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We study the kinematics of the M87 jet using the first year data of the KVNand VERA Array (KaVA) large program, which has densely monitored the jet at 22and 43 GHz since 2016. We find that the apparent jet speeds generally increasefrom $\approx0.3c$ at $\approx0.5$ mas from the jet base to $\approx2.7c$ at$\approx20$ mas, indicating that the jet is accelerated from subluminal tosuperluminal speeds on these scales. We perform a complementary jet kinematicanalysis by using archival Very Long Baseline Array monitoring data observed in$2005-2009$ at 1.7 GHz and find that the jet is moving at relativistic speedsup to $\approx5.8c$ at distances of $200-410$ mas. We combine the two kinematicresults and find that the jet is gradually accelerated over a broad distancerange that coincides with the jet collimation zone, implying that conversion ofPoynting flux to kinetic energy flux takes place. If the jet emission consistsof a single streamline, the observed trend of jet acceleration ($\Gamma\proptoz^{0.16\pm0.01}$) is relatively slow compared to models of a highly magnetizedjet. This indicates that Poynting flux conversion through the differentialcollimation of poloidal magnetic fields may be less efficient than expected.However, we find a non-negligible dispersion in the observed speeds for a givenjet distance, making it difficult to describe the jet velocity field with asingle power-law acceleration function. We discuss the possibility that the jetemission consists of multiple streamlines following different accelerationprofiles, resulting in jet velocity stratification.

Kinematics of the M87 jet in the collimation zone: gradual acceleration and velocity stratification