Letter to the Editor

2I/Borisov: A C₂-depleted interstellar comet

¹ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
e-mail: copitom@eso.org
² Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK
³ Astrophysics Research Centre, Queen’s University Belfast, Belfast BT7 1NN, UK
⁴ STAR Institute, Université de Liège, Allée du 6 Aout, 19C, 4000 Liège, Belgium
⁵ Oukaimeden Observatory, Cadi Ayyad University, Marrakech, Morocco
⁶ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁷ Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
⁸ ESA NEO Coordination Centre, Largo Galileo Galilei 1, 00044 Frascati, RM, Italy
⁹ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone, RM, Italy

Received: 20 October 2019
Accepted: 22 October 2019

Abstract

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 (B − V = 0.82 ± 0.02, V − R = 0.46 ± 0.03, R − I = 0.44 ± 0.03, B − R = 1.28 ± 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 1.6 × 10²⁴ and 2.1 × 10²⁴ molec/s. No other species are detected. We determine three-sigma upper limits for C₂, C₃, and OH production rates of 6 × 10²³ molec/s, 2 × 10²³ molec/s and 2 × 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.