Polycyclic aromatic hydrocarbon radiative properties and formation in carbon-rich stellar outflows

The infrared emission and radiative properties of gas-phase polycyclic aromatic hydrocarbon (PAH) molecules are investigated: the infrared fluorescence spectrum of azulene (;The formation of PAH molecules in carbon-rich stellar outflows is modelled by a detailed chemical kinetic approach. This method uses a complete and up-to-date chemical scheme based on soot formation in combustion systems. Strong periodic shocks in the inner stellar envelope are considered, and their effects on the gas density and flow velocity are evaluated. The boundary molecular concentrations at the shock formation radius are derived from thermal equilibrium calculations including PAHs. The "inverse greenhouse" effect is included, and calculations are carried out for aromatics up to the size of pyrene. PAH concentration profiles are obtained for several envelope models.;The PAH formation yields are small compared to values derived from the observed dust content of stellar envelopes. Furthermore, the yields are extremely sensitive to gas density and temperature. It is therefore unlikely that aromatic molecules are generated in the stellar outflow, itself. However, another mechanism is suggested: PAHs may form and accumulate in gas parcels subjected to quasi-ballistic trajectories induced by shocks, close to the photosphere.