Jos de Bruijne

A refurbished convergent‐point method for finding moving groups in the Hipparcos Catalogue*

The Hipparcos data allow a major step forward in the research of ‘moving groups’ in the Solar neighbourhood, as the common motion of group members causes converging proper motions. Previous knowledge of these coherent structures in velocity space has always been limited by the availability, reliability and accuracy of ground-based proper-motion measurements. A refurbishment of Jones’ convergent point method is presented, which takes full advantage of the quality of the Hipparcos data. The original implementation of this method determines the maximum-likelihood convergent-point on a grid on the sky and simultaneously selects group members from a given set of stars with positions and proper motions. The refurbished procedure takes into account the full covariance matrix of the Hipparcos measurements instead of standard errors only, allows for internal motions of the stars, and replaces the grid-based approach by a direct minimization. The method is tested on Monte Carlo simulations of moving groups, and applied to the Hyades. Despite the limited amount of data used by the convergent-point method, the results for stars in and around the cluster-centre region agree very well with those of the recent comprehensive study by Perryman et al.

Gaia astrometric CCDs and focal plane

ESAs Gaia astrometry mission is due for launch in 2011. The astrometric instrument focal plane will have an area of up to 0.5m2 and will contain more than 100 CCDs. These will be operated in Time Delay and Integration mode in order to track and observe sources whilst the telescopes continuously scan the sky. Gaias target for astrometric precision of a few millionths of an arc second, places extreme demands on focal plane thermo--mechanical stability and electronics performance. The CCDs themselves are large area, back illuminated, full--frame, four phase devices. They require maximum efficiency for observing the majority of (faint) objects, yet must simultaneously be able to handle very bright objects that will regularly cross the field of view. Achieving the final astrometric precision will also require excellent noise performance and MTF. In addition to demanding excellent performance from each CCD, they will need to be produced in large numbers which raises production and yield issues. When analyzing Gaia data it will be essential to understand and calibrate CCD behaviour correctly, including the expected performance degradation due to radiation damage. This is being addressed through comprehensive testing and the development of CCD models.

Gaia: 1,000 million stars with 100 CCD detectors

Gaia is the next space-astrometry mission of the European Space Agency, following up on the success of the Hipparcos mission. With a focal plane containing more than 100 large-area CCD detectors, Gaia will survey the sky and repeatedly observe the brightest 1,000 million (one billion) objects, down to 20th magnitude, during its 5-year nominal lifetime. Gaias science data will comprise absolute astrometry, broad-band photometry, and low-resolution spectro-photometry. Medium-resolution spectroscopic data (resolving power 11,500) will be obtained for the brightest 150 million sources, down to 17th magnitude. The extreme thermo-mechanical stability of the spacecraft, combined with the selection of the L2 Lissajous point of the Sun-Earth/Moon system for operations, allows stellar parallaxes (distances) to be measured with standard errors less than 10 micro-arcsecond (μas) for stars brighter than 13th magnitude, 20-30 μas for stars at 15th magnitude, and around 300 μas at magnitude 20. Photometric standard errors are in the milli-magnitude regime. The spectroscopic data will allow the measurement of radial velocities with errors at the level of 15 km s-1 at magnitude 17. Gaias primary science goal is to unravel the kinematical, dynamical, and chemical structure and evolution of the Milky Way. In addition, Gaias data will touch many other areas of research, for instance stellar physics, solar-system bodies, fundamental physics, and exo-planets. The Gaia spacecraft is currently undergoing its critical design review (CDR). With a launch foreseen in the second half of 2012, the final catalogue is expected in 2020. The science community in Europe, organized in the Gaia Data Processing and Analysis Consortium (DPAC), is responsible for the processing of the Gaia data. This formidable task is in full preparation. The calibration of the data presents exciting challenges, in particular in the area of radiation-damage-induced charge-transfer inefficiency (CTI).

On the accuracy of mass measurement for microlensing black holes as seen by Gaia and OGLE

We investigate the impact of combining Gaia astrometry from space with precise, high cadence OGLE photometry from the ground. For the archival event OGLE3-ULENS-PAR-02, which is likely a black hole, we simulate a realistic astrometric time-series of Gaia measurements and combine it with the real photometric data collected by the OGLE project. We predict that at the end of the nominal 5 years of the Gaia mission, for the events brighter than

Optimising the Gaia scanning law for relativity experiments

G\approx15.5

Validation of a CCD cosmic ray event simulator against Gaia in-orbit data

mag at the baseline, caused by objects heavier than 10

Structure and colour–magnitude diagrams of Scorpius OB2 based on kinematic modelling of Hipparcos data

M_{\odot}

Scale-free dynamical models for galaxies: flattened densities in spherical potentials

, it will be possible to unambiguously derive masses of the lenses, with accuracy between a few to 15 per cent. We find that fainter events (

A parameter database for large scientific projects: application to the Gaia space astrometry mission

G<17.5

A Gaia study of the Hyades open cluster

) can still have their lens masses determined, provided that they are heavier than 30