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

Abstract

Aims. Chemical networks used for models of interstellar clouds contain many reactions, some of them with poorly determined rate coefficients and/or products. In this work, we report a method for improving the predictions of molecular abundances using sensitivity methods and ab initio calculations. Methods. Based on the chemical network osu.2003, we used two different sensitivity methods to determine the most important reactions as a function of time for models of dense cold clouds. Of these reactions, we concentrated on those between C and C 3 and between C and C 5 , both for their effect on specific important species such as CO and for their general effect on large numbers of species. We then used ab initio and kinetic methods to determine an improved rate coefficient for the former reaction and a new set of products, plus a slightly changed rate coefficient for the latter. Results. Putting our new results in a pseudo-time-dependent model of cold dense clouds, we found that the abundances of many species are altered at early times, based on large changes in the abundances of CO and atomic C. We compared the effect of these new rate coefficients/products on the comparison with observed abundances and found that they shift the best agreement from 3 × 10 4 yr to (1−3) × 10 5 yr.

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“…As in any nucleation based work, we also assume that grain formation and growth take place by the addition of single carbon atoms, and that grain erosion as well takes place by removing one atom at a time from a cluster. Reactions that cause the splitting of a cluster in two fragments are not considered, even though they might be important, especially at low temperatures (see, e.g., Wakelam et al 2009). Another important issue is whether the chemical and nucleation approaches are fully consistent with each other.…”

RECOMMENDED THERMAL RATE COEFFICIENTS FOR THE C + H3 + REACTION AND SOME ASTROCHEMICAL IMPLICATIONS

“…Moreover, our rate coefficient for this channel is larger than that of Reaction (1) Reducing the uncertainty of the rate coefficient for Reaction (1) has been identified by Wakelam et al (2009Wakelam et al ( , 2010 as being critically important in order to more reliably predict the abundances for a large number of species observed in dense molecular clouds. Similarly, Vasyunin et al (2008) has shown that the uncertainty in this rate coefficient hinders our ability to reliably predict chemical abundances in protoplanetary disks.…”