Abstract
Abstract 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 ( p r / 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.
Highlights
Our knowledge of the content of the Oort cloud is highly fragmentary; all inferences are based upon the small subset of its members that are torqued into orbits with perihelia q 10 au and, until recently, only the subset of these that develop comae bright enough to be noticed as comets
The cometary activity makes the objects easier to find and makes it easier to identify the composition of the surface volatiles, but it can obscure the properties of the nuclear body
Comet BB was discovered as part of the search for transNeptunian objects (TNOs) in the 80,000 exposures taken by the Dark Energy Survey (DES) in the period 2013–2019 described fully in Bernardinelli et al (2021)
Summary
Our knowledge of the content of the Oort cloud is highly fragmentary; all inferences are based upon the small subset of its members that are torqued into orbits with perihelia q 10 au and, until recently, only the subset of these that develop comae bright enough to be noticed as comets. We will summarize the observations in which BB was discovered and the inferences about its composition and history that can be made from these and other images taken until the recent first announcement of detectable activity. Comet BB was discovered as part of the search for transNeptunian objects (TNOs) in the 80,000 exposures taken by the Dark Energy Survey (DES) in the period 2013–2019 described fully in Bernardinelli et al (2021). The DES search should not be used to estimate the density of Oort cloud comets like BB, though we can say that any object having rh > 29 au and mr < 23.8 for >2 yr of DES observing and lying within the 5000 deg footprint of DES (see Figure 5 of Bernardinelli et al 2021) would have a high probability of detection.
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