| Abstract : | One of the fundamental problems in astrochemistry is to understand the chemical evolution of protoplanetary disks and the origin of the primordial compositions of planetary systems like ours. Comets are the least-altered objects surviving from the protoplanetary disk that formed the Solar System. Hence they preserve the signature and aid in deducing the physical processes and the chemical stratification that prevailed due to the spatial and temporal variation of volatiles in the disk. The first interstellar comet observed in the Solar System is the comet 2I/Borisov, discovered on 30 August 2019. Observations of this comet show that the CO/H2O ratio is higher than that observed in Solar System comets at a heliocentric distance < 2.5 AU.
We aim to study the gas-phase coma of the comet 2I/Borisov and to see what effect the high CO/H2O ratio has on the coma chemistry. We use a multi-fluid chemical-hydrodynamical treatment for the coma, whereby the neutral species, ions and electrons are considered to flow as three separate fluids. The chemical network used to model the coma contains more than 400 chemical species connected by about 4500 reactions. Energy exchange between the three fluids due to elastic and inelastic scattering, and radiative losses are considered. Our model output results show that the presence of a large amount of CO in comet 2I/Borisov results in a high abundance of CO+ and HCO+ ions. These two ions affect the formation and destruction rates of other major ions such as H2O+, H3O+, N-bearing ions and large organic ions. We also find that the high presence of CO leads to a higher abundance of organic ions and neutrals such as CH3OH2+, CH3OCH4+ and CH3OCH3, as compared to a typical H2O-rich Solar System comet. |
