C/2014 UN271(Bernardinelli-Bernstein): The Nearly Spherical Cow of Comets

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C/2014 UN271 (Bernardinelli-Bernstein) is a comet incoming from the Oort cloud which is remarkable in having the brightest (and presumably largest) nucleus of any well-measured comet, and having been discovered at heliocentric distance rh≈29 au farther than any Oort-cloud member. We describe the properties that can be inferred from images recorded until the first reports of activity in June 2021. The orbit has i=95∘, with perihelion of 10.97 au to be reached in 2031, and previous aphelion at 40,400±260 au. Backwards integration of the orbit under a standard Galactic tidal model and known stellar encounters suggests this is a pristine new comet, with a perihelion of q≈18 au on its previous perihelion passage 3.5 Myr ago. The photometric data show an unresolved nucleus with absolute magnitude Hr=8.0, colors that are typical of comet nuclei or Damocloids, and no secular trend as it traversed the range 34--23 au. For r-band geometric albedo pr, this implies a diameter of 150(pr/0.04)−0.5 km. There is strong evidence of brightness fluctuations at ±0.2 mag level, but no rotation period can be discerned. A coma consistent with a ``stationary' 1/ρ surface-brightness distribution grew in scattering cross-section at an exponential rate from Afρ≈1 m to ≈150 m as the comet approached from 28 to 20 au. The activity is consistent with a simple model of sublimation of a surface species in radiative equilibrium with the Sun. The inferred enthalpy of sublimation matches those of CO2 and NH3. More-volatile species -- N2, CH4, and CO -- must be far less abundant on the sublimating surfaces.

astro-ph.EP, astro-ph.EP
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Astrophysical Journal Letters
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American Astronomical Society
STFC (ST/T003081/1)