The mechanisms of nucleation and growth of carbon dust particles in circumstellar
envelopes of carbon-rich stars in the red giant and AGB phases of their evolution are
poorly understood. It has been proposed that the transition of gas phase species to solid
particles, is achieved by the formation of a critical nucleus composed of two PAHs held
together by van der Waals forces. Some insights into the validity of the nucleation of PAH
molecules in the envelope can be gained through the investigation of the thermodynamics of
dimers, representing the first stage towards condensation. We have performed experiments
to identify the temperature range over which small PAH clusters form in saturated uniform
supersonic flows. The kinetics of the formation has also been investigated. The
experimental data have been combined with theoretical calculations. We unambiguously
demonstrate that the association of small PAHs such as pyrene
(C16H10) is slower than the destruction of the dimer in warm and
hot environments such as IRC +10216. Our findings challenge a formation model based on the
physical stacking of small PAH units in circumstellar shells of carbon rich stars.