C/2014 UN271 (Bernardinelli-Bernstein): the nearly spherical cow of comets

Abstract

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 $r_h\approx 29$ 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 $i=95^{\circ },$ with perihelion of 10.97 au to be reached in 2031, and previous aphelion at $40,400\pm 260$ 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 $q\approx 18$ 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 $\pm 0.2$ mag level, but no rotation period can be discerned. A coma consistent with a ``stationary' $1/\ddot{I}$ surface-brightness distribution grew in scattering cross-section at an exponential rate from $Af\ddot{I}\approx 1$ m to $\approx 150$ 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 $C{O}_2$ and $N{H}_3$. More-volatile species -- $N_2,$ $C{H}_4,$ and $CO$ -- must be far less abundant on the sublimating surfaces.

19 pages, submitted to ApJ Letters