Floor van Leeuwen

Analysis of the Hipparcos Measurements of υ Andromedae: A Mass Estimate of Its Outermost Known Planetary Companion

We present an analysis of Hipparcos astrometric measurements of υ Andromedae, a nearby main-sequence star around which three planet candidates have recently been discovered by means of radial velocity measurements. The stellar orbit associated with the outermost candidate has a period of 1269 ± 9 days and a minimum semimajor axis of 0.6 mas. Using the Hipparcos data together with the spectroscopic elements, we found a semimajor axis of 1.4 ± 0.6 mas. This implies a mass of 10.1 Jupiter masses for that planet of υ And.

The Hipparcos Mission

A review is presented of the European Space Agencys astrometric satellite project Hipparcos, for which the final data catalogues were published in June 1997. The emphasis is on those aspects that have or may have influenced the data as presented in the catalogues. It presents a brief review of the satellite, the aims of the mission with their relation to ground-based astrometry, and the mission history. This is followed by a description of the input data streams and a summary of the data reductions. Data files presented in the catalogues are described in the context of the data reductions, and explained in content and usage. The mission results comprise not only outstanding astrometric data on both single and double stars, but also an unique all-sky photometric survey which has been used for variability investigations. This has led to the discovery of thousands of variable stars. The Hipparcos mission was complemented by the Tycho experiment, providing a complete all-sky survey of astrometric and photometric parameters for one million stars down to magnitude 11, though with lower accuracies than obtained for the main mission. Astrometric and photometric data for a selection of 48 minor planets, the Jovian moon Europa and the Saturnian moons Titan and Iapetus were also obtained. The data quality verifications are reviewed and guidelines to the proper use of the Hipparcos data are provided, followed by some of the first scientific results of the mission. Experiences gained with this first ever space astrometry mission are considered in relation to a follow up mission for Hipparcos.A review is presented of the European Space Agencys astrometric satellite project Hipparcos, for which the final data catalogues were published in June 1997. The emphasis is on those aspects that have or may have influenced the data as presented in the catalogues. It presents a brief review of the satellite, the aims of the mission with their relation to ground-based astrometry, and the mission history. This is followed by a description of the input data streams and a summary of the data reductions. Data files presented in the catalogues are described in the context of the data reductions, and explained in content and usage. The mission results comprise not only outstanding astrometric data on both single and double stars, but also an unique all-sky photometric survey which has been used for variability investigations. This has led to the discovery of thousands of variable stars. The Hipparcos mission was complemented by the Tycho experiment, providing a complete all-sky survey of astrometric and photometric parameters for one million stars down to magnitude 11, though with lower accuracies than obtained for the main mission. Astrometric and photometric data for a selection of 48 minor planets, the Jovian moon Europa and the Saturnian moons Titan and Iapetus were also obtained. The data quality verifications are reviewed and guidelines to the proper use of the Hipparcos data are provided, followed by some of the first scientific results of the mission. Experiences gained with this first ever space astrometry mission are considered in relation to a follow up mission for Hipparcos.

Dynamic modelling of the Hipparcos attitude

We present a new method for a high-accuracy reconstruction of the attitude for a slowly spinning satellite. This method, referred to as the fully-dynamic approach, explores the possibility to describe the satellites attitude as that of a rigid body subject to continuous external torques. The method is tried out on the Hipparcos data and is shown to reduce the noise for the along-scan attitude reconstruction for that mission by about a factor two to three. The dynamic modelling is expected to give a more accurate representation of the satellites attitude than was obtained with a pure mathematical modelling. As such, it decreases the degrees of freedom in the a posteriori reconstruction. Some of the decrease is obtained through accumulating and subsequently implementing information on high frequency components in the solar radiation torques, which show to be systematic and predictable. This could be expected, as they are primarily linked to the external geometry and optical properties of the satellite. In the context of an astrometric mission, the methods presented here can only be applied as a final iteration step: the star positions that are used to reconstruct the attitude are also part of the scientific objectives of the mission. An estimate for the potential of a re-reduction of the Hipparcos data using the fully-dynamic model for the attitude reconstruction was obtained from test reductions of the first 24 months of mission data. Improvement of the accuracies of the astrometric parameters for all stars brighter than Hp=9.0 appears possible. The noise on the astrometric parameters for these stars was affected significantly by the along-scan attitude noise, which dominated for stars brighter than Hp=4.5. The possible improvement for stars brighter than about Hp=4.5 may, after iterations, be as much as a factor three. The reduced noise levels also allow a more accurate calibration and monitoring of instrument parameters, leading potentially to a better understanding of the instrument and the scientific data obtained with it.

