Given the extragalactic radio reference system, it is possible to extend this system to optical wavelengths by linking the provisional Hipparcos frame to the radio reference frame. Among the varying methods of frame connection we discuss here the usage of radio stars observed by the Very Large Array (VLA) relative to extragalactic radio sources. A general adjustment algorithm has been developed for the simultaneous determination of orientation angles and angular rates of rotation. From case studies we have derived the orientation angles of the provisional Hipparcos frame with respect to the VLA reference frame for radio star observations. Typical orientation angles about the three axes of the adopted equatorial coordinate system are -12.8, -19.1, +5.5 milliarcsec (mas), respectively. Evidence is shown that an offset exists between the VLA frame and the International Celestial Reference Frame (ICRF). Especially in the z-axis the offset is significant amounting to about 16 mas. From simulated data it is inferred that some 50 VLA observed radio stars are instrumental in linking optical and radio reference frames, with a precision of 0.5 mas/yr for the angular rates.

Distances to the recently-discovered planetary system candidates, 47 UMa, 70 Vir and 51 Peg, are presented, based on absolute trigonometric parallaxes from the global reduction of the ESA Hipparcos astrometry satellite data. Standard errors of the parallaxes are in the range 0.66-0.76 mas (milliarcsec), with resulting distances accurate to around 1 per cent. For 70 Vir, the Hipparcos parallax (55.22 +/- 0.73 mas) resolves a discrepancy of almost a factor of three in published trigonometric and photometric parallaxes. The residuals of the astrometric parameters are used in combination with the radial velocity data for these orbital systems to place upper limits on the companion masses of between 7-22 and 38-65 Jupiter masses (M(J)) for 47 UMa and 70 Vir respectively, with less stringent limits for 51 Peg. The Hipparcos data therefore provide confirmation of the existence of sub-stellar masses significantly below the brown dwarf limit (of about 0.08 M.) surrounding other stars.

We present a comparison of VLBI and Hipparcos astrometric parameters of several optically bright radio-emitting stars. The systematic discrepancies found in these preliminary data can be removed by performing a single global rotation between the extragalactic and Hipparcos reference frames. Two estimates of this rotation and its time derivative have been made by using seven and six link stars from the two Hipparcos reduction consortia FAST and NDAC, respectively. The three angles and annual rates of rotation between the two frames are determined at better than the milli-arcsec level. The angles of rotation found are relative to a VLBI extragalactic reference frame that is defined by the IERS VLBI coordinates of the extragalactic reference sources used for the differential VLBI measurement of the link stars. The post-fit residuals of the star coordinates and proper motion components after the adjustment of the global rotation indicate that the consistency between the Hipparcos and VLBI astrometric techniques is at the milli-arcsec level. This is the first cross-check between these two astrometric techniques of comparable precision. The level of agreement found is consistent with the expected accuracy of the technique, even though the astrometric parameters used are from preliminary reductions of the data for both techniques. The Hipparcos reference frame has been aligned with a quasi-FK5 frame in the process of its construction. The magnitudes of the rotation angles found are consistent with the degree of alignment known between the VLBI and FK5 frames. This VLBI link of the Hipparcos catalog is preliminary and we describe various improvements that will be implemented before the release of the Hipparcos catalog in 1996. Comparison of the VLBI and Hipparcos trigonometric parallaxes are also discussed.

The data analysis methods for the determination of the astrometric parameters from observations by the Hipparcos satellite as used in FAST and NDAC consortia for the construction of intermediate 18-month and 30-month solutions are presented. The quality of the results depends upon calibrations of the instrumental parameters, examples of which are given. Results for the 18 and 30 month reductions are presented: these include histograms of the rms errors of each astrometric parameters, the FAST-NDAC differences as a function of coordinate, magnitude or colour index and results of a first attempt at combining the two solutions. The actual and pre-launch accuracy predictions are compared and discussed.

We present an overview of the results obtained by FAST consortium in a preliminary reduction of the first 30 months of Hipparcos data. Precisions achieved are better than the nominal precisions announced and are representative of what can be expected for the definitive solution.

