Parsec-Scale Blazar Monitoring: Proper Motions

We present proper motions obtained from a dual-frequency, six-epoch, VLBA polarization experiment monitoring a sample of 12 blazars. The observations were made at 15 and 22 GHz at bimonthly intervals over 1996. A total of 10 of the 11 sources for which proper motion could be reliably determined are superluminal. Only J2005+77 has no superluminal components. Three sources (OJ 287, J1224+21, and J1512-09) show motion faster than 10h-1c, requiring γpattern of at least 10h-1 (H0 = 100h km s-1 Mpc-1). We compare our results to those in the literature and find motions outside the previously observed range for four sources. While some jet components exhibit significant nonradial motion, most motion is radial. In at least two sources there are components moving radially at significantly different structural position angles. In five of six sources (3C 120, J1224+21, 3C 273, 3C 279, J1512-09, and J1927+73) that have multiple components with measurable proper motion, the innermost component is significantly slower than the others, suggesting that acceleration occurs in the jet. In the motions of individual components we observe at least one decelerating motion and two "bending" accelerations that tend to align their motions with larger scale structure. We also discuss in detail our procedures for obtaining robust kinematical results from multifrequency VLBI data spanning several epochs.