The chemistry of C₂ and C₃ in the coma of Comet C/1995 O1 (Hale-Bopp) at heliocentric distances rh>2.9 AU

Abstract

The extraordinary activity of comet C/1995 O1 (Hale-Bopp) made it possible to observe the emission bands of the radicals C₂ and C₃ in the optical wavelengths range at heliocentric distances larger than 3 AU. Based on these observations, we perform an analysis of the formation of C₂ and C₃ in a comet coma at large heliocentric distances.We present the most complete chemical reaction network used until today, computing the formation of C₂ and C₃ from C₂H₂, C₂H₆, and C₃H₄ as their parent molecules. The required photodissociation rates of C₃H₂ and C₃ had to be derived based on the observations. The spatial distributions of C₂ and C₃ calculated with the chemical model show good agreement with the observations over the whole range of heliocentric distances covered in this work. Based on the production rates for C₂H₂, C₂H₆, and C₃H₄, abundance ratios are obtained for heliocentric distances rh ≥ 3 AU. In comet Hale-Bopp, C₂H₂ and C₂H₆ were measured directly by infrared observations only at heliocentric distance rh ≤ 3 AU (Dello Russo et al. 2001). The model presented here greatly extends the heliocentric distance range over which hydrocarbons can be studied in the coma of comet Hale-Bopp. We discuss possible indications of these abundance ratios for the formation region of comet Hale-Bopp.