Presentation #301.04 in the session “Distant Activity in Comets: C/2017 K2 & C/2014 UN271”.
The discovery of long period comet C/2014 UN271 (Bernardinelli-Bernstein) announced in June 2021 quickly gained broad attention. The discovery images suggested an inactive nucleus with an absolute magnitude of HV = 7.8 mag , which implied a diameter between 130 and 260 km assuming geometric albedos between 2% and 8%. Immediate follow-up observations with our Las Cumbres Observatory (LCO) Outbursting Objects Key project (LOOK)  as well as with SkyGems Namibia  revealed that the comet is currently active at 20.18 au. Moreover, there is building evidence that C/2014 UN271 had been active since 2018 and possibly even at the time it was first observed in 2014 (at 29 au) [4,5]. Prior to the observations of C/2014 UN271, the most distant comet discoveries were C/2010 U3 and C/2017 K2 which were made between 15 and 20 au but for which pre-discovery images indicate activity beyond 20 au [6,7]. C/2014 UN271 is significantly brighter than those comets at the same distance which provides an exceptional opportunity to characterize its very distant comet activity close to 20 au, and how it evolves as it approaches perihelion at 10.95 au in 2031. In this presentation we will report the results of our ongoing observing program with FORS2 on ESO’s 8-meter VLT. The VLT/FORS data are interpreted in combination with targeted observations with the 4.1-m SOAR and the long-term monitoring campaign within the LOOK Key Project. Our multi-band imaging and spectroscopic observations allow us to characterize the comet’s dust activity in terms of spectral slope and coma morphology, including arc-like features. We will also present our efforts to characterize the comet’s short-term variability and rotation period. References:  https://minorplanetcenter.net/mpec/K21/K21M53.html  Kokotanekova, R., et al. (2021), ATel, 14733  https://minorplanetcenter.net/mpec/K21/K21M83.html  https://minorplanetcenter.net/mpec/K21/K21M83.html  Farnham, T. (2021), ATel, 14759  Hui, M.-T., et al. (2019) AJ 157  Jewitt, D. et al. (2021) AJ 161, 188.