Carbon Chain Depletion of 2I/Borisov (1910.03222v2)

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 to October 26th at 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 20th and Q(CN) = 1.1 - 1.9 * 10^24 mol/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 mol/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 similiarity 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 clarify the usefulness of the approach.

• The paper identifies strong carbon chain depletion in 2I/Borisov by analyzing its CN and non-detectable C2 emissions.
• It employs spectroscopic data from three major telescopes and a Haser model to quantify CN production rates ranging from 5.0 ± 2.0×10^24 to 1.1–1.9×10^24 molecules/s.
• The findings align 2I/Borisov with a depleted comet taxonomy, suggesting similar formation processes to Jupiter Family Comets and expanding interstellar formation models.

Spectroscopic Analysis and Classification of Comet 2I/Borisov: Carbon Chain Depletion Observations

The paper titled "Carbon Chain Depletion of 2I/Borisov" explores the spectroscopic characteristics of the comet 2I/Borisov, the second identified interstellar object. This research provides a critical spectroscopic analysis, aiming to classify 2I/Borisov within the known comet taxonomies of the Solar System by investigating its gas composition and using this classification to test the broader applicability of current planetesimal formation models to other stellar systems.

Key Observations and Methodology

The observational data were acquired using three major telescopes: the Bok Telescope, the MMT Telescope, and the Large Binocular Telescope. Spectroscopic and imaging observations were systematically conducted between September 20 and October 26, 2019. The paper focused on analyzing the comet's gas emissions, primarily the CN (cyanogen) and potential C2 emissions, employing a Haser model to elucidate production rates. This model assists in converting integrated flux measures from spectral data to molecular production rates.

Findings and Numerical Results

The detection of CN emissions was consistent across the observation dates, with noted production rates ranging, for example, from Q(CN) = 5.0 ± 2.0 × 10^24 molecules/s on September 20 to Q(CN) = 1.1–1.9 × 10^24 molecules/s on later dates. Crucially, the paper reported no detectable C2 emissions, establishing an upper limit on C2 production, e.g., Q(C2) < 1.6 x 10^23 molecules/s on October 10. The critical production rate ratio (Q(C2)/Q(CN)) was found to be less than 0.095, categorizing 2I/Borisov as 'depleted' in terms of its Carbon chain. Compared to Solar System comets, this places 2I/Borisov into a category showing significant C2 depletion, akin only to a handful of known depleted comets.

Taxonomical and Theoretical Implications

The paper posits that the strong carbon depletion detected in 2I/Borisov aligns it with the depleted taxonomic group within Solar System comet classifications, suggesting similarities with Jupiter Family Comets (JFCs). The findings imply that the conditions in interstellar cometary formation might share characteristics with those of the depleted comets found in the Solar System, potentially extending our understanding of planetesimal and comet formation theories beyond our stellar neighborhood.

The absence of C2 and the precise measurement of the CN/C2 ratio propose noteworthy implications for the chemical processes involved in comet formation both in our Solar System and interstellar environments. The paper suggests further investigation into CN and C2 correlation and their relationship with other carbon chain molecules like C3 could lead to more profound insights into the protosolar disk processes.

Future Directions

The continuation of spectroscopic monitoring of 2I/Borisov is emphasized to ascertain whether this interstellar object maintains its depletion levels across different conditions or if variations would suggest alternative gas release mechanisms. The identification of its depletion levels also raises the importance of verifying whether such characteristics are consistent with its predicted formation environment. Moreover, this paper lays the groundwork for future comparative analysis with other interstellar objects that may be discovered.

In conclusion, this research on 2I/Borisov significantly contributes to our understanding of interstellar objects, particularly regarding their classification and chemical compositions in relation to known Solar System comet taxonomies. It challenges existing formation models and suggests intricate similarities across cosmic formation environments, urging further investigation and interstellar comparison for more comprehensive insights.