Processing of 18 months of data of the Hipparcos mission by the consortium FAST markes the end of the zero-iteration of data reduction based on the approximative astrometric star parameters of the input reference catalogue. Significant increase of accuracy is expected from the first iteration owing to the updated reference catalogue containing improved astrometric parameters of single and double stars augmented by revised stellar magnitudes. -This is a report on the intermediate status of the determination of astrometric parameters reached at the end of the zero-iteration. The astrometric parameters are routinely determined from the preprocessed raw measurements of the Hipparcos telescope and consist of two components of angular position, two components of proper motion and the trigonometric parallax. Afterwards the results of the mass production undergo a thorough process of evaluation: The adjusted parameters are subjected to statistical tests and are verified in relation to external ground-based data of high accuracy. The following options are discussed here: (1), comparison with the fundamental stars of FK5, (2), comparison with radio stars having positions obtained from radio interferometry, (3), comparison with ground-based trigonometric parallaxes of nearby stars and with photometric parallaxes of distant stars. At this intermediate stage the external accuracies of positions and parallaxes, of a few milli-arcseconds (mas), support the prediction that the Hipparcos project fulfills or even surpasses the mission objectives. Bearing in mind the currently short baseline of time also the present proper motion accuracy of several mas/yr gives all confidence in the success of the project.

We present an overview of the results obtained by FAST consortium in a preliminary reduction of the first 30 months of Hipparcos data. Precisions achieved are better then the nominal precisions announced and are representative of what can be expected for the definitive solution

Using the first 12 months of HIPPARCOS mission data already reduced, a synthesis solution was obtained by both CERGA and ARI teams. The reduction includes two steps. In the first one the origins of individual reference circles are determined, thus constituting the HIPPARCOS celestial reference frame. This solution uses a subset of some 30 000 bright and evidently single stars in order to minimize possible corruptive effects on the system. In the second step, the abscissa measurements of all observed stars are substituted in the system and then processed star by star in a least squares adjustment providing the astrometric parameters (positions and parallaxes).The results of sphere solution and astrometric parameter determination are discussed. In this context error ellipses and internal errors are calculated. External errors are derived by comparison with suitable astrometric catalogues. Especially, for estimating the external errors of the HIPPARCOS measured FK5 stars the instrumental system was tied to the FK5 system by rigid rotations followed by comparisons of the positions with those of FK5. The mean internal errors of the HIPPARCOS positions of FK5 stars is 1.5 milliarcseconds (mas) while the standard deviation of the differences HIPPARCOS minus FK5 reaches 90 mas. For the 5022 measured parallaxes the mean internal and external errors are 3.8 mas and 16.7 mas, respectively. — The results of precision and comparisons with external data show that the accuracy predicted for the final catalogue can be reached.

Optical positions of some 30 radio stars derived from 12 months of HIPPARCOS measurements are compared with their radio positions obtained with the Very Large Array (VLA). — Once the lengths of arcs between optical and radio positions of pairs of stars are calculated the differences of the arcs are formed. They provide an estimate of the coincidence of the optical and radio emission centres. — From the comparison of optical and radio positions infinitesimal rotation angles of the HIPPARCOS frame with respect to the VLA extragalactic reference frame are determined by rigid rotations. After taking account of the relative orientation of the frames the standard deviations of the remaining residuals are approximately of the order of the VLA observation errors, thus demonstrating the reliability of the HIPPARCOS results. However, they also indicate some data noise very likely caused by the low accuracy of optical proper motions used to bridge the HIPPARCOS-radio epoch differences up to 9 years, and possible discrepancies of radio-optical emission centres of some stars.

This paper gives the main features of the data analysis methods adopted by FAST in the calibration and processing of the Hipparcos data. The structure of the data reduction software is described and the actual processing and evaluation is presented.

For a test under real conditions the Hipparcos-measured star abscissae on 60 reference great circles have been processed covering partly the period from December 1989 to February 1991. After adapting the abscissae to a provisional instrumental system the determination of positions of some 4000 programme stars has been attempted in the course of the operational qualification of the data reduction procedures of the Consortium FAST. – Following the FAST Consortium’s decision to work exclusively in the ecliptic frame of reference the results are given in ecliptic longitude (λ) and latitude (β).

AbstractThis paper outlines the details of astrometric parameter determination of double stars from HIPPARCOS observations as prepared in the FAST consortium. Methods of parameter estimation are developed for relative astrometry and absolute astrometry. The capability of the procedures will be demonstrated using simulated observations, in describing the particular steps of a Monte Carlo simulation which have been executed at the Istituto di Astrofisica Spaziale (IAS) and at the Astronomisches Rechen–Institut (ARI).