Interstellar carbon chemistry: Reaction rates of neutral atomic carbon with organic molecules

A sensitivity study of the neutral-neutral reactions C + C$_{\sf 3}$ and C + C$_{\sf 5}$ in cold dense interstellar clouds

“…The calculated capture rate constant obtained by this procedure at 298 K is equal to 2.0 × 10 −10 cm 3 s −1 with very little temperature dependence in the 10-300 K range, in good agreement with theoretical treatments for reactions between atomic carbon and assorted alkenes and alkynes (Clary et al 1994) and with the experimental results of Chastaing et al (2001) for such reactions. This thermal rate coefficient, however, does not take into account the fraction of fine-structure states leading to the attractive ground state C 4 surface.…”

“…Based on experiments, the assumption that the rate coefficient is independent of the number of hydrogen atoms on the C 5 reactant, and using the fine structure factor in Eq. (10) , we estimate the rate coefficient to form the adduct and subsequent products to be 1.2 ± (0.8) × 10 −10 cm 3 s −1 for 10-300 K. This value is somewhat less than that obtained from the capture methodology of Clary et al (1994). Compared with the value in osu.2003 (k RA = 1.4 × 10 −10 cm 3 s −1 ), the rate coefficient for the reaction C + C 5 is approximately the same but the products are C 3 + C 3 instead of C 6 .…”

“…Exothermic fragment channels exist for all of these adducts. The rate coefficients for these reactions are once again estimated from experimental values of analogous systems, which in general are smaller than the capture values, derived from the formalism of Clary et al (1994), and further reduced by a factor that allows for the multiplicity of the potential energy surfaces that correlate with the reagents and also by a factor for the multiplicity of the exit channels if relevant. Correlation involving the fine-structure states of atomic C is ignored.…”

NON-THRESHOLD, THRESHOLD, AND NONADIABATIC BEHAVIOR OF THE KEY INTERSTELLAR C + C2H2REACTION

“…In dense interstellar clouds, temperatures between 10 and 30 K are prevalent, photons from neighboring stars are unable to penetrate, and reactions between neutral atomic carbon and carbon-bearing molecules are important. The C + C 2 H 2 reaction is considered to be a key process in the synthesis of unsaturated carbon chains (Clary et al 1994;Herbst 1995aHerbst , 1995b since it has a marked non-Arrhenius behavior with a rate increasing with decreasing temperature from 300 K to 15 K (Clary et al 1994;Chastaing et al 2001).…”

Reactions between neutral species at low temperatures: Laboratory results and astrophysical modelling

“…For the reactions of unsaturated molecules with non-polar radicals, including the important case of C atoms (Clary et al 1994), the position is less clearcut, as there have been no measurements of the rate coefficients at low temperatures. Nevertheless, it is likely that when such reactions are rapid at room temperature (A; « 10" 11 cm 3 molecule -1 s -1 ), then they will occur at or close to the collisionally determined rate below 100 K. The measurement of rate coefficients for reactions of C atoms in this temperature range is another target for future CRESU experiments.…”