Kinematic control of the inertiality of the system of Tycho-2 and UCAC2 stellar proper motions

Based on the Ogorodnikov-Milne model, we analyze the proper motions of Tycho-2 and UCAC2 stars. We have established that the model component that describes the rotation of all stars under consideration around the Galactic y axis differs significantly from zero at various magnitudes. We interpret this rotation found using the most distant stars as a residual rotation of the ICRS/Tycho-2 system relative to the inertial reference frame. For the most distant (d approximate to 900 pc) Tycho-2 and UCAC2 stars, the mean rotation around the Galactic y axis has been found to be M-13 - =-0.37 +/- 0.04 mas yr(-1). The proper motions of UCAC2 stars with magnitudes in the range 12-15(m) are shown to be distorted appreciably by the magnitude equation in mu(alpha) cos delta, which has the strongest effect for northern-sky stars with a coefficient of -0.60 +/- 0.05 mas yr(-1) mag(-1). We have detected no significant effect of the magnitude equation in the proper motions of UCAC2 stars brighter than approximate to 11(m).