The Hipparcos catalogue is more than a ``simple'' catalogue of positions, parallaxes and proper motions. It also contains the data that were used to construct these quantities: the Intermediate Astrometric Data or abscissa residuals (Vol. 1, Sect. 2.8) - for further treatment of single stars; the Transit Data (Vol. 1, Sect. 2.9) - for further analysis of double stars; Solar System Objects abscissae (Vol. 1, Sect. 2.7); and the Hipparcos Epoch Photometry Annex (Vol. 1, Sect. 2.5) (records of the fully calibrated mean signal intensity and modulation amplitude). All of these are available as ASCII files on the Hipparcos CD-ROM set and can be used either on their own - for a better understanding of the astrometric data - or used in connection with ground-based observations to derive alternative or general solutions for individual objects or for groups of objects.
The abscissa data formed the basic astrometric measurements for single stars, where we define a single star as a star for which the modulated signal was not significantly disturbed by the presence of a component. The abscissa data were collected over intervals set by the orbital period of the satellite (10.6 hours) and referred to a reference great circle, defined for each orbital period by the position of its pole in the final International Celestial Reference System (ICRS). The abscissae are one dimensional positional measurements, meridians of the great circle, where the position of the meridian is very well established (to an accuracy of a few mas), but the position of the object along the meridian is not known (except of course for the a priori knowledge of the positions of objects which were used in obtaining the observations). The abscissae, as observed over the length of the mission, were fitted with a set of astrometric parameters (usually position, parallax and a linear proper motion). Differences between observed and the predicted abscissae based on such models are referred to as abscissa residuals, and were made available in the Intermediate Astrometric Data file on the Hipparcos CD-ROM. For solar system objects the data were only presented in the form of abscissa data. The present paper focusses on when and how to use these intermediate astrometric data, for individual as well as groups of single stars.
Two reasons led to the decision to release the abscissa data: firstly, a small number of solutions provided in the final catalogue are not satisfactory, in particular when proper motions were fitted with a first or even second order function of time, or when a so-called stochastic solution had to be applied (see Sect. 2.4); secondly, correlations between errors on the abscissa estimates exist between measurements on the same reference great circle, most importantly when separations are less than 2-3 degrees (see Vol. 3, Chap. 17 and Sect. 2.2). The publication of the intermediate astrometric data has made it possible to incorporate the original Hipparcos abscissa data in a new solution when, at a later stage, additional observations clarify or constrain the nature of the object. Their publication also allows a proper determination of mean proper motions and parallaxes for stars in clusters and in the Magellanic Clouds, by allowing to fully take into account existing correlations.
The abscissa data allow for a complete reconstruction of the Hipparcos solutions for single stars (see Sect. 4. Thus, if for some reason at a later stage a particular abscissa measurement is considered suspect, then the solution can be repeated without this measurement. Similarly, abscissae rejected for the published solution are still included in the data file and can be recognized from the records and reintroduced if appropriate. If for some reason the parallax or the proper motion of an object is more accurately known already, a new solution can be obtained incorporating this information. In nearby star clusters, ground-based studies have over the years provided differential proper motions with precisions ten times higher than Hipparcos and could be used as external constraints on a new solution. Incorporating this kind of data is, however, not simple due to the distortions that appear to be present in such proper motion determinations (see e.g. Tian et al. 1996).
The present paper shows examples of how the intermediate astrometric data can be used in alternative models for the astrometric parameters of individual objects or groups of objects. These models are of two kinds: constrained solutions, where one or more parameters are supplemented with or replaced by information obtained from other sources than the Hipparcos Catalogue (Sect. 4); and ``combined solutions'', where parameters for a selection of stars are solved for in one solution while, when necessary, removing correlations between abscissa residuals (Sect. 6). The latter method may also include constrained parameters, when part of the astrometric parameter solutions for a group of stars is replaced by the solution for a few common parameters, shared by all stars in that group. A simple example is the stars in the LMC. Instead of solving for each star a parallax and a proper motion, it becomes possible to eliminate the parallax from the solution (a parallax of 0.02 mas could not be detected by Hipparcos), and solve for a collective proper motion for all LMC stars together (see e.g. Kroupa & Bastian 1997). The possibility to combine astrometric solutions for groups of stars, provides, in addition, the means to detect intrinsically weaker signals in luminosity calibrations.
A brief review of those aspects of the Hipparcos data that have an immediate bearing on the use and understanding of the intermediate astrometric data is presented in Sect. 2. This is followed by Sect. 3 explaining the information provided in the intermediate astrometric data file, and the proper implementation of these data. Sect. 5 presents methods for analyzing data on the solar system objects. A more extensive review is presented by VL98, and in Volume 3.
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