A sensitivity study of the neutral-neutral reactions C + C and C + C in cold dense interstellar clouds

¹ Université de Bordeaux, Laboratoire d'Astrophysique de Bordeaux, BP 89 33271 Floirac Cedex, France e-mail: wakelam@obs.u-bordeaux1.fr
² CNRS/INSU, UMR 5804, BP 89 33271 Floirac Cedex, France
³ Institut des Sciences Moléculaires CNRS UMR 5255, Université Bordeaux 1, 33405 Talence, France
⁴ Department of Physics, The Ohio State University, Columbus, OH 43210, USA
⁵ Departments of Astronomy and Chemistry, The Ohio State University, Columbus, OH 43210, USA
⁶ Université Montpellier II, Groupe de Recherches en Astronomie et Astrophysique du Languedoc, CNRS, UMR 5024, place Eugène Bataillon, 34095 Montpellier, France
⁷ The Chemical Physics Program, The Ohio State University, Columbus, OH 43210, USA

Received: 15 September 2008
Accepted: 19 December 2008

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 C₃ and between C and C₅, 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  yr to  yr.