A search for the origin of the interstellar comet 2I/Borisov
  The discovery of the second interstellar object 2I/Borisov on 2019 August 30
raises the question of whether it was ejected recently from a nearby stellar
system. Here we compute the asymptotic incoming trajectory of 2I/Borisov, based
on both recent and pre-discovery data extending back to December 2018, using a
range of force models that account for cometary outgassing. From Gaia DR2
astrometry and radial velocities, we trace back in time the Galactic orbits of
7.4 million stars to look for close encounters with 2I/Borisov. The closest
encounter we find took place 910kyr ago with the M0V star Ross 573, at a
separation of 0.068pc (90% confidence interval of 0.053-0.09pc) with a relative
velocity of 23km/s. This encounter is nine times closer than the closest past
encounter identified for the first interstellar object 1I/'Oumuamua. Ejection
of 2I/Borisov via a three-body encounter in a binary or planetary system is
possible, although such a large ejection velocity is unlikely to be obtained
and Ross 573 shows no signs of binarity. We also identify and discuss some
other recent close encounters, recognizing that if 2I/Borisov is more than
about 10Myr old, our search would be unlikely to find its parent system.
