Reaction Networks for Interstellar Chemical Modelling: Improvements and Challenges

We survey the current situation regarding chemical modelling of the synthesis of molecules in the interstellar medium. The present state of knowledge concerning the V. Wakelam ( ) Observatoire Aquitain des Sciences de l’Univers, Université de Bordeaux, BP 89, 33271 Floirac Cedex, France e-mail: [email protected] V. Wakelam Laboratoire d’Astrophysique de Bordeaux, CNRS, UMR 5804, BP 89, 33271 Floirac Cedex, France I.W.M. Smith University Chemical Laboratories, Lensfield Road, Cambridge CB2 1EW, UK E. Herbst Departments of Physics, Astronomy, and Chemistry, The Ohio State University, Columbus, OH 43210, USA J. Troe Institut für Physikalische Chemie, Universität Göttingen, Tammannstr. 6, 37077 Göttingen, Germany J. Troe Max-Planck-Institut für Biophysikalische Chemie, Am Fassberg 11, 37077 Göttingen, Germany W. Geppert Department of Physics, University of Stockholm, Roslagstullbacken 21, 10691 Stockholm, Sweden H. Linnartz · K. Öberg · H.M. Cuppen Sackler Laboratory for Astrophysics, Leiden Observatory, University of Leiden, P.O. Box 9513, 2300 RA Leiden, The Netherlands K. Öberg Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 42, Cambridge, MA 02138, USA E. Roueff Universit Paris 7 and Observatoire de Paris, LUTH, UMR 8102 CNRS, Place J. Janssen, 92195 Meudon, France 14 V. Wakelam et al. rate coefficients and their uncertainties for the major gas-phase processes—ion-neutral reactions, neutral-neutral reactions, radiative association, and dissociative recombination—is reviewed. Emphasis is placed on those key reactions that have been identified, by sensitivity analyses, as ‘crucial’ in determining the predicted abundances of the species observed in the interstellar medium. These sensitivity analyses have been carried out for gas-phase models of three representative, molecule-rich, astronomical sources: the cold dense molecular clouds TMC-1 and L134N, and the expanding circumstellar envelope IRC +10216. Our review has led to the proposal of new values and uncertainties for the rate coefficients of many of the key reactions. The impact of these new data on the predicted abundances in TMC-1 and L134N is reported. Interstellar dust particles also influence the observed abundances of molecules in the interstellar medium. Their role is included in gas-grain, as distinct from gas-phase only, models. We review the methods for incorporating both accretion onto, and reactions on, the surfaces of grains in such models, as well as describing some recent experimental efforts to simulate and examine relevant processes in the laboratory. These efforts include experiments on the surface-catalyzed recombination of hydrogen atoms, on chemical processing on and in the ices that are known to exist on the surface of interstellar grains, and on desorption processes, which may enable species formed on grains to return to the gas-phase.