The influence of individual stars on the long-term dynamics of comets
  C/2014 UN271 and C/2017 K2
  In June 2021 the discovery of an unusual comet C/2014 UN271
Bernardinelli-Bernstein has been announced. Its cometary activity beyond Uranus
orbit also has refreshed interest in similar objects, including C/2017K2
PanSTARRS. Another peculiarity of these objects is the long interval of
positional data, taken at large heliocentric distances. These two comets are
suitable candidates for a detailed investigation of their long-term motion
outside the planetary zone. Using the selected orbital solutions, we aim at
estimating the orbital parameters of their orbits at the previous perihelion
passage. This might allow us to discriminate between dynamically old and new
comets. To follow the dynamical evolution of long-period comets far outside the
planetary zone, it is necessary to take into account both the perturbation
caused by the overall Galactic gravitational potential and the actions of
individual stars appearing in the solar neighborhood. To this aim, we applied
the recently published methods based on stellar perturbers ephemerides. For
C/2014 UN271 we obtained a precise orbital solution that can be propagated to
the past and to the future. For C/2017 K2 we have to limit ourselves to study
only the past motion since some signs of nongravitational effects can be found
in recent positional observations. Therefore, we use a specially selected
orbital solution suitable for past motion studies. Using these starting orbits,
we propagated both comets to their previous perihelia. We also investigated the
future motion of C/2014 UN271. Orbital evolution of these two comets appears to
be sensitive to perturbations from several stars that closely approach the Sun.
Unfortunately, some of these stars have 6D data with uncertainties too large to
obtain definitive results for the studied comets; however, it appears that both
comets were probably outside the planetary zone in the previous perihelion.
