2I/Borisov: A C${}_2$ depleted interstellar comet

Abstract

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. 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. Broadband imaging confirms that the dust coma colours (B-V=0.82$\pm$0.02, V-R=0.46$\pm$0.03, R-I=0.44$\pm$0.03, B-R=1.28$\pm$0.03) are the same as for Solar System comets. We detect CN emission in all spectra and in the TRAPPIST narrow-band images with production rates between 1.6$\times 10^{24}$ and 2.1$\times 10^{24}$ molec/s. No other species are detected. We determine three-sigma upper limits for C${}_2$, C${}_3$, and OH production rates of 6$\times 10^{23}$ molec/s, 3$\times 10^{23}$ molec/s and 2$\times 10^{27}$ molec/s, respectively, on October 02. There is no significant increase of the CN production rate or A(0)f$\rho$ during our observing period. Finally, we place a three-sigma upper limit on the Q(C${}_2$)/Q(CN) ratio of 0.3 (on October 13). From this, we conclude that 2I is highly depleted in C${}_2$, and may have a composition similar to Solar System carbon-chain depleted comets.

Revised version, Accepted for publication in A&A Letters

References in corpus (5)

• Interstellar Interloper 1I/2017 U1: Observations from the NOT and WIYN Telescopes
• An Observational Upper Limit on the Interstellar Number Density of Asteroids and Comets
• Spectroscopy and thermal modelling of the first interstellar object 1I/2017 U1 'Oumuamua
• 1I/2017 U1 (`Oumuamua) is Hot: Imaging, Spectroscopy and Search of Meteor Activity
• Ensemble Properties of Comets in the Sloan Digital Sky Survey

Cited by in corpus (8)

• Pre-discovery Activity of New Interstellar Comet 2I/Borisov Beyond 5 AU
• Detection of a Water Tracer in Interstellar Comet 2I/Borisov
• Detection of Diatomic Carbon in 2I/Borisov
• Carbon Chain Depletion of 2I/Borisov
• A search for the origin of the interstellar comet 2I/Borisov
• Searching for water ice in the coma of interstellar object 2I/Borisov
• FLAMINGOS-2 infrared photometry of 2I/Borisov
• Is Interstellar Object 2I/Borisov a Stardust Comet? Predictions for the Post Perihelion Period