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. We investigate the problem of residual rotation of the Hipparcos/Tycho-2 system with respect to extragalactic objects by comparing the derived absolute proper motions of the XPM stars with those of the stars representing the Hipparcos Celestial Reference Frame system. The accuracy of the solution for this problem strongly depends on the accuracy of the proper motions used. In this case, systematic errors of the data in use are determinative rather than random ones. A correct solution is possible only under the condition that the systematic errors in the absolute proper motions of the XPM stars are eliminated. As was shown in Fedorov et al. (2011, hereafter Paper III), the magnitude equation in the XPM catalogue has been eliminated both in the bright and in the faint parts of the stellar magnitude range, while testing confirms the absence of the system rotation within an accuracy of several tenths of a milliarcsecond. This is a reason to treat the system of the XPM catalogue as an independent realization of the International Celestial Reference System, and to use it as a standard when comparing with other systems of coordinates.