A possible superluminal motion of the VLBI core in 3C345

Abstract

The recent VLBI observations of the QSO 3C345 have shown that the apparent motions of knots c 6 and C 7 are northwestward. This is approximately parallel to the apparent motion of knot C 4 during the period 1981–1984. The observed apparent motions of knots C 6 and C 7 are consistent with the prediction of the “core motion model” proposed. In this paper we use all the VLBI data available for 3C345 and a more stringent method to determine the apparent motion of the VLBI core of 3C345 in the N-S direction and the common apparent trajectory along which the knots move. The results show that the apparent N-S motion of the VLBI core in 3C345 is oscillatory. The derived maximal speed of the apparent motion is 0.10–0.15 mas/yr (i.e. 2 c or 3 c, h0 = 100 km −1 sec −1 Mpc −1). The VLBI monitoring observations of the apparent trajectories for knots C 6 and C 7 before 1993 is decisive to the test of the model. Direct measurements of the possible apparent motion of the VLBI core by VLBI phasereferencing technique would be most desirable.