Determining the Orientation Parameters of the ICRS/UCAC2 System Using
  the Kharkov Catalog of Absolute Stellar Proper Motions
  The absolute proper motions of about 275 million stars from the Kharkov XPM
catalog have been obtained by comparing their positions in the 2MASS and
USNO--A2.0 catalogs with an epoch difference of about 45 yr for
northern-hemisphere stars and about 17 yr for southern-hemisphere stars. The
zero point of the system of absolute proper motions has been determined using
1.45 million galaxies. The equatorial components of the residual rotation
vector of the ICRS/UCAC2 coordinate system relative to the system of
extragalactic sources have been determined by comparing the XPM and UCAC2
stellar proper motions: \omega_{x,y,z}=(-0.06,0.17,-0.84)+/-(0.15,0.14,0.14)
mas yr^{-1}. These parameters have been calculated using about 1 million
faintest UCAC2 stars with magnitudes R_{UCAC2}>16^m and J>14^m.7,for which the
color and magnitude equation effects are negligible.
