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 M₁₃⁻=−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 μ_α 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 ≈11^m.