Oort cloud Ecology II: Extra-solar Oort clouds and the origin of
  asteroidal interlopers
  We simulate the formation and evolution of Oort clouds around the 200 nearest
stars (within 16pc according to the Gaia DR2) database. This study is performed
by numerically integrating the planets and minor bodies in orbit around the
parent star and in the Galactic potential. The calculations start 1\,Gyr ago
and continue for 100Myr into the future. In this time frame, we simulate how
asteroids (and planets) are ejected from the star's vicinity and settle in an
Oort cloud and how they escape the local stellar gravity to form tidal steams.
A fraction of 0.0098 to 0.026 of the asteroids remain bound to their parent
star. The orbits of these asteroids isotropizes and circularizes due to the
influence of the Galactic tidal field to eventually form an Oort cloud between
10^4 and 2 10^5au. We estimate that 6% of the nearby stars may have a planet in
its Oort cloud. The majority of asteroids (and some of the planets) become
unbound from the parent star to become free floating in the Galactic potential.
These soli lapides remain in a similar orbit around the Galactic center as
their host star, forming dense streams of rogue interstellar asteroids and
planets.
  The Solar system occasionally passes through such tidal streams, potentially
giving rise to occasional close encounters with object in this stream. The two
recently discovered objects, 1I/(2017 Q3) 'Oumuamua and 2I/(2019 Q4) Borisov,
may be such objects. Although the direction from which an individual solus
lapis originated cannot easily be traced back to the original host, multiple
such objects coming from the same source might help to identify their origin.
At the moment the Solar system is in the bow or wake of the tidal stream of 10
of the nearby stars which might contribute considerably to the interaction
rate. (abridged)
