2I/Borisov: A C 2-depleted interstellar comet

Aims. The discovery of the first active interstellar object 2I/Borisov provides an unprecedented opportunity to study planetary formation processes in another planetary system. In particular, spectroscopic observations of 2I allow us to constrain the composition of its nuclear ices. Methods. We obtained optical spectra of 2I with the 4.2 m William Herschel and 2.5 m Isaac Newton telescopes between 2019 September 30 and October 13, when the comet was between 2.5 au and 2.4 au from the Sun. We also imaged the comet with broadband filters on 15 nights from September 11 to October 17, as well as with a CN narrow-band filter on October 18 and 20, with the TRAPPIST-North telescope. Results. Broadband imaging confirms that the dust coma colours (Ba -a Va =a 0.82a ±a 0.02, Va -a Ra =a 0.46a ±a 0.03, Ra -a Ia =a 0.44a ±a 0.03, Ba -a Ra =a 1.28a ±a 0.03) are the same as for Solar System comets. We detect CN emission in all spectra and in the TRAPPIST narrow-band images with production rates between 1.6a ×a 1024 and 2.1a ×a 1024 molec/s. No other species are detected. We determine three-sigma upper limits for C2, C3, and OH production rates of 6a ×a 1023 molec/s, 2a ×a 1023 molec/s and 2a ×a 1027 molec/s, respectively, on October 02. There is no significant increase of the CN production rate or A(0)fρ during our observing period. Finally, we place a three-sigma upper limit on the Q(C2)/Q(CN) ratio of 0.3 (on October 13). From this, we conclude that 2I is highly depleted in C2, and may have a composition similar to Solar System carbon-chain depleted comets.