Carbon Chain Depletion of 2I/Borisov

The composition of comets in the solar system comes in multiple groups thought to encode information about their formation in different regions of the outer protosolar disk. The recent discovery of the second interstellar object, 2I/Borisov, allows for spectroscopic investigations into its gas content and a preliminary classification of it within the solar system comet taxonomies to test the applicability of planetesimal formation models to other stellar systems. We present spectroscopic and imaging observations from 2019 September 20 through October 26 from the Bok, MMT telescope (formerly the Multiple Mirror Telescope, Mount Hopkins, Arizona), and Large Binocular Telescopes. We identify CN in the comet's spectrum and set precise upper limits on the abundance of C₂ on all dates in October. We use a Haser model to convert our integrated fluxes to production rates and find Q(CN) = (1.1-1.9) ∗ 10²⁴ mols s^-1 increasing over 2019 October 1 to 26, consistent with contemporaneous observations. We set our lowest upper limit on a C₂ production rate, Q(C₂) < 1.6 ∗ 10²³ mols s^-1 on 2019 October 10. The measured upper limit ratio for that date Q(C₂)/Q(CN) < 0.1 indicates that 2I/Borisov is strongly in the (carbon-chain) "depleted" taxonomic group if there is any C₂ production at all. Most "depleted" comets are Jupiter-family comets (JFCs), perhaps indicating a similarity in formation conditions between the most depleted of the JFCs and 2I/Borisov. More work is needed to understand the applicability of our knowledge of solar system comet taxonomies onto interstellar objects and we discuss future work that could help to clarify the usefulness of the approach.