Insights into the condensation of PAHsin the envelope of IRC +10216

¹ Institut de Physique de Rennes, Équipe: “Astrochimie Expérimentale”, UMR CNRS 6251, Université de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France
² Current address: Zarelab, Chemistry Department, Stanford University, CA 94305, USA
³ Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, IL 60439, USA
⁴ Current address: Institut des Sciences Moléculaires, UMR CNRS 5255, Université Bordeaux 1, 33405 Talence, France

Abstract

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.