Formation and destruction of polycyclic aromatic hydrocarbon clusters in the interstellar medium

¹ Centre d'Étude Spatiale des Rayonnements, CNRS-Université Paul Sabatier, Observatoire Midi-Pyrénées, 9 Av. du colonel Roche, BP 4346, 31028 Toulouse Cedex 4, France e-mail: mathias.rapacioli@chemie.tu-dresden.de
² Laboratoire de Chimie et Physique Quantiques, IRSAMC, Université Paul Sabatier, 118 Route de Narbonne, 31062 Toulouse Cedex, France
³ Laboratoire de Photophysique Moléculaire, CNRS Bât. 210, Université Paris-Sud, 91405 Orsay Cedex, France

Received: 11 April 2006
Accepted: 20 June 2006

Abstract

Aims.The competition between the formation and destruction of coronene clusters under interstellar conditions is investigated theoretically.

Methods.The unimolecular nucleation of neutral clusters is simulated with an atomic model combining an explicit classical force field and a quantum tight-binding approach. Evaporation rates are calculated in the framework of the phase space theory and are inserted in an infrared emission model and compared with the growth rate constants.

Results.It is found that, in interstellar conditions, most collisions lead to cluster growth. The time evolution of small clusters (containing up to 312 carbon atoms) was specifically investigated under the physical conditions of the northern photodissociation region of NGC 7023. These clusters are found to be thermally photoevaporated much faster than they are reformed, thus providing an interpretation for the lowest limit of the interstellar cluster size distribution inferred from observations. The effects of ionizing the clusters and density heterogeneities are also considered. Based on our results, the possibility that PAH clusters could be formed in PDRs is critically discussed.