The First Detection of Molecular Activity in the Largest Known Oort Cloud Comet: ALMA Imaging of C/2014 UN271 (Bernardinelli–Bernstein) at 16.6 au from the Sun

Abstract

We report observations of comet C/2014 UN271 (Bernardinelli–Bernstein) carried out on UT 2024 March 8 and 17 at a heliocentric distance (rH) of 16.6 au using the Atacama Large Millimeter/submillimeter Array (ALMA). The CO (J = 2–1) line at 230 GHz was detected along with continuum emission from its dust coma and large (∼140 km) nucleus, revealing the nature of the activity drivers and outgassing kinematics of the largest Oort cloud comet discovered to date. This work presents spectrally integrated flux maps, autocorrelation spectra, production rates, and parent scale lengths for CO and a stringent upper limit for the H2CO production rate. CO outgassing displayed multiple active jets that evolved from one epoch to the next. The continuum emission was compact and spatially unresolved and is consistent with thermal emission from the large nucleus and a tentative detection of a dust coma. Complementary optical observations provided activity context for the ALMA epochs, indicating that UN271 underwent an outburst in late February before returning to a quiescent brightness in mid- to late March. These results represent the first secure detection of molecular activity reported in the literature for C/2014 UN271 and highlight the dynamic nature of this distantly active small world.