The chemistry of C₂and C₃in the coma of Comet C/1995 O1 (Hale-Bopp) at heliocentric distancesr_h≥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 C 2 and 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 C 2 and 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 C 2 and C 3 from C 2 H 2 , C 2 H 6 , and C 3 H 4 as their parent molecules. The required photodissociation rates of C 3 H 2 and C 3 had to be derived based on the observations. The spatial distributions of C 2 and 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 C 2 H 2 , C 2 H 6 , and C 3 H 4 , abundance ratios are obtained for heliocentric distances r h ≥ 3 AU. In comet Hale-Bopp, C 2 H 2 and C 2 H 6 were measured directly by infrared observations only at heliocentric distance r h ≤ 3 AU . 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.

The chemistry of C3and C2in cometary comae

“…Among these kinds of reactions are, as already described, photochemical and electron impact reactions. Furthermore, various collisional reactions between neutrals and ions are listed by Helbert (2002) in the formation network of C 3 and C 2 . Although these reactions were not found to be significant, they should nevertheless be included in the study of other comets, as we attempt in this work.…”

“…In this respect, the most abundant species in the coma, such as water and carbon oxides (CO and CO 2 ), are the most important. In this work, all reactions included in the study of H05, and listed with their Arrhenius coefficients in the appendix of Helbert (2002), were included when computing the column densities of C 3 and C 2 . Furthermore, all reactions presented by Schmidt et al (1988), with updates from Huebner et al (1992), were also included as background reactions in the reaction network used in this work.…”

“…For realistic production rates of C 2 H 2 and C 2 H 6 , this assumption is justified. However, if the production rates of these parent species reach unrealistically high values, their densities in the innermost coma become sufficiently high to build up species bearing three carbon atoms via collisional ion-neutral reactions included in the model of Helbert (2002). This effect could be important for C 2 H 6 in particular.…”

“…Differences in the best-fit model parameters between the two different position angles then provides a rough estimate of the uncertainties introduced by the violation of the simplifying assumptions made in this work. Helbert et al (2005) presented the most elaborate study of the formation of C 3 and C 2 , and the only one making use of a detailed chemical modeling of the cometary coma. The natural first step is therefore to compare the performance of the simpler model described above with the results presented there for comet Hale-Bopp.…”

“…This was confirmed using the values of the solar 10.7 cm radio flux, and the Brussels International Sunspot Number, as provided by the Space Environment Center of the National Oceanic and Atmospheric Administration 1 . Furthermore, these observations have a relatively low level of noise and were the easiest to be digitised from the figure presented in Helbert (2002). As the error bars of the different data points overlap, the errors could not be digitised as well.…”