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

Abstract

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≈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 12–15m are shown to be distorted appreciably by the magnitude equation in μα cos δ, 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 ≈11m.