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

The extraordinary activity of comet C/1995 O1 (Hale-Bopp) made it possible to observe the emission bands of the radicals $\rm C_2$ and $\rm C_3$ in the optical wavelengths range at heliocentric distances larger than 3 AU. Based on these observations, we perform an analysis of the formation of $\rm C_2$ and $\rm C_3$ 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 $\rm C_3H_2$ and $\rm C_3$ had to be derived based on the observations. The spatial distributions of $\rm C_2$ and $\rm C_3$ 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 $r_{\rm h} \ge 3$ AU. In comet Hale-Bopp, C2H2 and C2H6 were measured directly by infrared observations only at heliocentric distance $r_{\rm h} \le 3$ 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.