Characterizing the Distant Outbursts of the Largest Known Comet C/2014 UN271 (Bernardinelli–Bernstein)

The discovery of the long-period comet C/2014 UN271 (Bernardinelli–Bernstein) was announced in June 2021 [1] and quickly brought attention to this unique and remarkable object. Archival observations revealed that the comet had been active as far out as ~26 AU on its inbound orbit [2], making it one of only four comets with clear evidence of inbound activity beyond 20 AU [3]. Remarkably, targeted ALMA and HST observations confirmed the initially hypothesized large nucleus of the comet [4]. B-B’s radius is estimated to exceed 60 km [5,6], distinguishing it as the largest comet observed to date.

Comet B-B will reach perihelion in 2031, providing us with a unique window into its distant activity evolution between ~20 and ~11 AU. Motivated by this opportunity, we initiated a ground-based optical observing campaign. We have monitored B-B during its observability windows for the past three years, covering heliocentric distances between ~20 and ~17 AU. Our dataset consists of photometric and spectroscopic observations with ground telescopes ranging between one and eight meters in diameter, and our main goal is to characterize the distant-activity mechanisms of the comet.

Previously, we reported the results of our observing program with FORS2 on ESO’s 8-meter VLT in July and August 2021 [7]. These data revealed a peculiar coma morphology, including arc-like features that were no longer observable in the HST frames from January 2022 [6]. The coma morphology from 2021 was associated with a major outburst identified through our long-term monitoring campaign with one-meter telescopes within the LOOK Project [8]. The LOOK dataset from 2021/2022 revealed that UN271 experiences frequent outbursts—sudden and significant increases in the comet's brightness, followed by a gradual decline in coma brightness, attributed to the expansion of the released material beyond the photometric aperture [8]. Comet outbursts are characteristic of short-period comets, but they have only been observed for UN271 and C/2010 U3 (Boattini) [9] at large heliocentric distances.

The outburst activity of comet B-B has repeated in the following two observing windows, motivating us to analyze our extensive dataset in an attempt to identify its driving mechanisms. In this work, we will present the brightness evolution from three years of LOOK monitoring and discuss our findings from the search for periodicity in the photometric variation. Our dataset includes multiple epochs of deep imaging with VLT/FORS2 and SOAR, allowing us to explore the coma morphology in its quiescent state as well as during outburst. Our analysis of the comet outbursts also includes a coma color study from VLT/MUSE and FORS2 spectra, as well as SOAR, LOOK, and FORS2 photometry.

The combination of these observations reveals how the coma’s brightness, morphology, and spectral slope evolve and allows us to investigate the mechanisms behind the comet’s activity pattern. We will present our results from the first three years of the monitoring campaign and discuss our hypotheses for the outbursting activity in the context of other teams' JWST and ALMA spectroscopic observations of the comet in 2023/2024 that investigate the comet’s volatile content.

References:
[1] https://minorplanetcenter.net/mpec/K21/K21M53.html; [2] Farnham, T., et al. (2021) PSJ, 2, 6, id.236, 8; [3] Hui, M.-T., et al. (2024) AJ, 167, 3, id.140, 16; [4] Bernardinelli, P. H., et al. 2021b, ApJ, 921, L37; [5] Lellouch, E. et al. (2022) A&,659, L1, 8; [6] Hui, M.-T., et al. (2022) ApJL, 929, 1, L12, 7; [7] Kokotanekova et al.. (2022) EPSC; [8] Kelley, M. S. P., et al. (2023) ApJL, 933, 2, id.L44, 10; [9] Hui, M.-T., et al. (2019) AJ 157.