Abstract
On the basis of Hipparcos data, whose system is believed to be quasi-inertial to within $\pm 0.25 \,\mathrm{mas} \,\mathrm{yr}^{-1}$ and to represent an internally consistent reference frame, we carried out an analysis of the SPM 2.0 proper-motion system. Comparing the Hipparcos proper motions with those of the SPM 2.0 Catalog, which provides absolute proper motions of objects measured directly relative to external galaxies, we found that the two components of the spin vector, $\omega_x$ and $\omega_z$, are apparently less than the value of $0.25 \,\mathrm{mas} \,\mathrm{yr}^{-1}$ for the uncertainty of the Hipparcos inertiality, while the component $\omega_y$ is almost twice that value. Inspecting the magnitude - and color-dependent pattern of the proper-motion difference, we found that the SPM 2.0 Catalog seems to suffer a slight color systematics with respect to the Hipparcos proper-motion system, though the result is inconclusive due to the small number of the Hipparcos and SPM 2.0 common stars.
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