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)