weight standard deviations of leastsquares solutions for cluster parameters. Reconstructed mean clusterparallax and proper motion values are generally in very good agreementwith earlier Hipparcos-based determination, although the Gaia meanparallax for the Pleiades is a significant exception. We have no currentexplanation for that discrepancy. Most clusters are observed to extendto nearly 15 pc from the cluster centre, and it will be up to futureGaia releases to establish whether those potential cluster-member starsare still dynamically bound to the clusters.Conclusions. The Gaia DR1 provides the means to examine open clustersfar beyond their more easily visible cores, and can provide membershipassessments based on proper motions and parallaxes. A combined HRdiagram shows the same features as observed before using the Hipparcosdata, with clearly increased luminosities for older A and F dwarfs.