![]() Planetary ephemerides have been developed and improved over centuries. In addition, the use of radio stars is also a prospective method to obtain independent and accurate orientation between the Gaia frame and the ICRF. The suggestion is made to use astrometric radio sources with optical magnitude to 20 m rather than to 18 m as currently planned for the GCRF–ICRF link. Additional possibilities to improve the GCRF–ICRF link accuracy are discussed. ![]() In this paper, a brief overview of recent work on the GCRF–ICRF link is presented. Therefore, the link accuracy between the ICRF and GCRF should be obtained with similar error level, which is not a trivial task due to relatively large systematic and random errors in source positions at different frequency bands. Both systems, ideally, should be a realization of the ICRS (International Celestial Reference System) at micro-arcsecond level accuracy. The main objectives of the investigations in this direction during the next few years are the preparation of a comparison and the mutual orientation and rotation between the optical Gaia Celestial Reference Frame (GCRF) and the 3rd generation radio International Celestial Reference Frame (ICRF3), obtained from VLBI observations. The current state of the link problem between radio and optical celestial reference frames is considered. This is expected if a neutron star or a black hole lurk in these systems. The space velocities of LSI 61303 and Cyg X1 determined with our VLBI proper motions, radial velocities and distances, are 58 +/- 6 and 70 +/- 3 km s(-1), respectively, and are relatively high. This would argue for in situ acceleration of the energetic electrons responsible for the synchrotron emission detected by VLBI as proposed by Vestrand (1983). For LSI 61303, this scale is consistent with the size of the free-free absorption region of the enshrouding material beyond which radio radiation can escape. The linear scale of the rms of these residuals is > 10(13) cm, more than 10 times the stellar separations in these systems. The two X-ray binaries in our program, LSI61303 and Cyg X1, exhibit larger than expected post-fit position residuals. The distances of the stars HD199178, IM Peg and AR Lac were uncertain by as much as 50% before our observations and are now 116 +/- 4, 97 +/- 6, 41.7 +/- 0.6 pc, respectively. The close binary UX Ari is the only star that exhibits an acceleration larger than 3sigma and the most plausible cause is the gravitational interaction of a third body. The proper motions of HR5110, HR1099 and IM Peg, regarded as possible guide stars for the NASA Gravity Probe B space mission, have formal precisions of 0.16, 0.31 and 0.40 milliarcsecond per year, respectively, and the mission requirement is 0.15 milliarcsecond per year. The orthogonality of the 2 orbital planes in the ternary system Algol is supported by new astrometric evidences. The distance to the nearby Tau-Auriga star forming region is 148 +/- 5 pc, determined directly through the VLBI trigonometric parallax of the Pre-Main-Sequence star HD283447 of this region. In addition to the Hipparcos link, these observations have provided several new results. The mean astrometric precision achieved relative to the calibrators is 0.36 milliarcsecond and the highest precision is for the RS CVn close binary sigma (2) CrB with formal uncertainties of 0.12 milliarcsecond for its relative position, 0.05 milliarcsecond for its annual proper motion and 0.10 milliarcsecond for its trigonometric parallax. ![]() These astrometric parameters are absolute because they are directly measured relative to the distant quasars used as VLBI phase reference calibrators. We present the VLBI positions, proper motions and trigonometric parallaxes from this program in the ICRF (International Celestial Reference Frame). ![]() Multiple-epoch phase-referenced VLBI observations of 11 radio-emitting stars have been conducted as part of an astrometric program to link the Hipparcos optical reference frame to the radio extragalactic reference frame. ![]()
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