Résumé
Polycycle Aromatic Hydrocarbons (PAH) have been suggested as a link in one of the possible routes from molecules to grains in carbon-rich stellar atmospheres. Carbon grains, except SiC, may form by conversion of C2H2 into PAHs. The most obvious site of this PAH production is the outer atmosphere of carbon stars. We present the first calculations of equilibrium partial pressures of PAHs and other complex carbon-bearing molecules in stellar photosphere models for carbon si-ars, We have included 38 atomic species and 338 molecules in chemical equilibrium. Our computed models have the following parameters: T-eff=(2800K, 2600K, 2400K), Z=Z(.) log(g) epsilon [-1,0.5], C/O epsilon [1.1,7.0]. The results of the hydrostatic model calculations show that or atmospheric regions with T>1000K the partial pressures of PAHs and other large molecules are negligible small. This result is independent of the choice of fundamental stellar parameters.
For shallower depths than our hydrostatic photosphere models, however, a systematic chemical equilibrium exploration of the T-P-gas plane, T epsilon[750K, 1000K], logP(gas) epsilon[-5,3] (P-gas in dyn/cm(2)), discloses PAHs as the species containing the largest fraction of carbon atoms not bound in CO for a range of temperatures around 850K, More carbon is contained in PAHs than in CO at these temperatures for C/O greater than or similar to 5.0.
Chemical equilibrium considered in a dynamical atmospheric structure confirms these results. The higher column density of PAHs is sufficient. for the dynamic model to have an effect on the structure of the model (levitation) and on the emergent spectrum. However, some studies (Frenklach & Feigelson 1989) on kinetic PAH formation require a residence time at favourable T and P-gas which is longer than the rime scales of realistic wind models. In the framework of our hydrostatic models we confirm that the conditions for PAH formation (T, P-gas or residence time) are not met, and we can point al the lower boundary condition needed for the wind models. Therefore either the observed dust grains form via other routes, or more complex stellar environments are required.