Claus Fabricius

The Tycho double star catalogue

We report the discovery of 13 251 visual double stars, mostly with separations between 0.3 and 1 arcsec, from a dedicated re-reduction of the Tycho data from the star mapper of the ESA Hipparcos satellite. The new doublesare combined with 18 160 WDS systems identified in the Tycho-2 Catalogue, and 1220 new Tycho-2 doubles, to form the Tycho Double Star Catalogue, TDSC, a catalogue of absolute astrometry and B T , V T photometry for 66219 components of 32631 double and multiple star systems. We also include results for 32 263 single components for systems unresolved in TDSC, and a supplement gives Hipparcos and Tycho-1 data for 4777 additional components. The TDSC thus contains a total of 103 259 entries. Cross identifications are given to WDS, HD, Hipparcos and Tycho-2.

Internal kinematics of the TW Hya association of young stars

Thirty one probable kinematic members of the nearby TW Hya association of young stars are selected from the RASSBSC/Tycho-2 sample of stars luminous in X-rays, detected by ROSAT. Eight of them have been listed already as members of the association, and 23 are new candidates. The association occupies a volume of some 10 6 pc 3 , the nearest member being at a distance of only 17.5 pc from the Sun. The lower bound internal velocity dispersion is estimated at 0.8 km s -1 , which is considerably larger than is expected in a gravitationally bound open cluster. The total mass of the group is estimated at 31 solar masses. The centre of mass lies at 73 pc from us in the direction close to the position of the prototype star TW Hya. A realistic kinematical model fitting both the observed trigonometric parallaxes and radial velocities involves a linear expansion of the group with the rate 0.12 km s -1 pc -1 , which defines a dynamical age of 8.3 Myr, in good agreement with previous age estimations for T Tauri members. Likewise the nearby Carina-Vela moving group of X-ray stars, the TW Hya association appears to be an outpost of the Gould Belt structure rather than an isolated open cluster.

Henry Draper catalogue identifications for Tycho-2 stars

We present identications in the Tycho-2 Catalogue, for 99.8 per cent of the stars in the Henry Draper Catalogue and for 96 per cent of the Henry Draper Extensions.

Variable stars in the Tycho photometric observations I. Detection

We present an original way to detect variable stars in the Tycho photometric observations. A modied Kolmogorov-Smirnov statistical test has been constructed, based on a model of most of the satellites parameters, leading to the dispersion in the measurements. This allows us to build a treatment taking into account truncated detections and censored measurements, and to search for variability in the faint part of the catalogue. The selection threshold of variable stars has been calibrated to minimize false alarm rate. Reliable results were thus obtained for stars as faint as 11 mag. One thousand and ninety-one suspected variable stars have been listed, 407 of which are already known in Hipparcos, GCVS or NSV, and 684 of which are suspected variable stars to be identied soon.

Passband reconstruction from photometry

Based on an initial expectation from laboratory measurements or instrument simulations, photometric passbands are usually subject to refinements. These refinements use photometric observations of astronomical sources with known spectral energy distribution. This work investigates the methods for and limitations in determining passbands from photometric observations. A simple general formalism for passband determinations from photometric measurements is derived. The results are applied to the passbands of HIPPARCOS, Tycho, and Gaia DR1. The problem of passband determination is formulated in a basic functional analytic framework. For the solution of the resulting equations, functional principal component analysis is applied. We find that, given a set of calibration sources, the passband can be described with respect to the set of calibration sources as the sum of two functions, one which is uniquely determined by the set of calibration sources, and one which is entirely unconstrained. The constrained components for the HIPPARCOS, Tycho, and Gaia DR1 passbands are determined, and the unconstrained components are estimated.

The micrometers of the Carlsberg Automatic Meridian Circle

A new moving slit star micrometer has been developed for the 18 cm Carlsberg Automatic Meridian Circle on La Palma. A zenith mean error of 0inf.sup15 for one transit and a limiting magnitude of 14inf.supm8 is obtained. It has a high efficiency and is well suited for improving the FK5 system and extending it to much fainter stars. A new collimator alignment micrometer and the necessary calibrations are also described.

Double Stars and Astrometric Uncertainties inGaiaDR1

A significant number of double stars with separations up to 2.5 arcsec are present in the Gaia Data Release 1 astrometric catalogs. Limiting our analysis to a well-studied sample of 1124 doubles resolved by Hipparcos, provided with individual Tycho component photometry, and cross-matched with the TGAS catalog, we estimate a rate of at least 3% for brighter double stars in Gaia DR1, which should be resolved in the future data releases. Gaia astrometric results are affected by unresolved duplicity. The variance-normalized quadratic differences of proper motion between Gaia and Hipparcos do not follow the expected chi^2 distribution and show signs of powerful degradation in the components aligned with the axes of the double systems. This concerns only pairs with separation below 1.2 -- 1.5 arcsec, which mostly remain unresolved in Gaia DR1. On the other hand, the orthogonal proper motion components and parallaxes do not have any detectable perturbation, as well as all astrometry for separations above 1.5 arcsec. Gaia parallaxes do not seem to be perturbed by duplicity, with Gaia - Hipparcos differences being systematically smaller than the expectation. The rate of incorrectly identified, or swapped, companions is estimated at 0.4%.

Astrometry from Space: New Design of the GAIA Mission

An interferometric astrometric mission, aiming at accuracies at around the10 microarcsec level, was recommended as a high priority concept within thenew ESA Horizon 2000+ scientific programme. The original outline concept forsuch a mission, GAIA, presented its general feasibility but did not addressmany questions of implementation or optimisation. Another concept of aninterferometer for a scanning astrometric satellite is presented. It containsa simpler optical telescope and a more efficient detector system. The designutilizes the full resolution of all light in the dispersed fringes of aFizeau interferometer. A preliminary optimization of the satellite indicatesthat two telescope units with a baseline of 100 cm will achieve a precisionof 3, 8, 22, 68, 302 microarcsec for parallaxes of stars with V = 12, 14, 16, 18, 20 mag, respectively, from a 5 year mission. Simultaneousspectrophotometry of the entire spectrum of each star will be obtained with aresolution corresponding to intermediate band photometry. The expectedprecision of this photometry is about 0.003 mag for V = 16. The performance is good in crowded fields, at least up to one star per 5 arcsec2. A Hipparcos-type beam combiner of 150 cm width is placed in front of atelescope with 4 square apertures of 50 cm. The assumed focal length is f = 60 m and the field 0.5 degree diameter. The detector consists of CCDs used for time delayed integration (drift-scan.)

Optical reference frame defined by Carlsberg Meridian Catalogue La Palma Number 4

The contribution of the Carlsberg Meridian Catalogue La Palma Number 4 to the determination of the optical reference frame is discussed. This catalogue of almost 51000 stars provides one of the most accurate optical reference frames at the present epoch, having a density of 1 star per square degree, and an average accuracy of 0.″12 in position and 0.″003 per year in proper motion for stars with V 11 in the fields of benchmark extragalactic radio sources which can be used in linking the optical reference frame defined by the FK5 to the extragalactic frame.

First Results from the New Micrometer on the Carlsberg Automatic Meridian Circle

A new micrometer has been in operation on the Carlsberg Automatic Meridian Circle since May 1988. The zenith mean error for one observation has improved from 0.″19 to 0.″14 and the limiting magnitude from 13m. 2 to 14m. 8. The first meridian observations of Pluto and observations of nine extragalactic objects are briefly discussed.