C/2014 UN271 (Bernardinelli-Bernstein): the nearly spherical cow of comets
Creators: name: Bernardinelli, Pedro H.
name: Bernstein, Gary M.
name: Montet, Benjamin T.
name: Weryk, Robert
name: Wainscoat, Richard
name: Aguena, M.
name: Allam, S.
name: Andrade-Oliveira, F.
name: Annis, J.
name: Avila, S.
name: Bertin, E.
name: Brooks, D.
name: Burke, D. L.
name: Rosell, A. Carnero
name: Kind, M. Carrasco
name: Carretero, J.
name: Cawthon, R.
name: Conselice, C.
name: Costanzi, M.
name: da Costa, L. N.
name: Pereira, M. E. S.
name: De Vicente, J.
name: Diehl, H. T.
name: Everett, S.
name: Ferrero, I.
name: Flaugher, B.
name: Frieman, J.
name: García-Bellido, J.
name: Gaztanaga, E.
name: Gerdes, D. W.
name: Gruen, D.
name: Gruendl, R. A.
name: Gschwend, J.
name: Gutierrez, G.
name: Hinton, S. R.
name: Hollowood, D. L.
name: Honscheid, K.
name: James, D. J.
name: Kuehn, K.
name: Kuropatkin, N.
name: Lahav, O.
name: Maia, M. A. G.
name: Marshall, J. L.
name: Menanteau, F.
name: Miquel, R.
name: Morgan, R.
name: Ogando, R. L. C.
name: Paz-Chinchón, F.
name: Pieres, A.
name: Malagón, A. A. Plazas
name: Rodriguez-Monroy, M.
name: Romer, A. K.
name: Roodman, A.
name: Sanchez, E.
name: Schubnell, M.
name: Serrano, S.
name: Sevilla-Noarbe, I.
name: Smith, M.
name: Soares-Santos, M.
name: Suchyta, E.
name: Swanson, M. E. C.
name: Tarle, G.
name: To, C.
name: Troxel, M. A.
name: Varga, T. N.
name: Walker, A. R.
name: Zhang, Y.
Publisher: name: arXiv
Year: 2021
Subjects: subject: Earth and Planetary Astrophysics (astro-ph.EP)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: 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 $r_h\approx29$ 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 $i=95^\circ,$ with perihelion of 10.97 au to be reached in 2031, and previous aphelion at $40,400\pm260$ 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 $q\approx18$ au on its previous perihelion passage 3.5 Myr ago. The photometric data show an unresolved nucleus with absolute magnitude $H_r=8.0,$ 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 $p_r,$ this implies a diameter of $150 (p_r/0.04)^{-0.5}$ km. There is strong evidence of brightness fluctuations at $\pm0.2$ 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 $A f ρ\approx1$ m to $\approx150$ 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 $CO_2$ and $NH_3$. More-volatile species -- $N_2,$ $CH_4,$ and $CO$ -- must be far less abundant on the sublimating surfaces.
description: 19 pages, submitted to ApJ Letters