The Influence Of Individual Stars On The long-Term Dynamics Of Comets C/2014 UN271 And C/2017 K2

Context. In June 2021, the discovery of an unusual comet C/2014 UN271 (Bernardinelli-Bernstein) was announced. Its cometary activity beyond the orbit of Uranus has also refreshed interest in similar objects, including C/2017 K2 (PANSTARRS). Another peculiarity of these objects is the long interval of positional data, taken at large heliocentric distances. Aims. These two comets are suitable candidates for a detailed investigation of their long-term motion outside the planetary zone. Using the carefully selected orbital solutions, we aim to estimate the orbital parameters of their orbits at the previous perihelion passage. This might allow us to discriminate between dynamically old and new comets. Methods. 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 the ephemerides of stellar perturbers. Results. For C/2014 UN271, we obtained a precise orbital solution that can be propagated into the past and the future. For C/2017 K2, we have to limit ourselves to studying the past motion because 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. Conclusions. The orbital evolution of these two comets appears to be sensitive to perturbations from several stars that closely approach the Sun. To the detriment of our analysis, the errors on the 6D data for some of these stars are too large to obtain definitive results for the studied comets; nevertheless, we deduce that both comets were probably outside the planetary zone in the previous perihelion.