The carbon monoxide-rich interstellar comet 2I/Borisov (2004.08972v1)

Abstract: Interstellar comets offer direct samples of volatiles from distant protoplanetary disks. 2I/Borisov is the first notably active interstellar comet discovered in our solar system[1]. Comets are condensed samples of the gas, ice, and dust that were in a star's protoplanetary disk during the formation of its planets and inform our understanding on how chemical compositions and abundances vary with distance from the central star. Their orbital migration moves volatiles[2], organic material, and prebiotic chemicals in their host system[3]. In our solar system, hundreds of comets have been observed remotely, and a few have been studied up close by space missions[4]. However, knowledge of extrasolar comets has been limited to what could be gleaned from distant, unresolved observations of cometary regions around other stars, with only one detection of carbon monoxide[5]. Here we report that the coma of 2I/Borisov contains significantly more CO than H2O gas, with abundances of at least 173%, more than three times higher than previously measured for any comet in the inner (<2.5 au) solar system[4]. Our ultraviolet observations of 2I/Borisov provide the first glimpse into the ice content and chemical composition of the protoplanetary disk of another star that is substantially different from our own.

Analysis of the Carbon Monoxide-Rich Interstellar Comet 2I/Borisov

The paper presents a comprehensive analysis of the comet 2I/Borisov, highlighting its characteristics as the first notably active interstellar comet observed in our solar system with a distinctly high carbon monoxide to water (CO/H<sub\>2</sub>O) ratio. Extensive ultraviolet observations using the Hubble Space Telescope (HST) have revealed that this interstellar comet's coma has CO abundances at least 173%, more than triple the levels documented for any comet within the inner solar system (<2.5 au). This paper provides valuable insights into the comet's chemical composition, origin, and thermal history, with significant implications for understanding the differences between our solar system and other protoplanetary disks.

Key Findings and Observations

1. Chemical Composition and Production Rates: Observations made between December 2019 and January 2020 using the HST's Cosmic Origins Spectrograph reported CO production rates repurposing around 30-50 kg/s, evidently remaining consistent even after 2I/Borisov's perihelion passage. In stark contrast, H<sub\>2</sub>O production rates peaked near perihelion and declined sharply afterward. These observations elucidate the inherent high CO/H<sub\>2</sub>O abundance in comparison to comets from our solar system, typically ranging from 0.2% to 23%, with average values near 4%.
2. Implications of CO Abundance: The analysis concluded that the substantial CO content, which persists post-perihelion from 2I/Borisov indicates a primordial quality, suggesting the comet formed in regions with temperatures below 25 K. The high CO to H<sub\>2</sub>O ratio along with a near sixfold increase in carbon relative to oxygen compared to solar system comets infers a chemically distinct formation environment. Such peculiarities imply that the protoplanetary disk where 2I/Borisov originated possessed a unique enrichment of carbon.
3. Dynamical and Evolutionary Considerations: Given 2I/Borisov’s characteristics, such a comet is likely representative of a typical interstellar comet rather than being an exception. Consequently, the paper posits that massive dynamical interactions in the host system could have facilitated the ejection and subsequent preservation of comets like 2I/Borisov beyond its CO snowline.

Implications and Future Directions

The insights gained from 2I/Borisov offer profound implications for astrophysics and planetary science by challenging existing paradigms about comet formation and volatile preservation in protoplanetary environments. Despite its interstellar origins, 2I/Borisov shares several properties with solar system comets, such as dust color and brightness trends. Yet, the heightened CO/H<sub\>2</sub>O ratio distinctly differentiates it, implying varied chemical processes and conditions in its original system. These findings urge the necessity to revisit models of protoplanetary disk compositions, particularly around M-type stars where similar properties could prevail, influencing snowline dynamics and volatile retention.

Further studies should focus on identifying other interstellar cometary bodies with similar volatile compositions to discern patterns indicating distinct solar system contrasts. Investigation into high CO-content cometary nuclei provides an opportunity to broaden the understanding of diverse chemical environments present during comet formation. Future observations, potentially complemented by advanced spectroscopic techniques, could refine these evolutionary narratives, offering insights into the broader galactic chemical landscape.

The paper exemplifies the power of detailed spectral analysis in unveiling the profound compositional characteristics of interstellar objects, serving as a catalyst for further exploration into the cosmic origins and evolutionary dynamics of cometary bodies across diverse astrophysical settings.