Hipparcos-VLBI: the way towards an ideal primary celestial system

Abstract

The astrometric satellite HIPPARCOS of the European Space Agency (ESA) was launched in August 1989. As a consequence of a technical defect, the satellite could not reach the operational geostationary orbit, and it is now in an extremely elliptical orbit around the Earth. Despite the bad observing conditions, HIPPARCOS began to develop the new observing program, and its results promise to be of a higher quality than those expected for the nominal mission. The celestial frame of HIPPARCOS satellite will be materialized by the positions and proper motions of about 120000 stars relative to arbitrary origins. The astrometric precision of its frame will be one order of magnitude higher than that of the optical astronomy from Earth. The precision expected for HIPPARCOS nominal mission is 0.002" for the positions, yearly proper motion and parallaxes, decreasing towards the limit magnitude of the satellite (m=13). However, this precision will be intrinsic to HIPPARCOS sphere, and it will be completely achieved only if its reference frame becomes inertial by means of its link to another stable, non-rotating frame of similar quality. The technique of VLBI determines the coordinates at the level of 0.001" (see Arias, this volume). The quasars and the galactic nuclei are not affected by apparent proper motions, and therefore they constitute the most stable celestial reference frame available at present. The link between the VLBI and HIPPARCOS reference frames will: a) stop the rotation of the reference frame produced by the satellite, b) densify the extragalactic reference frame in optical frequencies, c) unify the radio and optical coordinate systems and allow direct comparison of the images of the celestial objects obtained with radio and optical techniques with the same angular resolution. In absence of regional deformations, the link between two frames with the same origin can be mathematically expressed by a simple rotation. If the relative orientation of the frames evolve linearly with time, the link is given by a global rotation at an arbitrary epoch, and by the components of the angular velocity of rotation of one frame relative to the other. The method to link the VLBI and HIPPARCOS frames has been developed by Arias (1990). We have studied the link between HIPPARCOS and VLBI frames on the basis of observations of radio stars. We have simulated HIPPARCOS observations in a realistic way. Lastrade et al. (1988) and White et al. (1990) provided the VLBI positions and proper motions of 14 stars between +62° and -75° in declination. The relative orientation at the initial epoch is determined with a precision at the level of 0.001". The precision of the components of the angular velocity is in the range 0.0007" - 0.0009". Concerning the distribution of objects, the inclusion of radio stars at negative declination, with uncertainties several times greater than those in the North, does not improve the link and introduces a bias in the angles of orientation of the frames.