  We carry out two-fluid, two-dimensional global hydrodynamic simulations to
test whether protostellar infall can trigger Rossby wave instability (RWI) in
protoplanetry disks. Our results show that infall can trigger the RWI and
generate vortices near the outer edge of the mass landing on the disk (i.e.
centrifugal radius). We find that the RWI is triggered under a variety of
conditions, although the details depend on the disk parameters and the infall
pattern. The common key feature of triggering the RWI is the steep radial
gradient of the azimuthal velocity induced by the local increase in density at
the outer edge of the infall region. Vortices form when the instability enters
the nonlinear regime. In our standard model where self-gravity is neglected,
vortices merge together to a single vortex within $\sim 20$ local orbital
times, and the merged vortex survives for the remaining duration of the
calculation ($> 170$ local orbital times). The vortex takes part in outward
angular momentum transport, with a Reynolds stress of $\lesssim10^{-2}$. Our
two-fluid calculations show that vortices efficiently trap dust particles with
stopping times of the order of the orbital time, locally enhancing the dust to
gas ratio for particles of the appropriate size by a factor of $\sim 40$ in our
standard model. When self-gravity is considered, however, vortices tend to be
impeded from merging and may eventually dissipate. We conclude it may well have
that protoplanetary disks have favorable conditions for vortex formation during
the protostellar infall phase, which might enhance early planetary core
formation.
