Characterising face-on accretion onto and the subsequent contraction of
  protoplanetary discs
  Observations indicate that stars generally lose their protoplanetary discs on
a timescale of about 5 Myr. Which mechanisms are responsible for the disc
dissipation is still debated. Here we investigate the movement through an
ambient medium as a possible cause of disc dispersal. The ram pressure exerted
by the flow can truncate the disc and the accretion of material with no
azimuthal angular momentum leads to further disc contraction. We derive a
theoretical model from accretion disc theory that describes the evolution of
the disc radius, mass, and surface density profile as a function of the density
and velocity of the ambient medium. We test our model by performing
hydrodynamical simulations of a protoplanetary disc embedded in a flow with
different velocities and densities. We find that our model gives an adequate
description of the evolution of the disc radius and accretion rate onto the
disc. The total disc mass in the simulations follows the theoretically expected
trend, except at the lowest density where our simulated discs lose mass owing
to continuous stripping. This stripping may be a numerical rather than a
physical effect. Some quantitative differences exist between the model
predictions and the simulations. These are at least partly caused by numerical
viscous effects in the disc and depend on the resolution of the simulation. Our
model can be used as a conservative estimate for the process of face-on
accretion onto protoplanetary discs, as long as viscous processes in the disc
can be neglected. The model predicts that in dense gaseous environments, discs
can shrink substantially in size and can, in theory, sweep up an amount of gas
of the order of their initial mass. This process could be relevant for planet
formation in dense environments.
