• DOI:10.3847/2041-8213/ac32d3
• Corpus ID: 237581632

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

@article{Bernardinelli2021C2014U,
  title={C/2014 UN271 (Bernardinelli-Bernstein): The Nearly Spherical Cow of Comets},
  author={Pedro H. Bernardinelli and Gary M. Bernstein and Benjamin T. Montet and Robert J. Weryk and Richard J. Wainscoat and Michel Aguena and Sahar Allam and Felipe Andrade-Oliveira and James T. Annis and Santiago {\'A}vila and E. Bertin and David Brooks and David L. Burke and A. Carnero Rosell and Matias Carrasco Kind and Jorge Carretero and R. Cawthon and C. J. Conselice and Matteo Costanzi and L. N. da Costa and M. E. S. Pereira and Juan Carlos de Vicente and H. Thomas Diehl and S. Everett and I. Ferrero and Brenna L. Flaugher and Joshua A. Frieman and Juan Garc'ia-Bellido and Enrique Gazta{\~n}aga and David W. Gerdes and Daniel Gruen and Robert A. Gruendl and Julia Gschwend and Gaston R. Guti{\'e}rrez and Samuel R. Hinton and Devon L. Hollowood and Klaus Honscheid and David J. James and Kyler W. Kuehn and Nikolay Kuropatkin and Ofer Lahav and Marcio A. G. Maia and Jennifer L. Marshall and Felipe Menanteau and Ramon Miquel and Robert Morgan and R. L. C. Ogando and Francisco Paz-Chinch{\'o}n and A. Pieres and Andr'es A. Plazas Malag'on and Mario Rodr{\'i}guez-Monroy and A. K. Romer and Aaron Roodman and E. Sanchez and Michael S. Schubnell and S. Serrano and Ignacio Sevilla-Noarbe and M. Smith and Marcelle Soares-Santos and Eric Suchyta and Mollye E. C. Swanson and Gregory G. Tarl{\'e} and C. To and Michael Troxel and T. N. Varga and Alistair R. Walker and Y. Zhang},
  journal={The Astrophysical Journal Letters},
  year={2021},
  volume={921}
}

• P. Bernardinelli, G. Bernstein, +64 authors Y. Zhang
• Published 20 September 2021
• Physics, Geology
• The Astrophysical Journal Letters

Comet C/2014 UN271 (Bernardinelli-Bernstein), incoming from the Oort cloud, is remarkable in having the brightest (and presumably largest) nucleus of any well-measured comet and having been discovered at the heliocentric distance r h ≈ 29 au, farther than any Oort cloud comet. In this work, we describe the discovery process and observations and the properties that can be inferred from images recorded until the first reports of activity in 2021 June. The orbit has i = 95°, with a perihelion of 10.97 au to be reached in 2031 and a previous aphelion at 40,400 ± 260 au. Backward integration of the orbit under a standard Galactic tidal model and known stellar encounters suggests a perihelion of q ≈ 18 au on its previous perihelion passage 3.5 Myr ago; hence, the current data could be the first ever obtained of a comet that has not been inside Uranus’s orbit in 4 Gyr. 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 the r-band geometric albedo p r , this implies a diameter of 150(pr/0.04)−0.5 km. There is strong evidence of brightness fluctuations at the ±0.2 mag level, but no rotation period can be discerned. A coma, nominally consistent with a “stationary” 1/ρ surface brightness distribution, grew in scattering cross section at an exponential rate from A f ρ ≈ 1 to ≈150 m as the comet approached from 28 to 20 au. The activity rate is consistent with a very 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, such as N2, CH4, and CO, must be far less abundant on the sublimating surfaces.