Carbon Chain Depletion of 2I/Borisov

Abstract

The composition of comets in the Solar System come 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 20th through October 26th from the Bok, MMT, and LBT telescopes. We identify CN in the comet's spectrum and set precise upper limits on the abundance of C2 on all dates. We use a Haser model to convert our integrated fluxes to production rates and find Q(CN) = 5.0 +/-2.0 * 10^24 mol/s on September 20 th and Q(CN) = (1.1 -1.9) * 10^24 mols/s on later dates, both consistent with contemporaneous observations. We set our lowest upper limit on a C2 production rate, Q(C2) < 1.6 10^23 mols/s on October 10th. The measured ratio upper limit for that date Q(C2)/Q(CN) < 0.095 indicates that 2I/Borisov is strongly in the (carbon chain) 'depleted' taxonomic group. The only comparable Solar System comets have detected ratios near this limit, making 2I/Borisov statistically likely to be more depleted than any known comet. Most 'depleted' comets are Jupiter Family Comets, 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.

Water Production Rates and Activity of Interstellar Comet 2I/Borisov

“…For the visits from Nov. 1 to Dec. 21, we find colors of = 19%, 14% and 13% per 100 nm between 260 and 547 nm (α = 0.054, 0.062, 0.064). These are consistent with most published results for the same wavelength range Lin et al 2020;Guzik et al 2019), but not consistent with −8 ± 7% per 100 nm reported by Kareta et al (2020). If the dust color is assumed to be constant during the short observation campaign, reddening can be considered as its mean value, 15%.…”

Early Observations of the Interstellar Comet 2I/Borisov

“…We note the non-detection of C2 lines could be due to our shallow spectrum, or because the comet is at lower activity at this epoch; the surface activity could increase significantly as it approaches perihelion. We note that our independent spectroscopic observations lead to the results similar to [7,14,15]. To summarize, both imaging and spectroscopy show that 2I/Borisov shares similar properties with comets in the solar system, hence it is unremarkable except for it being ejected from another star.…”

Characterization of the Nucleus, Morphology, and Activity of Interstellar Comet 2I/Borisov by Optical and Near-infrared GROWTH, Apache Point, IRTF, ZTF, and Keck Observations

“…However, the prediscovery photometry may also be compatible with CO 2 -driven activity where CO 2 can become active at >13 au (Womack et al 2017;Ye et al 2020). As discussed in Section 3.4, a significant production rate of H 2 O is inferred from the observed production of CN and C 2 gas (Fitzsimmons et al 2019;Kareta et al 2020) and is ∼100 kg s −1 comparable, though larger than the H 2 O∼20 kg s −1 production inferred from the detection of the [O I] 6300 Å line taken at further heliocentric distances (McKay et al 2020). Since our photometric light curve suggests that the activity of 2I is partially driven by CO, we expect the mass loss of CO to also be much higher than the mass loss from dust in the ∼10-100 kg s −1 range as it approaches perihelion.…”

“…The second ISO, 2I, seems on all accounts like an ordinary comet compared to the comets of the solar system, though it is depleted in some chemical species relative to solar system comets (Kareta et al 2020;Opitom et al 2019;Bannister et al 2020) and has an excess of CO (Bodewits et al 2020;Cordiner et al 2020). If it were not for its significantly hyperbolic orbit, 2I probably would not have warranted an in-depth scientific investigation.…”

“…Initial spectroscopic observations have revealed the presence of CN and C 2 gas in the coma of 2I with gas production rates comparable to solar system comets at similar heliocentric distances, r h (Fitzsimmons et al 2019;Kareta et al 2020;Opitom et al 2019). Using solar system comets as a guide, the production rate of CN observed in 2I implies a nuclear diameter of ∼6 km.…”