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Comet C/2014 UN271 (Bernardinelli-Bernstein) and the Rate of Long-Period Comet Apparitions

Presentation #304.02 in the session Comets and ISOs: Dynamics, Origins and Theory.

Published onOct 20, 2022
Comet C/2014 UN271 (Bernardinelli-Bernstein) and the Rate of Long-Period Comet Apparitions

Comet C/2014 UN271 (Bernardinelli-Bernstein) [henceforth BB] is a long-period comet [LPC] discovered by the Dark Energy Survey 29 au from the Sun. BB has been active at heliocentric distances of at least 26 au [1-3]. The comet is likely dynamically new, with a perihelion distance q > 17 au during its previous appearance ~3 Myr ago [1,4]. BB will pass perihelion near 11 au in 2031. The diameter (d) of its nucleus is estimated to be 100-150 km [1,5,6], making BB the largest LPC known. The discovery of such a big nucleus is surprising, even for a distant comet, given that (1) the size-frequency distribution (SFD) of large LPCs is said to fall rapidly with increasing size [7,8] (although see [9]) and (2) both surveys and dynamical models indicate that the perihelion distribution of LPCs only rises slowly out to q ~ 15 au [8,10]. Using the SFD inferred for LPCs observed by Pan-STARRS for LPCs as big as 20 km [8], and extrapolating to 100 km, we estimate that the average number of new LPCs at any given time with d > 100 km and q < 15 au within 30 au of the Sun is ~ 0.0005. If, instead, we use the SFD for ecliptic comets from [11], which is primarily based on Jupiter Trojans [12], we would expect 0.04 such bodies. It is not clear which estimate is better. [8] relies on photometry from many observers and a complex debiasing procedure; in most cases, the signal from the nucleus is much smaller than that from the coma [13]. [11] assumes that outer solar system small-body populations all derive from the same parent, the primordial massive Kuiper Belt, modified by collisional disruption. Models [14,15] find that if the dynamical instability occurred early and icy planetesimals are weak, comets smaller than ~10 km should be fragments, while larger comets are primordial and should have a shallower SFD. In this case the expected number of “BBs” would be between 0.0005 and 0.04. The Vera Rubin Observatory is expected to measure the SFD of LPCs up to ~100 km directly, so we should soon know just how lucky we are.

We thank Alan Chamberlin and Davide Farnocchia for discussions.

[1] Bernardinelli et al., ApJ Lett 921, L37, 2021. [2] Farnham et al., PSJ 2, 236, 2021. [3] Kelley et al., ApJ Lett 933, L44, 2022. [4] Dybczyński and Królikowska, A&A 660, A100, 2022. [5] Lellouch et al., A&A 659, L1, 2022. [6] Hui et al., ApJ Lett 929, L12, 2022. [7] Silsbee and Tremaine, AJ 152, 103, 2016. [8] Boe et al., Icarus 333, 252, 2019. [9] Bauer et al., AJ 154, 53, 2017. [10] Vokrouhlický et al., AJ 157, 181, 2019. [11] Nesvorný et al., ApJ 845, 27, 2017. [12] Uehata et al., AJ 163, 213, 2022. [13] Hui and Li, PASP 130, 104501, 2018. [14] Bottke et al., LPSC 53, 2678, 2022. [15] Benavidez et al., MNRAS 514, 4876, 2022.

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