Europlanet Science Congress 2022
Palacio de Congresos de Granada, Spain
18 – 23 September 2022
Europlanet Science Congress 2022
Palacio de Congresos de Granada, Spain
18 September – 23 September 2022
EPSC Abstracts
Vol. 16, EPSC2022-989, 2022, updated on 23 Sep 2022
Europlanet Science Congress 2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Observations during a 20-au outburst of the largest observed Oort-cloud comet C/2014 UN271 (Bernardinelli-Bernstein)

Rosita Kokotanekova1, Michael S. P. Kelley2, Carrie E. Holt2, Cyrielle Opitom3, Silvia Protopapa4, Matthew M. Knight5, Tim Lister6, Michele T. Bannister7, Colin Snodgrass3, and Alan Fitzsimmons8
Rosita Kokotanekova et al.
  • 1Bulgarian Academy of Sciences, Institute of Astronomy and National Astronomical Observatory, Sofia, Bulgaria (
  • 2University of Maryland, USA
  • 3University of Edinburgh, UK
  • 4Southwest Research Institute, USA
  • 5United States Naval Academy, USA
  • 6Las Cumbres Observatory, USA
  • 7University of Canterbury, New Zealand
  • 8Queen’s University Belfast, UK

The discovery of long period comet C/2014 UN271 (Bernardinelli-Bernstein) announced in June 2021 quickly suggested an inactive nucleus with an absolute magnitude of HV = 7.8 mag [1], 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) [2] as well as with SkyGems Namibia [3] revealed that the comet was active at 20.18 au. Evidence was quickly found that C/2014 UN271 had been active since 2018 and possibly even active at the time it was first observed in 2014 (at 29 au) [4,5]. After the discovery announcement, follow-up observations with ALMA and HST determined that Bernardinelli-Bernstein has a cometary albedo (0.033 ± 0.009) and an effective diameter of 137 ± 17 km, distinguishing it as the largest observed Oort-cloud comet [6,7].

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 [8,9]. C/2014 UN271 was significantly brighter than those comets at the same distance, which provided an exceptional opportunity to characterize its very distant comet activity close to 20 au. In this presentation we will report the results of our observing program with FORS2 on ESO’s 8-meter VLT in July and August 2021. The VLT/FORS data are interpreted in combination with targeted observations with the 4.1-m SOAR and the long-term monitoring campaign with 1-m facilities within the LOOK Project [10,11]. 

The multi-facility long-term photometric monitoring of C/2014 UN271 enabled our team to identify three outbursts between June and September 2021, indicating that the comet's optical brightness was dominated by cometary outbursts during the VLT observing run. Our VLT/FORS2 multi-band imaging and spectroscopic observations allowed us therefore to characterize the comet’s outburst 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: [1] [2] Kokotanekova, R., et al. (2021), ATel, 14733 [3] [4] [5] Farnham, T. (2021), ATel, 14759 [6] Lellouch, E. et al. (2022) A&,659, L1, 8 [7] Hui, M.-T., et al. (2022) ApJL, 929, 1, L12, 7 [8] Hui, M.-T., et al. (2019) AJ 157 [9] Jewitt, D. et al. (2021) AJ 161, 188 [10] Lister et al., submitted [11] Kelley et al., submitted.

How to cite: Kokotanekova, R., Kelley, M. S. P., Holt, C. E., Opitom, C., Protopapa, S., Knight, M. M., Lister, T., Bannister, M. T., Snodgrass, C., and Fitzsimmons, A.: Observations during a 20-au outburst of the largest observed Oort-cloud comet C/2014 UN271 (Bernardinelli-Bernstein), Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-989,, 2022.


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