A Detailed Analysis of the Operational Orbit of the Hipparcos Satellite

An analysis of the orbital evolution of the ESAs Hipparcos satellite is presented. Hipparcos operated between August 1989 and March 1993 in a highly elliptical orbit: a geostationary transfer orbit with increased perigee height. The requirements of the scientific mission included high accuracy knowledge of the position and velocity vectors of the spacecraft as a function of time. Through a study of the variations in the total orbital energy, the loss of energy during the mission as a result of non-conservative forces is recovered. These are explained as largely due to atmospheric drag during perigee passages. Apparent variations in the drag coefficient are in agreement with orientation variations of the satellite during those perigee passages. Two different models used for calculating the atmospheric drag give significantly different results, confirming earlier findings by other users of those models.An analysis of the orbital evolution of the ESAs Hipparcos satellite is presented. Hipparcos operated between August 1989 and March 1993 in a highly elliptical orbit: a geostationary transfer orbit with increased perigee height. The requirements of the scientific mission included high accuracy knowledge of the position and velocity vectors of the spacecraft as a function of time. Through a study of the variations in the total orbital energy, the loss of energy during the mission as a result of non-conservative forces is recovered. These are explained as largely due to atmospheric drag during perigee passages. Apparent variations in the drag coefficient are in agreement with orientation variations of the satellite during those perigee passages. Two different models used for calculating the atmospheric drag give significantly different results, confirming earlier findings by other users of those models.

Impact of basic angle variations on the parallax zero point for a scanning astrometric satellite

Context. Determination of absolute parallaxes by means of a scanning astrometric satellite such as Hipparcos or Gaia relies on the short-term stability of the so-called basic angle between the two viewing directions. Uncalibrated variations of the basic angle may produce systematic errors in the computed parallaxes. Aims. We examine the coupling between a global parallax shift and specific variations of the basic angle, namely those related to the satellite attitude with respect to the Sun. Methods. The changes in observables produced by small perturbations of the basic angle, attitude, and parallaxes were calculated analytically. We then looked for a combination of perturbations that had no net effect on the observables. Results. In the approximation of infinitely small fields of view, it is shown that certain perturbations of the basic angle are observationally indistinguishable from a global shift of the parallaxes. If these kinds of perturbations exist, they cannot be calibrated from the astrometric observations but will produce a global parallax bias. Numerical simulations of the astrometric solution, using both direct and iterative methods, confirm this theoretical result. For a given amplitude of the basic angle perturbation, the parallax bias is smaller for a larger basic angle and a larger solar aspect angle. In both these respects Gaia has a more favourable geometry than Hipparcos. In the case of Gaia, internal metrology is used to monitor basic angle variations. Additionally, Gaia has the advantage of detecting numerous quasars, which can be used to verify the parallax zero point.

THE OPERATIONAL ENVIRONMENT OF THE HIPPARCOS MISSION

We investigate links between the observational environment as experienced by the Hipparcos satellite and the performance of the spacecraft and payload instrumentation, with particular emphasis on finding out whether some of these effects may have been inadequately represented in instrument calibrations and could thus have affected the scientific results of the mission. Scan-coverage and radiation effects are primarily random effects with only some long-term systematics. However, long- (days to weeks) and short-term (hours) temperature variations reflected in the performance of some of the spacecraft instrumentation. It is shown that only a small sign of some long-term thermal variations could be detected in the payload instrumentation. These findings further limit the scope left for the occurrence of large-scale correlated errors in the Hipparcos astrometric data. On the other hand, a number of great circles were identified which showed a highly significant drift of the basic angle, which had not been detected in the preparation of the published data. The data from these circles may have, in some cases, led to, very localised, slightly anomalous results, in particular where stars are accidentally affected by two or more of such circles.

The Pleiades question, the definition of the zero-age main sequence, and implications

Determinations and measurements of the parallax of the Pleiades, obtained with ground-based studies and with Hipparcos data, are reviewed. A number of uncertainties in both sets of data are found. Although a further correction for abscissa correlations brings the Hipparcos determination closer to the ground-based value, the difference still seems too large. The new Hipparcos parallax determinations seem to reinforce a possible age related effect. A new reduction of the Hipparcos data is in progress. It reduces significantly the contribution of the attitude noise, providing higher accuracies and lower correlation levels for the brighter stars. To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html