2005A&A...442.1107H

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 r_h≥3AU. In comet Hale-Bopp, C₂H₂ and C₂H₆ were measured directly by infrared observations only at heliocentric distance r_h≤3AU (Dello Russo et al., 2001, Icaus, 153, 162). 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.