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How to accurately establish the link (orientation and spin) between the Gaia Celestial Reference Frame (GCRF) and International Celestial Reference Frame (ICRF) is a burning question of fundamental astrometry in the coming years. The quasars with both Gaia and VLBI positions were expected to play the key role for the link. However, the reality is rather more complicated. There is an indication that the bright GCRF (G < 13 mag) is rotating with respect to the quasars. VLBI astrometry of radio stars is the most valuable way to examine the GCRF for bright stars. The link accuracy is currently limited by a few radio stars, and by issues related to the astrophysical nature of radio stars. We are going to measure about 30 new radio stars using VLBI phase-referencing observations during the Gaia mission, to improve the link robustness and accuracy at the level of 20 micro-arcsecond and 5 micro-arcsecond/year for orientation and spin, respectively. In addition, the Gaia parallax zero point would be estimated with accuracy better than 10 micro-arcsecond. We propose to observe 73 candidate radio stars in VLBI snap-shot mode, to select about 25 targets for follow-up VLBI astrometric observations. |