The Highly Unusual Behavior and Chemical Composition of Interstellar Comet 2I/Borisov

Abstract

We report on imaging, spectroscopy, and narrowband photometry of interstellar comet 2I/Borisov obtained at Lowell Observatory from 2019 September 13 to December 16. We measured chemical composition on four nights before perihelion, from October 4 to November 25 (2.466 to 2.028 au), with CN detections on all nights. Our measurements from October 27 represent the earliest reported detections of C$_2$ and C$_3$, where C$_2$-to-CN and C$_3$-to-CN log production rate ratios were $-$0.7 and $-$1.3, respectively. Both ratios increased by nearly 4$\times$ to $-$0.3 and $-$0.9 by November 25, an extremely large increase with heliocentric distance that is unprecedented in the Lowell Observatory comet database. We additionally observed OH and NH on November 25, with a derived H$_2$O production rate of 4.5$\times $10$^{26}$ mol s$^{-1}$ and very high log production rate ratios for CN-to-OH ($-$2.0) and NH-to-OH ($-$1.7). Monitoring from broadband $R$ images from September 13 to December 16 suggests dust ($A(0^{\circ})f\rho$) production peaked around October 19, 50 days before its December 8 perihelion passage. Narrowband images from October 4 and 27 reveal a nearly symmetric CN distribution in Borisov's coma, suggesting activity over its entire sunward hemisphere and possibly its entire nucleus. Our results, combined with those from others, reveal a chemical composition and behavior with heliocentric distance unlike any other comet we have observed.