Residual rotation of the Hipparcos/Tycho-2 system as determined from the data of the XPM catalogue

From a comparison of absolute proper motions of stars from the XPM catalogue with those of the same stars from the PPMXL, UCAC3, Tycho-2 and XC1 catalogues, the equatorial components of the rotation vector of these coordinate systems are determined with respect to the coordinate system specified by positions and proper motions of XPM. These parameters are calculated with the use of about 90 million stars from the UCAC3 catalogue and about 300 million stars from the PPMXL catalogue. It is shown that the Hipparcos Celestial Reference Frame represented by the Tycho-2, PPMXL, UCAC3 and XC1 catalogues has a significant rotation component ωz = (−1.8 ± 0.16) mas yr−1 about the equatorial axis directed to the celestial pole. The result is confirmed by an analysis of the formal proper motions of the extragalactic sources contained in the considered catalogues. Key words: catalogues – astrometry – reference systems. 1 IN T RO D U C T I O N We investigate the problem of residual rotation of the Hippar-cos/Tycho-2 system with respect to extragalactic objects by compar