MOJAVE XII: Acceleration and Collimation of Blazar Jets on Parsec Scales
  We report on the acceleration properties of 329 features in 95 blazar jets
from the MOJAVE VLBA program. Nearly half the features and three-quarters of
the jets show significant changes in speed and/or direction. In general,
apparent speed changes are distinctly larger than changes in direction,
indicating that changes in the Lorentz factors of jet features dominate the
observed speed changes rather than bends along the line of sight. Observed
accelerations tend to increase the speed of features near the jet base,
$\lesssim 10-20$ parsecs projected, and decrease their speed at longer
distances. The range of apparent speeds at fixed distance in an individual jet
can span a factor of a few, indicating that shock properties and geometry may
influence the apparent motions; however, we suggest that the broad trend of jet
features increasing their speed near the origin is due to an overall
acceleration of the jet flow out to de-projected distances of order $10^2$
parsecs, beyond which the flow begins to decelerate or remains nearly constant
in speed. We estimate intrinsic rates of change of the Lorentz factors in the
galaxy frame of order $\dot{\Gamma}/\Gamma \simeq 10^{-3}$ to $10^{-2}$ per
year which can lead to total Lorentz factor changes of a factor of a few on the
length scales observed here. Finally, we also find evidence for jet collimation
at projected distances of $\lesssim 10$ parsecs in the form of the non-radial
motion and bending accelerations that tend to better align features with the
inner jet.
