Precessional Constant Determined from Optical Astrometry

Abstract

We have analyzed proper motions of about 30000 K-M giants chosen from the astrometrie catalogue ACRS Part 1 (Corbin and Urban 1991). To avoid localized velocity fields, these K-M giants are chosen from the heliocentric distance interval 0.5 - 1.0 kpc. Starting from the initial trial of the luni-solar precessional correction Δρ ~ —0”.3/cy, we have found that the motion of stars are well expressed in terms of the Oort constants for the K-M giants. After confirming that the K-M giants are in a steady-state, we have applied the velocity-field model of the plane-parallel galactic rotation with non-zero Oort constants. Then, we have found a rational set of the corrections to the FK5 system: Δρ = -0”.27 ± 0”.03/cy and Δe + Δλ = —0”.12 ± 0”.03/cy, as is shown in Table 1 (Miyamoto and Sóma 1993). In consequence, the FK5 system is still rotating.