Changes in orientation and shape of protoplanetary discs moving through
  an ambient medium
  Misalignments between the orbital planes of planets and the equatorial planes
of their host stars have been observed in our solar system, in transiting
exoplanets, and in the orbital planes of debris discs. We present a mechanism
that causes such a spin-orbit misalignment for a protoplanetary disc due to its
movement through an ambient medium. Our physical explanation of the mechanism
is based on the theoretical solutions to the Stark problem. We test this idea
by performing self-consistent hydrodynamical simulations and simplified
gravitational $N$-body simulations. The $N$-body model reduces the mechanism to
the relevant physical processes. The hydrodynamical simulations show the
mechanism in its full extent, including gas-dynamical and viscous processes in
the disc which are not included in the theoretical framework. We find that a
protoplanetary disc embedded in a flow changes its orientation as its angular
momentum vector tends to align parallel to the relative velocity vector. Due to
the force exerted by the flow, orbits in the disc become eccentric, which
produces a net torque and consequentially changes the orbital inclination. The
tilting of the disc causes it to contract. Apart from becoming lopsided, the
gaseous disc also forms a spiral arm even if the inclination does not change
substantially. The process is most effective at high velocities and
observational signatures are therefore mostly expected in massive star-forming
regions and around winds or supernova ejecta. Our $N$-body model indicates that
the interaction with supernova ejecta is a viable explanation for the observed
spin-orbit misalignment in our solar system.
