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Astron. Astrophys. 323, 163-176 (1997)

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PAHs in circumstellar envelopes

I. Processes affecting PAH formation and growth

T. Allain 1, 2, E. Sedlmayr 2 and S. Leach 1

1 Département Atomes et Molécules en Astrophysique, CNRS-URA812, Observatoire de Paris-Meudon, F-92195 Meudon, France
2 Institut für Astronomie und Astrophysik, Technische Universität Berlin, PN 8-1, Hardenbergstrasse 36, D-10623 Berlin, Germany

Received 24 May 1996 / Accepted 17 September 1996


Analytical expressions are presented to estimate the interactions of gas particles with polycyclic aromatic hydrocarbons (PAHs) described as large flat plates. This modelization enables us to determine the collision rate of gas particles with PAHs, the drag force of gas particles on PAHs and the energy exchanged between PAHs and gas particles.

These 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 determination of PAH temperatures in the envelope is also carried out. Finally, the results are discussed in the context of the role of PAHs in the formation of dust particles.

Key words: ISM: molecules – molecular processes – stars: circumstellar matter – stars: carbon rich

Send offprint requests to: S. Leach

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998