PAHS IN CIRCUMSTELLAR ENVELOPES .1. PROCESSES AFFECTING PAH FORMATIONAND GROWTH

Analytical expressions are presented to estimate the interactions of g as particles with polycyclic aromatic hydrocarbons (PAHs) described as large flat plates. This modelization enables us to determine the coll ision rate of gas particles with PAHs, the drag force of gas particles on PAHs and the energy exchanged between PAHs and gas particles. Thes e results are applied to the case of the circumstellar envelopes of C- stars where PAHs are considered to form and grow. As a result of these calculations, the drift velocities of PAHs are estimated and found to be negligible in comparison with the gas thermal velocity. The determ ination of PAH temperatures in the envelope is also carried out. Final ly, the results are discussed in the context of the role of PAHs in th e formation of dust particles.