利用射电星实现光学和射电天球参考架的连接

在光学天体测量卫星Gaia时代，如何建立射电和光学天球参考架（ICRF和GCRF）之间的精确连接（定向和旋转），是基本天文学亟待解决的问题。在暗端（G星等>15），具有射电（VLBI）和光学（Gaia）观测的河外类星体可用来实现ICRF与GCRF的连接。而在亮端（G星等<13），由于缺少天体测量精度与Gaia相当的射电星，导致ICRF与GCRF的精确连接非常困难，因而难以精确评估GCRF的一致性。在Gaia服役期间，开展射电星的VLBI天体测量，可有效提高参考架连接的精度和稳健性。本项目将利用国内外VLBI网，通过观测获得约30颗射电星的精确天体测量参数，并结合以往射电星的VLBI观测数据，实现ICRF与GCRF在亮端的精确连接，将定向和旋转的精度分别提高至30微角秒和10微角秒/年，高于当前精度一个量级。并将Gaia视差零点的精度提高至10微角秒，远好于当前水平。

How to accurately establish the link (orientation and spin) between radio and optical celestial reference frames (International Celestial Reference Frame - ICRF and Gaia Celestial Reference Frame - GCRF) is a burning question of fundamental astrometry in the coming years for Gaia era. At faint magnitude (G>15), the quasars with both Gaia and VLBI positions can help to link ICRF and GCRF. However, at bright magnitude (G<13), it is hard to link the frames accurately and assess the consistency of GCRF, due to the scarcity of comparison data of radio stars with high astrometric accuracy. VLBI astrometry of radio stars during the Gaia mission would improve the accuracy and robustness of the link effectively. We propose to measure about 30 radio stars to determine their accurate astrometric parameters, using domestic and international VLBI networks. Combined our measured data with previous VLBI data of radio stars, we expect to link ICRF and GCRF accurately, at the level of better than 30 micro-arcsecond and 10 micro-arcsecond/year for orientation and spin, respectively. This would be about one magnitude better than the current link accuracy. In addition, the Gaia parallax zero point would be estimated with an accuracy of 10 micro-arcsecond, which is much better than the current level.